Nitrogen is essential for life and its biogeochemical cycle involves many processes. There are four main types of nitrate reductases - eukaryotic assimilatory, bacterial cytoplasmic assimilatory, membrane-bound respiratory, and periplasmic dissimilatory. They contain molybdenum and molybdenum cofactor in their active sites and catalyze the reduction of nitrate to nitrite. Bacterial nitrate reductases are further classified based on their electron donors as either ferredoxin-dependent or NADH-dependent. Genes encoding the enzymes for nitrate assimilation are often organized in operons along with those for nitrate and nitrite transporters.
Plants assimilate mineral nutrients by incorporating them into organic compounds. This requires complex biochemical reactions that are highly energy-demanding, such as the assimilation of nitrogen and sulfur which uses 12-16 ATPs per reaction. Nitrogen fixation converts atmospheric nitrogen gas into ammonium or nitrate that plants can absorb. Nitrate assimilation is a two-step process where nitrate is first reduced to nitrite then to ammonium. Ammonium is rapidly converted to glutamine and glutamate to avoid toxicity.
1. The document discusses nitrogen metabolism in plants and the nitrogen cycle. It describes how plants absorb nitrogen from the soil, the forms of nitrogen absorbed, and the processes of nitrogen fixation, nitrification, and denitrification.
2. Key parts of the nitrogen cycle described are ammonification, nitrification, nitrate assimilation, and biological and non-biological nitrogen fixation. Nitrogen is required for important molecules like amino acids and proteins.
3. The nitrogen cycle is essential for life and involves the natural processes by which nitrogen circulates between the atmosphere, soil, plants, and animals.
This document discusses nitrogen metabolism in living organisms. It notes that nitrogen is an important component of amino acids, proteins, enzymes, vitamins, and hormones. While nitrogen gas (N2) makes up 78% of the atmosphere, most organisms cannot use it directly. The nitrogen cycle converts atmospheric nitrogen into usable forms through processes like nitrogen fixation. Nitrogen fixation involves reducing nitrogen gas into compounds like ammonia, which is an anaerobic process requiring reducing conditions. There are two types of nitrogen fixation - abiological fixation such as the industrial Haber process, and biological fixation where nitrogen-fixing bacteria convert nitrogen gas to ammonia using the nitrogenase enzyme.
This document discusses nitrogen metabolism, including nitrogen fixation, amino acid anabolism and catabolism, and purine catabolism. Nitrogen fixation incorporates nitrogen from the atmosphere into organic compounds through processes like symbiotic nitrogen fixation by microorganisms in plants. The urea cycle removes nitrogen from the body in the form of urea. Purine catabolism produces uric acid as its end product in humans and other primates, which can accumulate and cause gout.
The nitrogen cycle describes the biological and physical processes by which nitrogen is converted between its various chemical forms and circulates between terrestrial, marine, and atmospheric ecosystems, driven largely by microbial activity such as nitrogen fixation, nitrification, and denitrification; nitrogen is essential but scarce for biological use, and the nitrogen cycle maintains its abundance through these processes of conversion between its inert atmospheric form and bioavailable forms incorporated into living tissues.
Molecular biology of bioluminescence mypptNeha verma
1. Bioluminescence is the production and emission of light by living organisms. It occurs in many marine organisms like bacteria, fish, squid, jellyfish, and coral.
2. The document discusses the process of bioluminescence in bacteria. It involves the reaction of luciferase, an enzyme, with luciferin substrates - reduced flavin mononucleotide and a fatty aldehyde. This reaction produces light in the form of blue-green photons.
3. Gene expression of the lux operon controls the biochemical process. LuxR and LuxI proteins regulate the expression of luciferase and fatty acid reductase enzyme complex genes through a feedback loop involving acyl-homoserine lact
Minerals are essential inorganic nutrients that are required for plant growth and development. There are two types of minerals: macronutrients which are needed in large amounts and micronutrients which are needed in trace amounts. Hydroponics is a method used to study mineral requirements of plants by growing them in nutrient solutions. Essential minerals meet certain criteria and are involved in key metabolic processes in plants. Nitrogen is an important macronutrient that plants obtain through nitrogen fixation by symbiotic bacteria in root nodules of legumes or free-living soil bacteria. The nitrogen is then assimilated into amino acids and proteins in plants.
Plants assimilate mineral nutrients by incorporating them into organic compounds. This requires complex biochemical reactions that are highly energy-demanding, such as the assimilation of nitrogen and sulfur which uses 12-16 ATPs per reaction. Nitrogen fixation converts atmospheric nitrogen gas into ammonium or nitrate that plants can absorb. Nitrate assimilation is a two-step process where nitrate is first reduced to nitrite then to ammonium. Ammonium is rapidly converted to glutamine and glutamate to avoid toxicity.
1. The document discusses nitrogen metabolism in plants and the nitrogen cycle. It describes how plants absorb nitrogen from the soil, the forms of nitrogen absorbed, and the processes of nitrogen fixation, nitrification, and denitrification.
2. Key parts of the nitrogen cycle described are ammonification, nitrification, nitrate assimilation, and biological and non-biological nitrogen fixation. Nitrogen is required for important molecules like amino acids and proteins.
3. The nitrogen cycle is essential for life and involves the natural processes by which nitrogen circulates between the atmosphere, soil, plants, and animals.
This document discusses nitrogen metabolism in living organisms. It notes that nitrogen is an important component of amino acids, proteins, enzymes, vitamins, and hormones. While nitrogen gas (N2) makes up 78% of the atmosphere, most organisms cannot use it directly. The nitrogen cycle converts atmospheric nitrogen into usable forms through processes like nitrogen fixation. Nitrogen fixation involves reducing nitrogen gas into compounds like ammonia, which is an anaerobic process requiring reducing conditions. There are two types of nitrogen fixation - abiological fixation such as the industrial Haber process, and biological fixation where nitrogen-fixing bacteria convert nitrogen gas to ammonia using the nitrogenase enzyme.
This document discusses nitrogen metabolism, including nitrogen fixation, amino acid anabolism and catabolism, and purine catabolism. Nitrogen fixation incorporates nitrogen from the atmosphere into organic compounds through processes like symbiotic nitrogen fixation by microorganisms in plants. The urea cycle removes nitrogen from the body in the form of urea. Purine catabolism produces uric acid as its end product in humans and other primates, which can accumulate and cause gout.
The nitrogen cycle describes the biological and physical processes by which nitrogen is converted between its various chemical forms and circulates between terrestrial, marine, and atmospheric ecosystems, driven largely by microbial activity such as nitrogen fixation, nitrification, and denitrification; nitrogen is essential but scarce for biological use, and the nitrogen cycle maintains its abundance through these processes of conversion between its inert atmospheric form and bioavailable forms incorporated into living tissues.
Molecular biology of bioluminescence mypptNeha verma
1. Bioluminescence is the production and emission of light by living organisms. It occurs in many marine organisms like bacteria, fish, squid, jellyfish, and coral.
2. The document discusses the process of bioluminescence in bacteria. It involves the reaction of luciferase, an enzyme, with luciferin substrates - reduced flavin mononucleotide and a fatty aldehyde. This reaction produces light in the form of blue-green photons.
3. Gene expression of the lux operon controls the biochemical process. LuxR and LuxI proteins regulate the expression of luciferase and fatty acid reductase enzyme complex genes through a feedback loop involving acyl-homoserine lact
Minerals are essential inorganic nutrients that are required for plant growth and development. There are two types of minerals: macronutrients which are needed in large amounts and micronutrients which are needed in trace amounts. Hydroponics is a method used to study mineral requirements of plants by growing them in nutrient solutions. Essential minerals meet certain criteria and are involved in key metabolic processes in plants. Nitrogen is an important macronutrient that plants obtain through nitrogen fixation by symbiotic bacteria in root nodules of legumes or free-living soil bacteria. The nitrogen is then assimilated into amino acids and proteins in plants.
This document discusses Rubisco, the most abundant enzyme on Earth, which catalyzes the first major step of photosynthesis and photorespiration. Rubisco exists in two forms with different functions - RuBP carboxylase, which catalyzes the first step of photosynthesis, and RuBP oxygenase, which catalyzes the first step of the wasteful process of photorespiration. The document outlines the conditions that favor each form and describes photorespiration in C3, C4, and CAM plants. It also discusses the significance and factors affecting photosynthesis.
This document summarizes a study that investigated the degradation of halogenated organic compounds (haloorganics) by iron porphyrin complexes as biomimetic models of marine microbial pathways. The study reacted various iodinated substrates with different iron porphyrin complexes and monitored the reactions using UV-Vis spectroscopy and gas chromatography-mass spectrometry. The results showed that the iodo substrates reacted fastest, followed by chloro and bromo substrates. Products identified included methane, ethylene, and the corresponding dehalogenated alkene. Reaction rates correlated with the redox potential of the iron complex, with higher redox potentials corresponding to faster rates. The study provides insights into abiotic marine degradation of haloorgan
This slideshow explains the details about Photosynthesis process. It has covered all the aspects such as definition, significance, photosystems, Hill reaction, Calvin cycle, HSK cycle, CAM pathway, Photorespiration, etc. of photosynthesis. This slide show will be useful to College students and the students who are appearing for various competitive examinations. .This slide show is equally beneficial to the students who want to pursue career in the biological sciences.
Overview of the pigment Chlorophyll, its sources, types, structure, photoreceptors, benefits, stability, degradation, preservation during food processing and technologies associated with it.
This article discusses how methanol (MeOH) affects ascorbate (AsA) levels and related gene expression in Oncidium orchids. The study found that a low concentration of MeOH (50 mM) triggered hydrogen peroxide (H2O2) production and increased expression of AsA biosynthesis genes. However, a high MeOH concentration was toxic and depleted AsA levels. The increased gene expression from low MeOH levels was blocked by inhibitors of alcohol oxidase and NADPH oxidase, suggesting H2O2 is a signaling molecule regulating AsA gene expression in response to MeOH. The results help explain the mechanism by which MeOH stimulates AsA production in Oncidium at the molecular level.
Reactive oxygen and nitrogen species (ROS/RNS) are highly reactive molecules that can damage cells. They include radicals like superoxide and hydroxyl, as well as non-radical species like hydrogen peroxide. ROS/RNS are produced through normal cellular processes but also from external sources like radiation and pollutants. They can cause oxidative damage to lipids, proteins, and DNA if not neutralized by antioxidant defenses. This oxidative stress has been linked to many diseases due to disruption of redox signaling and control in the cell.
1. Nitrification and denitrification can occur in the same environment because nitrate produced during nitrification is transported by water to anoxic environments where it is used in denitrification.
2. Biological nitrogen fixation requires more electrons than expected to produce ammonia from nitrogen gas. It also produces hydrogen gas, making it less energetically favorable than chemical nitrogen fixation.
3. Nodules on plant roots maintain an anaerobic environment for nitrogen-fixing bacteria through the high concentration of leghemoglobin which binds oxygen, keeping levels too low to inhibit nitrogenase.
Deleterious effects of RONS on biomoleculesNoor Lasheen
Reactive oxygen and nitrogen species (RONS) are produced naturally during metabolism but also as a response to stressors like radiation, heat, and inflammation. At high levels, RONS can damage biomolecules through oxidation, leading to oxidative stress. The main types of damage include lipid peroxidation of cell membranes, oxidation of DNA and RNA bases, protein carbonylation and nitration, and enzyme deactivation through cofactor oxidation. These molecular changes disrupt normal cellular functions and contribute to aging and diseases over time.
nitrate and sulfate reduction ; methanogenesis and acetogenesisjyoti arora
this powerpoint includes the following topics in brief:
1. Nitrate assimilatory reduction
2. Sulfate assimilatory reduction
3. Methanogenesis
4. Acetogenesis
This document discusses reactive oxygen species (ROS), specifically superoxide. It defines ROS as chemically reactive molecules containing oxygen that are produced naturally during cellular metabolism. Superoxide is formed as a byproduct of mitochondrial electron transport and can damage cells when overproduced. The document describes how hydroethidine fluorescence is used to selectively detect superoxide levels in mitochondria, finding increased fluorescence with antimycin stimulation. It concludes that precise superoxide detection aids understanding of its role in signaling and damage.
This document summarizes the synthesis and characterization of the oxomolybdenum mono-ene-1,2-dithiolate complex (Tp*)MoO(bdtCl2) (3). X-ray crystallography showed compound 3 crystallizes in the monoclinic space group P21/c, with a distorted octahedral coordination geometry around the Mo atom. IR, EPR, and electronic absorption spectroscopy indicate the chlorine substituents do not significantly perturb the electronic structure of the Mo(V) center, but solution redox potentials and gas-phase ionization energies are sensitive to remote substituent effects. The coordination environment of 3 is similar to the related complex (Tp*)MoO(
The document discusses the process of chlorophyll degradation in senescent plant tissues. It begins with an introduction to chlorophyll and degreening. The key points are:
1. Chlorophyll degradation is catalyzed by the enzyme chlorophyllase, which removes the phytol group from chlorophyll.
2. The breakdown products of chlorophyll degradation include pheophytin, red chlorophyll catabolite (RCC), and primary fluorescent chlorophyll catabolite (pFCC). pFCCs are further modified and transported to the vacuole where they spontaneously isomerize to non-fluorescent chlorophyll catabolites (NCCs).
3. The pathway and enzymes involved
The document discusses the chemical structure and metabolism of bacteria. It describes the principal elements that make up bacterial cells, including carbon, hydrogen, oxygen, nitrogen, phosphorus, and others. It also discusses macromolecules that constitute bacterial cells, such as proteins, RNA, DNA, lipids, and carbohydrates. Additionally, it outlines various environmental factors that influence bacterial growth, such as temperature, oxygen, pH, and osmotic pressure.
This document outlines a lecture on carbohydrate and nucleic acid chemistry. It begins by introducing carbohydrates as one of the four major classes of biological molecules, along with proteins, lipids, and nucleic acids. Carbohydrates serve important nutritional, structural, informational, and regulatory functions in living systems. They are classified based on their monomer units into monosaccharides, disaccharides, oligosaccharides, and polysaccharides. The lecture further discusses carbohydrate isomers, epimers, enantiomers, cyclization, and polysaccharides such as starch, glycogen, cellulose, and chitin. It then introduces nucleic acids DNA and RNA as polymers of nucleotides, highlighting their monomers, base pairing, and DNA
This document summarizes research on the reduction of manganese porphyrins by flavoenzymes and mitochondrial particles, and how this relates to the catalytic reduction of peroxynitrite. It finds that flavoenzymes like xanthine oxidase and glucose oxidase, as well as mitochondrial Complex I and II, catalyze the reduction of manganese porphyrins using substrates like NADH and succinate. Reduced manganese porphyrins can then rapidly react with peroxynitrite via a two-electron process, producing nitrite and manganese porphyrin radicals. This redox cycle can protect peroxynitrite-sensitive targets in submitochondrial particles.
1. The document summarizes how photosynthesis works through light-dependent and light-independent reactions. It describes the process of capturing energy from sunlight to make ATP and NADPH, and then using these products to fix carbon dioxide and produce sugars.
2. Photosynthesis takes place in chloroplasts in plant cells, which contain thylakoid membranes where the light-dependent reactions occur. These reactions use light energy to convert water to oxygen and produce ATP and NADPH.
3. The light-independent reactions then use ATP and NADPH to fix carbon dioxide and produce glucose through the Calvin cycle. This provides the sugars and starches that plants need for growth and energy storage.
Methanotrophs are unique bacteria that use methane as their sole carbon and energy source. They play several important roles in the environment. First, they oxidize methane produced by archaea, reducing the amount of this potent greenhouse gas that enters the atmosphere. Second, they can degrade various organic pollutants using monooxygenase enzymes. Finally, they can remediate heavy metals by transforming more toxic forms into less toxic forms or facilitating precipitation. Methanotrophs thus aid in carbon and pollutant cycling while also helping to mitigate climate change.
Free radicals are highly unstable chemical species with unpaired electrons that can damage cells. Reactive oxygen and nitrogen species are important free radicals generated through normal cellular processes and environmental exposures that can initiate chain reactions. Free radicals attack and degrade membranes, proteins, and nucleic acids. This can lead to lipid peroxidation, protein oxidation, and DNA damage implicated in various diseases. Cells employ antioxidant defenses and enzymes like catalase and superoxide dismutase to limit free radical damage.
This document provides an overview of inorganic nitrogen and sulfur metabolism. It discusses biological nitrogen fixation, the process by which molecular nitrogen is converted to ammonia. Key points include:
- Biological nitrogen fixation is carried out by bacteria that live freely or in symbiotic relationships with plants. Important symbiotic fixers include Rhizobium bacteria that form nodules on legume roots.
- Nitrogen fixation requires the nitrogenase enzyme, protection from oxygen, electron donors like ferredoxin, a source of ATP, and the metals molybdenum and iron. It occurs only in the absence of fixed nitrogen.
- Nodule formation on legume roots involves recognition between the plant and bacteria, infection
This document discusses nitrogen fixation and the nitrogen cycle. It describes the key enzyme nitrogenase, which consists of an iron protein and an iron-molybdenum protein. Nitrogenase facilitates the conversion of atmospheric nitrogen to ammonia through its metal components. Biological nitrogen fixation can occur through non-symbiotic bacteria or through a symbiotic relationship between rhizobia bacteria and legumes in root nodules. The process involves the reduction of nitrogen to ammonia using solar energy and coenzymes like ferredoxin.
CS_701_Nitrate Assimilation by arnold_damasoAr R Ventura
Nitrate assimilation is the formation of organic nitrogen compounds like amino acids from inorganic nitrogen compounds present in the environment. Organisms like plants, fungi and certain bacteria that cannot fix nitrogen gas (N2) depend on the ability to assimilate nitrate or ammonia for their needs.
Plants like castor reduce a lot of nitrate in the root itself, and excrete the resulting base. Some of the base produced in the shoots is transported to the roots as salts of organic acids while a small amount of the carboxylates are just stored in the shoot itself. However, about 99% of the organic nitrogen in the biosphere is derived from the assimilation of nitrate. NH4+ is formed as an end product of the degradation of organic matter, primarily by the metabolism of animals and bacteria, and is oxidized to nitrate again by nitrifying bacteria in the soil. Thus a continuous cycle exists between the nitrate in the soil and the organic nitrogen in the plants growing on it. While nearly all the ammonia in the root is usually incorporated into amino acids at the root itself, plants may transport significant amounts of ammonium ions in the xylem to be fixed in the shoots. This may help avoid the transport of organic compounds down to the roots just to carry the nitrogen back as amino acids.
This document discusses Rubisco, the most abundant enzyme on Earth, which catalyzes the first major step of photosynthesis and photorespiration. Rubisco exists in two forms with different functions - RuBP carboxylase, which catalyzes the first step of photosynthesis, and RuBP oxygenase, which catalyzes the first step of the wasteful process of photorespiration. The document outlines the conditions that favor each form and describes photorespiration in C3, C4, and CAM plants. It also discusses the significance and factors affecting photosynthesis.
This document summarizes a study that investigated the degradation of halogenated organic compounds (haloorganics) by iron porphyrin complexes as biomimetic models of marine microbial pathways. The study reacted various iodinated substrates with different iron porphyrin complexes and monitored the reactions using UV-Vis spectroscopy and gas chromatography-mass spectrometry. The results showed that the iodo substrates reacted fastest, followed by chloro and bromo substrates. Products identified included methane, ethylene, and the corresponding dehalogenated alkene. Reaction rates correlated with the redox potential of the iron complex, with higher redox potentials corresponding to faster rates. The study provides insights into abiotic marine degradation of haloorgan
This slideshow explains the details about Photosynthesis process. It has covered all the aspects such as definition, significance, photosystems, Hill reaction, Calvin cycle, HSK cycle, CAM pathway, Photorespiration, etc. of photosynthesis. This slide show will be useful to College students and the students who are appearing for various competitive examinations. .This slide show is equally beneficial to the students who want to pursue career in the biological sciences.
Overview of the pigment Chlorophyll, its sources, types, structure, photoreceptors, benefits, stability, degradation, preservation during food processing and technologies associated with it.
This article discusses how methanol (MeOH) affects ascorbate (AsA) levels and related gene expression in Oncidium orchids. The study found that a low concentration of MeOH (50 mM) triggered hydrogen peroxide (H2O2) production and increased expression of AsA biosynthesis genes. However, a high MeOH concentration was toxic and depleted AsA levels. The increased gene expression from low MeOH levels was blocked by inhibitors of alcohol oxidase and NADPH oxidase, suggesting H2O2 is a signaling molecule regulating AsA gene expression in response to MeOH. The results help explain the mechanism by which MeOH stimulates AsA production in Oncidium at the molecular level.
Reactive oxygen and nitrogen species (ROS/RNS) are highly reactive molecules that can damage cells. They include radicals like superoxide and hydroxyl, as well as non-radical species like hydrogen peroxide. ROS/RNS are produced through normal cellular processes but also from external sources like radiation and pollutants. They can cause oxidative damage to lipids, proteins, and DNA if not neutralized by antioxidant defenses. This oxidative stress has been linked to many diseases due to disruption of redox signaling and control in the cell.
1. Nitrification and denitrification can occur in the same environment because nitrate produced during nitrification is transported by water to anoxic environments where it is used in denitrification.
2. Biological nitrogen fixation requires more electrons than expected to produce ammonia from nitrogen gas. It also produces hydrogen gas, making it less energetically favorable than chemical nitrogen fixation.
3. Nodules on plant roots maintain an anaerobic environment for nitrogen-fixing bacteria through the high concentration of leghemoglobin which binds oxygen, keeping levels too low to inhibit nitrogenase.
Deleterious effects of RONS on biomoleculesNoor Lasheen
Reactive oxygen and nitrogen species (RONS) are produced naturally during metabolism but also as a response to stressors like radiation, heat, and inflammation. At high levels, RONS can damage biomolecules through oxidation, leading to oxidative stress. The main types of damage include lipid peroxidation of cell membranes, oxidation of DNA and RNA bases, protein carbonylation and nitration, and enzyme deactivation through cofactor oxidation. These molecular changes disrupt normal cellular functions and contribute to aging and diseases over time.
nitrate and sulfate reduction ; methanogenesis and acetogenesisjyoti arora
this powerpoint includes the following topics in brief:
1. Nitrate assimilatory reduction
2. Sulfate assimilatory reduction
3. Methanogenesis
4. Acetogenesis
This document discusses reactive oxygen species (ROS), specifically superoxide. It defines ROS as chemically reactive molecules containing oxygen that are produced naturally during cellular metabolism. Superoxide is formed as a byproduct of mitochondrial electron transport and can damage cells when overproduced. The document describes how hydroethidine fluorescence is used to selectively detect superoxide levels in mitochondria, finding increased fluorescence with antimycin stimulation. It concludes that precise superoxide detection aids understanding of its role in signaling and damage.
This document summarizes the synthesis and characterization of the oxomolybdenum mono-ene-1,2-dithiolate complex (Tp*)MoO(bdtCl2) (3). X-ray crystallography showed compound 3 crystallizes in the monoclinic space group P21/c, with a distorted octahedral coordination geometry around the Mo atom. IR, EPR, and electronic absorption spectroscopy indicate the chlorine substituents do not significantly perturb the electronic structure of the Mo(V) center, but solution redox potentials and gas-phase ionization energies are sensitive to remote substituent effects. The coordination environment of 3 is similar to the related complex (Tp*)MoO(
The document discusses the process of chlorophyll degradation in senescent plant tissues. It begins with an introduction to chlorophyll and degreening. The key points are:
1. Chlorophyll degradation is catalyzed by the enzyme chlorophyllase, which removes the phytol group from chlorophyll.
2. The breakdown products of chlorophyll degradation include pheophytin, red chlorophyll catabolite (RCC), and primary fluorescent chlorophyll catabolite (pFCC). pFCCs are further modified and transported to the vacuole where they spontaneously isomerize to non-fluorescent chlorophyll catabolites (NCCs).
3. The pathway and enzymes involved
The document discusses the chemical structure and metabolism of bacteria. It describes the principal elements that make up bacterial cells, including carbon, hydrogen, oxygen, nitrogen, phosphorus, and others. It also discusses macromolecules that constitute bacterial cells, such as proteins, RNA, DNA, lipids, and carbohydrates. Additionally, it outlines various environmental factors that influence bacterial growth, such as temperature, oxygen, pH, and osmotic pressure.
This document outlines a lecture on carbohydrate and nucleic acid chemistry. It begins by introducing carbohydrates as one of the four major classes of biological molecules, along with proteins, lipids, and nucleic acids. Carbohydrates serve important nutritional, structural, informational, and regulatory functions in living systems. They are classified based on their monomer units into monosaccharides, disaccharides, oligosaccharides, and polysaccharides. The lecture further discusses carbohydrate isomers, epimers, enantiomers, cyclization, and polysaccharides such as starch, glycogen, cellulose, and chitin. It then introduces nucleic acids DNA and RNA as polymers of nucleotides, highlighting their monomers, base pairing, and DNA
This document summarizes research on the reduction of manganese porphyrins by flavoenzymes and mitochondrial particles, and how this relates to the catalytic reduction of peroxynitrite. It finds that flavoenzymes like xanthine oxidase and glucose oxidase, as well as mitochondrial Complex I and II, catalyze the reduction of manganese porphyrins using substrates like NADH and succinate. Reduced manganese porphyrins can then rapidly react with peroxynitrite via a two-electron process, producing nitrite and manganese porphyrin radicals. This redox cycle can protect peroxynitrite-sensitive targets in submitochondrial particles.
1. The document summarizes how photosynthesis works through light-dependent and light-independent reactions. It describes the process of capturing energy from sunlight to make ATP and NADPH, and then using these products to fix carbon dioxide and produce sugars.
2. Photosynthesis takes place in chloroplasts in plant cells, which contain thylakoid membranes where the light-dependent reactions occur. These reactions use light energy to convert water to oxygen and produce ATP and NADPH.
3. The light-independent reactions then use ATP and NADPH to fix carbon dioxide and produce glucose through the Calvin cycle. This provides the sugars and starches that plants need for growth and energy storage.
Methanotrophs are unique bacteria that use methane as their sole carbon and energy source. They play several important roles in the environment. First, they oxidize methane produced by archaea, reducing the amount of this potent greenhouse gas that enters the atmosphere. Second, they can degrade various organic pollutants using monooxygenase enzymes. Finally, they can remediate heavy metals by transforming more toxic forms into less toxic forms or facilitating precipitation. Methanotrophs thus aid in carbon and pollutant cycling while also helping to mitigate climate change.
Free radicals are highly unstable chemical species with unpaired electrons that can damage cells. Reactive oxygen and nitrogen species are important free radicals generated through normal cellular processes and environmental exposures that can initiate chain reactions. Free radicals attack and degrade membranes, proteins, and nucleic acids. This can lead to lipid peroxidation, protein oxidation, and DNA damage implicated in various diseases. Cells employ antioxidant defenses and enzymes like catalase and superoxide dismutase to limit free radical damage.
This document provides an overview of inorganic nitrogen and sulfur metabolism. It discusses biological nitrogen fixation, the process by which molecular nitrogen is converted to ammonia. Key points include:
- Biological nitrogen fixation is carried out by bacteria that live freely or in symbiotic relationships with plants. Important symbiotic fixers include Rhizobium bacteria that form nodules on legume roots.
- Nitrogen fixation requires the nitrogenase enzyme, protection from oxygen, electron donors like ferredoxin, a source of ATP, and the metals molybdenum and iron. It occurs only in the absence of fixed nitrogen.
- Nodule formation on legume roots involves recognition between the plant and bacteria, infection
This document discusses nitrogen fixation and the nitrogen cycle. It describes the key enzyme nitrogenase, which consists of an iron protein and an iron-molybdenum protein. Nitrogenase facilitates the conversion of atmospheric nitrogen to ammonia through its metal components. Biological nitrogen fixation can occur through non-symbiotic bacteria or through a symbiotic relationship between rhizobia bacteria and legumes in root nodules. The process involves the reduction of nitrogen to ammonia using solar energy and coenzymes like ferredoxin.
CS_701_Nitrate Assimilation by arnold_damasoAr R Ventura
Nitrate assimilation is the formation of organic nitrogen compounds like amino acids from inorganic nitrogen compounds present in the environment. Organisms like plants, fungi and certain bacteria that cannot fix nitrogen gas (N2) depend on the ability to assimilate nitrate or ammonia for their needs.
Plants like castor reduce a lot of nitrate in the root itself, and excrete the resulting base. Some of the base produced in the shoots is transported to the roots as salts of organic acids while a small amount of the carboxylates are just stored in the shoot itself. However, about 99% of the organic nitrogen in the biosphere is derived from the assimilation of nitrate. NH4+ is formed as an end product of the degradation of organic matter, primarily by the metabolism of animals and bacteria, and is oxidized to nitrate again by nitrifying bacteria in the soil. Thus a continuous cycle exists between the nitrate in the soil and the organic nitrogen in the plants growing on it. While nearly all the ammonia in the root is usually incorporated into amino acids at the root itself, plants may transport significant amounts of ammonium ions in the xylem to be fixed in the shoots. This may help avoid the transport of organic compounds down to the roots just to carry the nitrogen back as amino acids.
The document discusses nitrogen fixation and the nitrogen cycle. It notes that while nitrogen gas makes up 78% of the atmosphere, plants cannot use it directly and must obtain nitrogen from the soil in the forms of nitrates and ammonium salts. Nitrogen fixation is carried out by both biological and non-biological processes, with biological nitrogen fixation being the primary means of fixing atmospheric nitrogen in the soil through the action of nitrogen-fixing bacteria and their enzyme nitrogenase. The nitrogenase enzyme converts atmospheric nitrogen gas into ammonia through an ATP-dependent process.
The document discusses nitrogen fixation and the nitrogen cycle. It notes that while nitrogen gas makes up 78% of the atmosphere, plants cannot use it directly and must obtain nitrogen from the soil in the forms of nitrates and ammonium salts. Nitrogen fixation is carried out through both biological and non-biological processes, with biological nitrogen fixation being the primary means of fixing atmospheric nitrogen in the soil through symbiotic bacteria like Rhizobium that form nodules on legume roots. The nitrogenase enzyme is responsible for this nitrogen fixation through an energy-intensive process requiring ATP.
This document discusses reactive oxygen species, reactive nitrogen species, and redox signaling. It covers the following key points:
- Nitric oxide is an important signaling molecule produced through the oxidation of L-arginine by nitric oxide synthase. There are three NOS isoforms.
- Reactive oxygen species include superoxide and hydrogen peroxide. Superoxide is produced by NADPH oxidase and can be converted to hydrogen peroxide. These molecules are involved in cell signaling but can also cause damage.
- Hydrogen sulfide is another gasotransmitter that regulates various physiological processes through protein persulfidation.
- Reactive species can modify protein cysteine residues through oxidation
The nitrogen cycle describes the transformation of nitrogen in nature. Nitrogen makes up 78% of the atmosphere but needs to be converted to usable forms for organisms. Bacteria fix nitrogen gas into ammonia through nitrogenase enzymes. Ammonia is further converted through nitrification, denitrification, and ammonification to move between nitrogen's many oxidation states. These processes are essential to life but also leave the nitrogen budget of the ocean unbalanced, with excess nitrogen escaping to the atmosphere through denitrification.
today's class PPT.pptx pre university collegeakholmes2104
Nucleic acids like DNA and RNA are polymers composed of nucleotides. A nucleotide contains a nitrogenous base, a pentose sugar, and a phosphate group. DNA contains the sugars deoxyribose and the bases adenine, guanine, cytosine, and thymine. RNA contains the sugar ribose and replaces thymine with uracil. Nucleotides are linked through phosphodiester bonds between the 5' carbon of one pentose sugar and the 3' carbon of the next, forming long chains that can be either DNA or RNA.
The document summarizes key aspects of nucleotides and nucleic acids. It begins by introducing nucleotides and their roles as energy carriers and structural components of enzymes and nucleic acids. It then describes the Watson-Crick model of DNA structure, including the double helix structure, base pairing between A-T and G-C, and antiparallel strands. It also notes that this structure allows for the duplication of genetic information during DNA replication through strand complementarity.
Nucleotides are components of nucleic acids DNA and RNA and have several important cellular functions. They contain a nitrogenous base, a pentose sugar, and one or more phosphate groups. The four major nucleotides that make up DNA contain the bases adenine, guanine, cytosine, and thymine. The four major nucleotides that make up RNA contain adenine, guanine, cytosine, and uracil instead of thymine. Nucleotides are linked together via phosphodiester bonds between the 5' phosphate of one nucleotide and the 3' hydroxyl group of the next, forming the backbones of nucleic acids.
The document summarizes the nitrogen cycle, which is the process by which nitrogen is converted between various chemical forms through biological and physical processes. It describes the major steps of the nitrogen cycle as nitrogen fixation, nitrogen assimilation, ammonification, nitrification, denitrification, and sedimentation. Nitrogen fixation involves converting nitrogen gas from the air into biologically useful forms through biological, physiochemical and industrial processes. Nitrification and denitrification are processes by which microorganisms convert nitrogen between its inorganic and organic forms.
Structure and Catalytic Function of Superoxide Dismutase.pptxSOUMYAJITBANIK20144
Superoxide dismutase (SOD) is a metalloenzyme that catalyzes the breakdown of the toxic superoxide radical into oxygen and hydrogen peroxide. There are three major types of SOD depending on the metal cofactor: copper-zinc SOD found in eukaryotes including humans, iron SOD found in prokaryotes and mitochondria, and manganese SOD also found in mitochondria. A fourth type uses nickel as its cofactor and is found in some prokaryotes. All SODs use similar catalytic mechanisms involving the oxidation and reduction of the metal cofactor to neutralize superoxide radicals via dismutation reactions.
Heterogeneous solid liquid catalysis of n-glycosylation by natural phosphate ...Alexander Decker
This document describes a study that investigated using natural phosphate doped with potassium iodide (NP/KI) as a heterogeneous solid-liquid catalyst for the one-step synthesis of several β-D-ribonucleosides. The reaction of 1-O-acetyl-2,3,5-tri-O-benzoyl-β-D-ribofuranoside with various nucleobases including uracil, thymine, cytosine, adenine, and N-acetyl-guanine in the presence of NP/KI produced the corresponding ribonucleosides in good yields ranging from 45-55% after refluxing overnight. The use of NP/KI as the catalyst resulted in higher yields
The document summarizes several biogeochemical cycles including nitrogen and phosphorus cycles. It describes how nitrogen and phosphorus cycle through ecosystems via biological and geological processes. For the nitrogen cycle, it outlines the five key steps of nitrogen fixation, assimilation, mineralization, nitrification, and denitrification. It provides details on the microorganisms involved in each step and factors that control the processes. The same level of detail is provided for the phosphorus cycle which involves mineralization, assimilation, precipitation of phosphorus compounds, and microbial solubilization of phosphorus.
Nitrogen is a universally occurring element in all the living beings.
Apart from water and mineral salts the next major substance in plant cell is protein (about 10-12% of the cell).
These proteins which are building blocks of the protoplasm are made up of nitrogenous substances called as the amino acids
During favorable conditions, the level of reactive spices in the cell is limited to what is required for normal cellular activities. They act as important components of signaling pathways. Plants control some important processes such as defense, hormonal signaling and development by using them as signaling molecules. And An equilibrium is steblished between antioxidant system and ros formation. But when plant feels an external stress like, drought,cold, salt etc. the level of reactive specease increases above the basal level a situation that we call oxidative stress. These reactive molecules during oxidative stress, they react with biomolecules like as carbohydrates, unsaturated lipids, proteins, nucleic acids. Proteins are the most abundant cellular targets of the oxidative species, more than DNA and lipids, making up 68% of the oxidized molecules in the cell. Ros reacts with proteins which results in protein modification called redox PTMs.
This document describes the synthesis and characterization of a manganese(III) complex intended to mimic the active site of superoxide dismutase (SOD) enzymes. A tetradentate ligand containing two quinoline groups (DQEA) was synthesized and characterized using NMR and IR spectroscopy. This ligand was then reacted with manganese(III) acetate to form the Mn(III) complex. The ability of this complex to catalyze the dismutation of superoxide, mimicking SOD activity, was confirmed using the Fridovich assay. Overall, the goal was to develop a low molecular weight SOD mimic that could function as a potential pharmaceutical for reducing oxidative stress.
The document provides an overview of nitrogen metabolism. It discusses (1) the importance of nitrogen in proteins and nucleic acids, (2) the key anabolic processes of nitrogen fixation, amino acid synthesis and protein synthesis, and (3) the main catabolic processes of proteolysis, nitrification and denitrification. Nitrogen is obtained from the atmosphere through nitrogen fixation by bacteria and is used to synthesize amino acids and proteins essential for plant structure and function.
Nitro compounds contain nitro (NO2) functional groups bonded to carbon atoms. They are usually prepared synthetically but some occur naturally. Nitro compounds are explosive, especially those with multiple nitro groups, because thermal decomposition releases nitrogen gas and large amounts of energy. Common explosives include picric acid, TNT, and styphnic acid. Nitro compounds are classified as aliphatic or aromatic depending on whether the nitro group is bonded to an alkyl or aryl group. They have important industrial uses as solvents or intermediates for dyes, drugs, and pesticides.
Similar to Nitrate Reductases: Structure, Functions, and Effect of Stress Factors (20)
Este documento presenta una propuesta didáctica para el aprendizaje de la fotosíntesis dirigida a estudiantes del Colegio José María Carbonell en Bogotá. Inicialmente, describe el contexto del colegio y sus estudiantes de estratos bajos con necesidades económicas. Luego, detalla la metodología que incluye revisar conceptos sobre fotosíntesis, su historia y las ideas previas de los estudiantes para desarrollar una unidad didáctica usando analogías. El objetivo es mejorar la enseñanza
Este documento describe el proceso de fotosíntesis en plantas. Explica que la fotosíntesis convierte la energía solar, el CO2 y el agua en carbohidratos mediante dos reacciones principales: la luminosa y la oscura. La luminosa utiliza la luz para producir ATP y NADPH, mientras que la oscura usa esta energía química para fijar el CO2 en carbohidratos. También describe factores como la luz, el punto de compensación, la saturación y la capacidad fotosintética máxima. Finalmente,
El documento describe tres mecanismos de fotosíntesis en plantas: el modo C3, el modo C4 y el metabolismo ácido de las crasuláceas (CAM). El modo C3 es el mecanismo primitivo, mientras que los modos C4 y CAM evolucionaron más tarde como adaptaciones a condiciones de calor, sequía y bajos niveles de dióxido de carbono. C4 y CAM permiten concentrar el CO2 alrededor de las células que realizan la fotosíntesis.
Este documento describe los aspectos básicos de la fotosíntesis. Explica que la fotosíntesis es un proceso mediante el cual las plantas, algas y bacterias utilizan la energía de la luz solar para sintetizar compuestos orgánicos. También describe que la fotosíntesis oxigenica libera oxígeno y utiliza dióxido de carbono, mientras que la fotosíntesis anoxigenica no produce oxígeno. Además, explica que el conocimiento de este proceso es fundamental para entender las relaciones entre los
Este documento describe varios métodos para estimar la transpiración de plantas, incluyendo lisímetros, potómetros e intercambio de gases. Los lisímetros miden la pérdida de peso de una planta en un contenedor sellado, mientras que los potómetros usan un indicador de burbujas de aire para medir el flujo de agua a través de ramas cortadas. Los sistemas de intercambio de gases usan analizadores infrarrojos de gases para medir la ganancia neta de vapor de agua que sale de las hojas.
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Este documento presenta los siete pasos para elaborar un póster científico de manera efectiva: 1) Planificar, 2) Componer, 3) Elaborar, 4) Revisar, 5) Imprimir, 6) Trasladar, y 7) Presentar. Explica cada paso en detalle, incluyendo aspectos como las preguntas para la planificación, los elementos de diseño como colores y letras, y consejos para la composición, elaboración y presentación del póster. El objetivo es producir un póster atractivo visualmente que comunique de man
La esferocitosis hereditaria es una enfermedad hereditaria causada por deficiencias moleculares en proteínas de la membrana eritrocitaria como la espectrina, ankirina y banda 3. Esto causa fragilidad osmótica de los eritrocitos y anemia hemolítica. Se hereda de forma autosómica dominante y su prevalencia es de 1 en 2000 personas. Los síntomas van desde asintomáticos hasta anemia severa, requiriendo tratamiento con ácido fólico y esplenectomía en casos graves.
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Este documento presenta las recomendaciones conjuntas de la EFLM y COLABIOCLI para la extracción de muestras de sangre venosa. El procedimiento de venopunción tiene gran variabilidad entre centros y países y es responsable de un alto porcentaje de errores preanalíticos. La guía establece recomendaciones detalladas para cada fase del proceso (preextracción, extracción y postextracción) basadas en evidencia y consenso de expertos. Su implementación requiere formación del personal, estandarización de los procedimientos y auditorías
Este documento proporciona directrices para realizar correctamente la prueba de sensibilidad antibiótica conocida como antibiograma de discos. Describe la importancia de usar cepas de control estandar para garantizar resultados confiables y la selección apropiada de antibióticos para probar dependiendo del microorganismo. También cubre aspectos técnicos como el almacenamiento y uso adecuado de los discos antibióticos.
analisis microbiologico de productos farmaceuticos no esterilesIPN
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Chlamydia es un género de bacterias intracelulares obligadas que incluye especies que infectan humanos y animales. Tienen una forma de cuerpo elemental infecciosa y una forma de cuerpo reticulado no infecciosa. Chlamydophila abortus causa abortos en ovejas, cabras y otros rumiantes al infectar la placenta, mientras que Chlamydophila felis causa conjuntivitis y queratitis en gatos. Ambas especies se transmiten principalmente a través de secreciones reproductivas y oculares, respectivamente. Su
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El documento describe el diagnóstico de pielonefritis infecciosa bovina en una zona entre Puebla y Veracruz en México. El diagnóstico se estableció basado en observaciones clínicas y pruebas de laboratorio que incluyeron el aislamiento e identificación del agente causal, Corynebacterium renale. Las lesiones patológicas incluyeron inflamación, hemorragias y úlceras en la vejiga urinaria.
Strategies for Effective Upskilling is a presentation by Chinwendu Peace in a Your Skill Boost Masterclass organisation by the Excellence Foundation for South Sudan on 08th and 09th June 2024 from 1 PM to 3 PM on each day.
বাংলাদেশের অর্থনৈতিক সমীক্ষা ২০২৪ [Bangladesh Economic Review 2024 Bangla.pdf] কম্পিউটার , ট্যাব ও স্মার্ট ফোন ভার্সন সহ সম্পূর্ণ বাংলা ই-বুক বা pdf বই " সুচিপত্র ...বুকমার্ক মেনু 🔖 ও হাইপার লিংক মেনু 📝👆 যুক্ত ..
আমাদের সবার জন্য খুব খুব গুরুত্বপূর্ণ একটি বই ..বিসিএস, ব্যাংক, ইউনিভার্সিটি ভর্তি ও যে কোন প্রতিযোগিতা মূলক পরীক্ষার জন্য এর খুব ইম্পরট্যান্ট একটি বিষয় ...তাছাড়া বাংলাদেশের সাম্প্রতিক যে কোন ডাটা বা তথ্য এই বইতে পাবেন ...
তাই একজন নাগরিক হিসাবে এই তথ্য গুলো আপনার জানা প্রয়োজন ...।
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How to Make a Field Mandatory in Odoo 17Celine George
In Odoo, making a field required can be done through both Python code and XML views. When you set the required attribute to True in Python code, it makes the field required across all views where it's used. Conversely, when you set the required attribute in XML views, it makes the field required only in the context of that particular view.
it describes the bony anatomy including the femoral head , acetabulum, labrum . also discusses the capsule , ligaments . muscle that act on the hip joint and the range of motion are outlined. factors affecting hip joint stability and weight transmission through the joint are summarized.
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Philippine Edukasyong Pantahanan at Pangkabuhayan (EPP) CurriculumMJDuyan
(𝐓𝐋𝐄 𝟏𝟎𝟎) (𝐋𝐞𝐬𝐬𝐨𝐧 𝟏)-𝐏𝐫𝐞𝐥𝐢𝐦𝐬
𝐃𝐢𝐬𝐜𝐮𝐬𝐬 𝐭𝐡𝐞 𝐄𝐏𝐏 𝐂𝐮𝐫𝐫𝐢𝐜𝐮𝐥𝐮𝐦 𝐢𝐧 𝐭𝐡𝐞 𝐏𝐡𝐢𝐥𝐢𝐩𝐩𝐢𝐧𝐞𝐬:
- Understand the goals and objectives of the Edukasyong Pantahanan at Pangkabuhayan (EPP) curriculum, recognizing its importance in fostering practical life skills and values among students. Students will also be able to identify the key components and subjects covered, such as agriculture, home economics, industrial arts, and information and communication technology.
𝐄𝐱𝐩𝐥𝐚𝐢𝐧 𝐭𝐡𝐞 𝐍𝐚𝐭𝐮𝐫𝐞 𝐚𝐧𝐝 𝐒𝐜𝐨𝐩𝐞 𝐨𝐟 𝐚𝐧 𝐄𝐧𝐭𝐫𝐞𝐩𝐫𝐞𝐧𝐞𝐮𝐫:
-Define entrepreneurship, distinguishing it from general business activities by emphasizing its focus on innovation, risk-taking, and value creation. Students will describe the characteristics and traits of successful entrepreneurs, including their roles and responsibilities, and discuss the broader economic and social impacts of entrepreneurial activities on both local and global scales.
2. 1152 MOROZKINA, ZVYAGILSKAYA
BIOCHEMISTRY (Moscow) Vol. 72 No. 10 2007
researchers. Studying periplasmic NRase (Nap) in P. pan-
totrophus by EXAFS (Extended X-ray Absorption Fine
Structure) pointed to the existence of di-oxo-condition in
Mo(VI) [11], whereas the Nap crystalline structure in D.
desulfuricans suggests mono-oxo condition in Mo(VI). It
should be noted that the amino acid sequence of Nap from
D. desulfuricans is similar to that of assimilatory enzymes.
There are a number of serine residues that increase the
probability of Mo–O–Ser ligands in Nar compared with
formation of Mo–S–Cys ligands found for Nap [7] and
predicted for Nas. Structural and molecular-genetic pecu-
liarities of different type NRases will be considered below.
ASSIMILATORY BACTERIAL NITRATE
REDUCTASE (Nas)
Two classes of assimilatory NRases have been found
in bacteria: ferredoxin- or flavodoxin-dependent Nas and
NADH-dependent Nas, the classification of which is
based on the cofactor structure (Fig. 5).
Ferredoxin- or flavodoxin-dependent bacterial and
cyanobacterial NRases contain in their active center Mo-
co and [Fe-S] clusters and are devoid of FAD or
cytochromes [12, 13]. NRases from Azotobacter vinelandii
and Plectonema boryanum consist of a single 105 kD sub-
unit incorporating a [4Fe-4S] center and molybdenum (1
atom/molecule) and use flavodoxin as a physiological
donor of electrons [14]. Analysis of amino acid sequence
of these enzymes has shown the presence of cysteine
residues at the N-terminus, which evidently binds the
[4Fe-4S] center. Ferredoxin-dependent NRases have
been found in A. chroococcum, Clostridium perfringens,
and Ectothiorhodospira shaposhnikovii [1].
NADH-dependent NRases of Klebsiella oxytoca
(pneumoniae) and Rhodobacter capsulatus are het-
erodimers consisting of the diaphorase FAD-containing
subunit of 45 kD and a catalytic subunit of 95 kD with
molybdenum cofactor and N-terminal [4Fe-4S] center
[15]. Besides, NRase of Klebsiella oxytoca (pneumoniae)
contains an additional [2Fe-2S] center bound to a C-ter-
minal cluster of cysteine residues, which resembles by its
amino acid sequence the NifU protein, the nifU gene
product, involved in formation of [Fe-S] clusters in the
nitrogenase active center [16].
This region is absent from the ferredoxin-dependent
NRase, whereas in the NADH-dependent NRases it may
play the role of the ferredoxin-similar domain carrying
out electron transfer. The NADH-dependent NRase of
Bacillus subtilis does not contain the NifU-like domain in
the catalytic subunit, but contains two tandem NifU-like
regions in the diaphorase subunit, which may be involved
in [4Fe-4S] or [3Fe-4S] binding [17].
Regulatory and structural genes responsible for
nitrate and nitrite uptake (transfer) and reduction are
usually located in the same operon. The nasFEDCBA
operon of K. oxytoca M5al encodes proteins responsible
for nitrate and nitrite assimilation [18, 19], and in this
case genes nasFED encode a periplasmic carrier that pro-
vides for nitrate and nitrite transfer [19], the NADH-
dependent NRase is encoded by genes nasC (diaphorase
subunit) and nasA (catalytic subunit), whereas gene nasB
is responsible for assimilatory nitrite reductase [20].
The operon encoding enzymes responsible for
nitrate assimilation is also well-studied and characterized
in A. vinelandii. Structural genes encoding flavodoxin-
dependent NRase of A. vinelandii (nasB) are transcribed
together with gene nasA responsible for synthesis of nitrite
reductase [20]. In most other studied bacteria, genes nirA,
nrtABCD, and narB, responsible, respectively, for NRase
Fig. 1. Biogeochemical cycle of nitrogen turnover.
–3 –1 +1 +3 +5
N2 N2O
NO2
–
NH4
+ NO3
–
NO2
–
NO2
–
Nitrogen fixation Denitrification
assimilatory and dissimilatory
nitrate reduction
nitrification
electron donor required
electron acceptor required
Fig. 2. Reactions of nitrogen reduction in pro- and eukaryotes.
NONO3
–
NO2
–
Denitrification
NarG or NapA
N2O N2
Dissimilatory reduction of nitrate to ammonium
NO3
– NarG Nir NH3NO2
–
NO3
– NapA Nir NH3NO2
–
Assimilatory reduction of nitrate to ammonium
Eucaryotes
NO3
– Euk-NR
Nir
NH3NO2
–
Procaryotes
NO3
–
Nir
NH3NO2
–Nas
3. NITRATE REDUCTASES: STRUCTURE, FUNCTIONS, AND EFFECT OF STRESS FACTORS 1153
BIOCHEMISTRY (Moscow) Vol. 72 No. 10 2007
synthesis, nitrate transport, and nitrite reductase synthe-
sis, make up a single operon [21, 22].
Nitrate transport into a cell involves carriers of the
NarK family and ATP-dependent transporters of the
ABC family (ATP-binding cassette transporters) [18, 23].
Bacillus subtilis is able to grow using nitrate as a
unique source of nitrogen. Assimilatory nitrate and nitrite
reductases are encoded by the locus containing genes
nasABCDEF. The nasA gene mutant was not able to grow
Fig. 3. Structure of molybdopterin-guanine-dinucleotide cofactor (MGD cofactor) (a) and active center of NRase (b): I, eukaryotic (Euk-
Nar); II, periplasmic (Nap); III, membrane-bound (Nar).
H2N
P
P P
N
H
N
N
H
HN
O
O
O O O
O–
O–
S
S
Mo
P
O O
S
S
H
N
N
H
O
O
O O O
O–
O–
O
H2N
HN
N
N
N
H
H H
H
O
HO HO
NH
O O
OH OH
HH
HN
N NH2
N
N N NH2
NH
O
O O
S S
SS
Mo
Ser
III
O
Mo
S
SS
S
S
Cys
II
Mo
S
S
S
O
O
Cys
I
a
b
Fig. 4. Scheme of catalytic cycle of nitrate reduction by Nap of P.
denitrificans.
NO3
–
NO2
–
ОNO2
–
OOO O
Н2O
Мо
(VI)
Мо
(IV)
Мо
(IV)3е– – 2Н
O
Fig. 5. Organization of genes encoding nitrate assimilation in bac-
teria Klebsiella oxytoca and Synechococcus PCC7942.
Klebsiella oxytoca
NO3
–
NO2
–
NO3
–
NO3
–
NO3
–
NO2
–
NH4
+
NH4
+
Synechococcus sp. PCC4972
R F E D C B A
periplasm
ntsB nirB nirA nrtaDCD narB
NasFED NrtABCD
NasA
NasC
NasB
NarB
NirA
Fd
cytoplasm
NAD(P)H
ATP ATP
MGD
4Fe4S
MGD
2Fe2S
4Fe4S FAD
2Fe2S
2Fe2S
FAD
4Fe4S
Fe Fe 4Fe4S
4. 1154 MOROZKINA, ZVYAGILSKAYA
BIOCHEMISTRY (Moscow) Vol. 72 No. 10 2007
on nitrate, but could grow on nitrite, thus indicating that
the NasA protein belongs to the family of the nitrogen
oxo-anion transporters (NarK) and is responsible for
nitrate transport. NasA homologs were also found in A.
aeolicus [24] and Ps. aeruginosa [25].
In Klebsiella and Synechococcus species the ATP-
dependent assimilatory uptake of nitrates with involve-
ment of ABC transporters is functioning. The transport
systems are encoded by nasFED genes in K. oxytoca [16,
19, 26] and by genes nrtABCD in Synechococcus sp.
PCC7942 [22]. Genes nasD, nrtD, and nrtC encode cyto-
plasmic protein subunits incorporating ATP-binding sites
of ABC transporters. Genes nasE and nrtB encode an
integral membrane subunit responsible for formation of a
channel for nitrate (and nitrite) transport through the
cytoplasmic membrane.
The level of nas gene transcription in enterobacteria
is under double control, namely, it is the repression by
ammonium via the general system of nitrogen regulation
(Ntr) and specific induction by nitrate or nitrite [18, 26].
In the photosynthesizing bacterium R. capsulatus assimi-
latory NRase is induced by nitrate and repressed by low
C/N ratio [27]. Positive regulation by nitrate requires
nitrate reduction to nitrite, evidently the real activator of
the nitrate assimilation gene transcription [28, 29].
BACTERIAL RESPIRATORY
MEMBRANE-BOUND
NITRATE REDUCTASE (Nar)
The main membrane-bound NRase (NarGHI) is
widespread among enterobacteria and is well studied in E.
coli [30]. The enzyme consists of three subunits (α, β, and
γ) and several cofactors and participates in creation of
transmembrane proton gradient, which is coupled with
nitrate reduction. It has been shown that subunits α and
β are localized in the cytoplasm, whereas subunit γ is
localized in the membrane and performs the attachment
of subunits α and β to the cytoplasmic side of the internal
membrane (Fig. 6).
The 19-28-kD subunit γ encoded by the narI gene is
a hydrophobic protein that forms five transmembrane α-
helical domains with N-terminus located in the periplasm
and C-terminus located in the cytoplasm and containing
two b-type hemes [31, 32]. The high-potential heme is
adjacent to the cytoplasmic side of the membrane, where-
as the low-potential heme is localized in the periplasm.
Subunit γ obtains electrons from quinone and transfers
them using b-hemes onto subunit β via appropriate
chains of histidine ligands [33]. Quinone pool oxidation
at the membrane periplasmic side forms a transmem-
brane electrochemical gradient used for ATP formation
with involvement of ATP synthase [34].
Subunit β (43-63 kD) encoded by narH gene is a
globular protein incorporating [Fe-S] clusters. One [3Fe-
4S] and three [4Fe-4S] clusters with different oxida-
tion–reduction potentials were detected by EPR in the αβ
complex isolated from E. coli. The high potential centers
[3Fe-4S] and [4Fe-4S] have the oxidation–reduction
potential of +60 and +80 mV, respectively, whereas two
low-potential centers [4Fe-4S] have oxidation–reduction
potentials of –200 and –400 mV, respectively [35]. It has
been supposed recently that only high-potential clusters
are able, for thermodynamic reasons, to transfer electrons,
whereas the role of the other two clusters is still unknown.
Subunit α (104-150 kD) coded by the narG gene
contains the [4Fe-4S] cluster and molybdenum cofactor,
just on which the nitrate reduction takes place [36].
Molybdenum can exist as Mo(VI), Mo(V), or Mo(IV),
and at a low and high pH levels there is pH-dependent
equilibrium between forms. X-Ray absorption spec-
troscopy has detected only a single Mo=O group in
NarGHI protein of E. coli. These results agree well with
data obtained during studies of crystalline structure of
DMSO reductase of Rhodobacter sphaeroides [37].
Structural analysis of the molybdenum center of NarGHI
suggests that the enzyme can be a member of the same
family of mononuclear molybdenum enzymes like
DMSO reductase [38]. Comparison of amino acid
sequence of NarGHI with available databases using the
BLOCK algorithm [39] has revealed a number of con-
served regions characteristic of all prokaryotic molyb-
dopterin-containing oxidoreductases. Three out of four
conserved cysteine residues, found in molybdopterin-
guanidine-dinucleotide-binding proteins, are located at
the N-terminus of the NarGHI α subunit.
Fig. 6. Organization of genes encoding for proteins of dissimilato-
ry membrane-bound bacterial nitrate reductase.
E. coli narA
2H+
NO3
–
NO2
–
M. tuberculosis
L X K G H J I G H J I
NarL
Periplasm
NO3
–
P. aeruginosa
L X K1 K2 G H J I
Cytoplasm
NarK
NarI
NarH
NarX
NarJ
NarG
Enr
cyt b
cyt b
2e
–
[4Fe-4S]
MGD
3[4Fe-4S]
3Fe-4S
NO3
–
+ 2H+
NO2
–
+ H2O
DNA
UQH2
UQ
5. NITRATE REDUCTASES: STRUCTURE, FUNCTIONS, AND EFFECT OF STRESS FACTORS 1155
BIOCHEMISTRY (Moscow) Vol. 72 No. 10 2007
Another membrane-bound NRase was found in E.
coli (and in this organism only) [30]. It was shown by dena-
turing electrophoresis in polyacrylamide gel that NarZYV
consists of two subunits with molecular mass of 150 and
60 kD encoded by narZ and narY genes and contains Mo-
co. Subunit γ of NarZYV encoded by narV was lost during
purification, but its existence was proved by spectral meth-
ods. The [Fe-S] clusters of NarZYV resemble by their
characteristics NarGHI, except the potential of the α-sub-
unit high-potential centers. Both enzymes catalyze nitrate
and chlorate reduction and are inhibited by azide.
Despite common properties, NarGHI and NarZYV
are characterized by different electrophoretic mobility
(Rf = 0.23 and 0.38 for NarGHI and NarZYV, respective-
ly). NarZYV is present in E. coli cells in minimal amounts
independently of the oxygen partial pressure.
Nitrate reduction is known to be controlled by oxy-
gen at several levels: at the level of gene expression and at
the level of nitrogen oxo-anion transport. In E. coli, P.
fluorescens, and P. stutzeri the narGHJI operon expression
is induced by anaerobiosis and by the presence of nitrate
or nitrite [40-42]. The regulatory transcription factor Fnr
(regulator of fumarate- and NRases) isolated from E. coli
is an anaerobic activator of narGHJI and in structure is a
dimer containing [Fe-S] clusters. The protein is inacti-
vated by molecular oxygen as a result of disintegration of
[4Fe-4S] clusters and is converted to the monomer form.
The Fnr protein monomer can be divided into three func-
tional domains: N-terminal sensor region incorporating
the [4Fe-4S] cluster (residues 1-50), central allosteric
domain contacting RNA polymerase, and C-terminal
regulatory domain incorporating the H–T–H region
[43]. In P. aeruginosa and some other Pseudomonas
species the Anr protein (anaerobic regulation of anaero-
bic deiminase and NRase) was identified as a structural
and functional homolog of Fnr [44, 45]. This regulator is
necessary for denitrification cascade induction during
growth on nitrogen oxides [44-46].
Despite significant interest of researchers in the
mechanism of nitrate uptake by denitrifying bacteria, this
problem is still poorly studied. In a number of articles dif-
ferent mechanisms of nitrate penetration were proposed
such as passive uniport, ATP-dependent uniport, trans-
membrane potential-dependent NO3
–
/H+
symport, and
NO3
–
/NO2
–
antiport [47].
BACTERIAL DISSIMILATORY PERIPLASMIC
NITRATE REDUCTASE (Nap)
It is supposed that nitrate reduction in the periplasm
plays the key role in aerobic denitrification in
Pseudomonas sp. [48], Paracoccus pantotrophus [49, 50],
and Rb. sphaeroides f. sp. denitrificans [51, 52], in adapta-
tion to growth under anaerobic conditions in Ralstonia
eutropha [53], in the maintenance of the oxidation–
reduction equilibrium upon nitrate usage as electron
acceptor for reducing energy removal in Rb. capsulatus,
Rb. sphaeroides, and Thiosphaera pantotropha [54, 55], or
for doubling the nitrate trace amounts in the case of E. coli
growing on a nitrate-deficient medium [56-58]. The nap
genes were also identified in Ps. putida [59], Ps. aeruginosa
[60], Ps. stutzeri, Shewanella putrefaciens, H. influenzae,
and quite recently in some pathogenic bacteria like
Salmonella typhimurium, Vibrio cholerae, Yersinia pestis,
and Campylobacter jejuni. Evidently, as soon as new infor-
mation concerning genomes of still not studied bacteria
will become available, we can expect a significant increase
in the number of species incorporating nap genes.
NRases localized in the periplasm are heterodimers
consisting of a catalytic 90-kD subunit, incorporating
molybdenum cofactor and one [4Fe-4S] center, and of a
13-19-kD subunit incorporating two-heme cytochrome c
[61-63] (Fig. 7). They are not involved in formation of
transmembrane potential.
Periplasmic NRase (NapABC enzyme) is encoded
by the napFDAGHBC operon, in which napA is responsi-
ble for synthesis of the catalytic subunit, and napB is the
two-heme cytochrome c involved in electron transport
onto molybdopterin [64]. Both subunits are synthesized
Fig. 7. Organization of genes encoding proteins of periplasmic
bacterial nitrate reductase.
Escherichia coli
2H+
E D A B C F D A G H B C
periplasm
NapA
NO3
–
+ 2H+
NO2
–
+ H2O
Paracoccus denitrificans
Rhodobacter sphaeroides Shewanella putrefaciens
K E F D A B C D A G H B
NapB
NapC NapG
NapE
NapK
2H+
NapH
NapF
NapD
p
G
X
X
C
C
X
X
G
p
MGD
[4Fe-4S]
2 x cytc
4x[4Fe-4S]
2e
_
4 x cytc
QH2
Q
4x[4Fe-4S]
2x[4Fe-4S]
QH2
Q?
cytoplasm
6. 1156 MOROZKINA, ZVYAGILSKAYA
BIOCHEMISTRY (Moscow) Vol. 72 No. 10 2007
as a precursor with N-terminal signal sequence that
defines their transfer through cytoplasmic membrane into
the periplasmic space. The NapC, membrane-bound
tetra-heme protein of cytochrome c-type, belongs to the
NapC/NirT family of periplasmic multi-heme proteins of
cytochrome c-type, which provide for the main,
cytochrome bc1-independent pathway of electron transfer
between quinone of cytoplasmic membrane and water-
soluble periplasmic oxidoreductases. The NapC/NirT
protein, a homolog of NrfH protein, which transfers elec-
trons between quinone and cytochrome c-nitrite reduc-
tase, was identified in Wolinella succinogenes [65]. The
cytoplasmic protein NapD is important for NapA matu-
ration [57]. Five different nap genes in different combina-
tions with napDABC were detected in well-studied nap
operons. They include napF, napG, and napH encoding
the [Fe-S] oxidation–reduction proteins, napE encoding
an integral membrane protein whose function is
unknown, and napK found only in Rb. sphaeroides [51].
Bacterial periplasmic NRase exhibits various physio-
logical functions and is expressed under different condi-
tions, although usually the NRase activity is revealed
under aerobic conditions, and the enzyme synthesis is not
repressed by ammonium [66, 67]. In E. coli maximal
expression of NRase is observed at low nitrate concentra-
tions under anaerobic conditions, in this case both Fnr
and NarP proteins are required [68-70]. On the other
hand, the Nap system of Ralstonia eutropha is maximally
expressed under aerobic conditions in the stationary
growth stage and is not induced by nitrate [53]. In P. pan-
totrophus maximal nap expression was found in cells
grown under aerobic conditions on such reduced carbon
sources as butyrate or caproate, but no expression was
detected in the case of growth on more oxidized sub-
strates like succinate or malate [71]. In the bacterium R.
sphaeroides DSM158 the Nap activity was stimulated, the
enzyme synthesis took place both under aerobic and
anaerobic conditions, but in the latter case it was more
efficient [52, 68].
The periplasmic localization of the enzyme raises
important questions concerning NRase transport. The
cytochrome c-containing enzyme subunit (NapB) incor-
porates a typical N-terminal signal sequence that is nec-
essary for Sec (secretion)-dependent transport [72, 73].
However, the catalytic NapA subunit can be assembled in
the cytoplasm and exported into the periplasm in a fold-
ed conformation via a Sec-independent pathway [72]. It
is supposed [73-75] that alternative transport systems may
involve unusually long and containing double arginine
site N-terminal signal sequences of NapA (and other
periplasmic molybdoenzymes), like it was supposed for
the pdf (proton driving force)-dependent pathways of
thylakoid import. The MGD cofactor is known to be syn-
thesized in the cytoplasm with involvement of five differ-
ent loci (moa, mob, mod, moe, and mog), but there are still
no data concerning the cofactor export in bacteria.
Besides, the size of cofactor and its complete internal
environment in the protein [7] suggest that the MGD
cofactor may be assembled in the cytoplasm before its
transport. It has been shown that the cofactor incorpora-
tion into apoprotein is a prerequisite for the trimethyl-
amine-N-oxidoreductase transfer in E. coli via Sec-inde-
pendent pathways [76]. Recently genes have been identi-
fied in E. coli that are necessary for Sec-independent
export of periplasmic protein that contains the MGD
cofactor [77, 78].
ASSIMILATORY EUKARYOTIC NITRATE
REDUCTASE (Euk-NRase)
The assimilatory NAD(P)H:nitrate reductase belongs
to the NRase family of the class of molybdenum-contain-
ing enzymes, which are widespread in plants, fungi, yeasts,
and algae [2, 79-81]. It is a water-soluble enzyme consist-
ing of a polypeptide with molecular mass of approximate-
ly 100 kD, bound to the molybdenum-molybdopterin
molecule (Mo-MPT), heme iron, and FAD. It is known
that dimerization of the enzyme is necessary for its activi-
ty, and thus the native enzyme is a homodimer having a
tendency to further dimerization to a homotetramer
(dimer of dimer). The enzyme has two active centers car-
rying out the internal electron transfer from FAD onto the
heme iron and then to Mo-MPT. On the first active center
electrons are transferred from FAD via NADH (or
NADPH) onto the enzyme; in the second active center
two electrons are transferred from reduced Mo(IV) to
nitrate and reduce the latter to nitrite and hydroxide.
Eukaryotic NRase exhibits its activity with such electron
acceptors as ferricyanide and cytochrome c by obtaining
electrons directly from FAD and heme iron, respectively.
The enzyme has only a limited similarity to prokaryotic
NRase that also contains Mo and pterin cofactor.
Bacterial cofactor (MGD) differs from eukaryotic MPT
by an additional nucleotide. In bacteria, the Mo atom
coordinates two pterins. All bacterial NRases contain the
[4Fe-4S] oxidation–reduction centers as electron carriers
from the electron donor to the enzyme. However, there is
still no known eukaryotic NRase that contains [Fe-S] cen-
ters as components of the electron transfer chain.
Studying genes encoding NRase of the yeast methy-
lotroph Hansenula polymorpha has shown that they are
located in the same locus with genes Yna1 and Yna2
responsible for transcription [82]. A similar gene structure
was also observed in different organisms, such as the fun-
gus Aspergillus [83, 84] or algae Chlamydomonas reinhardtii
[85, 86], but not in the fungus Neurospora crassa [87].
Despite the fact that many aspects of nitrate trans-
port (NT) in eukaryotes are still not clear, it is assumed
that a nitrate carrier really exists. Nitrate transport is car-
ried out in symport with protons, the driving force of
which is the transmembrane potential generated at the
7. NITRATE REDUCTASES: STRUCTURE, FUNCTIONS, AND EFFECT OF STRESS FACTORS 1157
BIOCHEMISTRY (Moscow) Vol. 72 No. 10 2007
expense of ATP hydrolysis by H+
-ATPase [88]. Individual
components of the NT system are regulated in a complex
way including induction by nitrate and repression follow-
ing a feedback mechanism (regulation by nitrate reduc-
tion products) [89, 90].
On the basis of expression condition and affinity to
nitrate all carriers can be classified as constitutive or
inducible with a high or low affinity to the substrate.
Studying primary amino acid sequence of nitrate carriers
NRT1 and NRT2 in algae and fungi has shown that they
belong to the superfamily of peptide transporters (MFS,
Major Facilitator Superfamily) [91]. NRT1 belongs to the
subgroup of nitrate/nitrite transporters, but effectively
transporting nitrate, histidine, and nitrite [92], whereas
NRT2 belongs to the subgroup of proton-dependent
oligopeptide transporters, transporting peptides, amino
acids, nitrate, chlorate, and nitrite [88].
Recently it has been found that eukaryotic cells are
able to carry out denitrification [93]. In particular, some
species of yeasts and fungi are able to perform the dissim-
ilatory reduction of nitrate to N2O [94-96]. The fungus
Cylindrocarpon tonkinense is able to denitrify NO3
–
despite
the presence of ammonium ions in the medium, the pres-
ence of sugars being the key condition for switching on
denitrification. In this case there is significantly increased
level of enzymes involved in NO3
–
metabolism such as
assimilatory NADPH- (or NADH)-dependent NO3
–
reductase, dissimilatory nitrite reductase, and NO reduc-
tase, but not ubiquinone-dependent dissimilatory NRase
[96]. Thus, the existence in C. tonkinense of a new type of
denitrification was shown, including assimilatory NRase
at the first step.
Zhou et al. have shown that the fungus Fusarium
oxysporum MT-811 was able to reduce nitrate to ammo-
nium under anaerobic conditions [97]. This process was
able to maintain growth of the fungus even under com-
pletely anaerobic conditions [97]. Cells grown under
microaerophilic conditions contained a noticeable num-
ber of intact mitochondria [98], which agrees well with
data on mitochondrial localization of the dissimilatory
denitrification process [99]. It was assumed previously
that soil fungi were obligate aerobes, but new data allow
one to consider them as facultative aerobes (or facultative
anaerobes).
Kurakov et al. have shown that dissimilatory NRase
is synthesized de novo under anaerobic conditions in the
fungus F. oxysporum 11dn1. The enzyme significantly dif-
fers by its properties (specific activity, molecular mass,
temperature optimum) from assimilatory enzyme [100].
Japanese scientists have identified a relationship
between activity of NRase found in mitochondrial frac-
tion of the fungus F. oxysporum MT-811 and ATP synthe-
sis and also solubilized and partially purified the enzyme
[99, 101]. Intact mitochondria of F. oxysporum exhibited
NRase activity with physiological respiratory substrates
(malate + pyruvate, succinate, formate) and with syn-
thetic electron donors (reduced methyl viologen, MVH).
Detergent treatment of mitochondrial fraction resulted in
a loss of NRase activity in the presence of respiratory sub-
strates, but had no effect on the MVH-dependent enzyme
activity. These results confirm the supposition concerning
the NRase relationship with the respiratory chain.
Besides, fungal NRase exhibited similarity by spectral
characteristics, molecular mass, and cofactor to dissimi-
latory NRase of E. coli and other denitrifying bacteria
containing molybdopterin, cytochrome b, and Fe-S pro-
teins.
ALTERATIONS OF NITRATE REDUCTASE
PROPERTIES AND STRUCTURE
CAUSED BY STRESS FACTORS
At this stage of investigations properties and struc-
ture of NRases of organisms (microorganisms, plants)
grown under stress conditions are still poorly studied.
Nevertheless, this information is extremely important
both for revealing general mechanisms of adaptation to
stress and for applied investigations with due regard to the
role of NRase in nitrogen turnover in nature.
It is known that many hyperthermophilic
(Pyrobaculum aerophilum [102], Halobacterium denitrifi-
cans [103]) and thermophilic (Thermus thermophilus
[104], Caldithrix abyssi [105], Thermovibrio ammonificans
[106]) bacteria are able to utilize nitrate as electron
acceptor under anaerobic conditions. The process of the
extremophile denitrification is studied insufficiently, but
NRase of some archaebacteria (P. aerophilum [107],
Haloarcula marismortui [108]) and certain Haloferax
species [109] has been studied.
Nitrate reductase of P. aerophilum is structurally
similar to the enzyme of Haloferax volcanii consisting of
three subunits of 100, 60, and 31 kD [110] and is a mem-
brane-bound enzyme, whereas NRases of Haloferax de-
nitrificans, Haloferax mediterranei, and Haloarcula maris-
mortui are localized in the periplasm [108, 109]. There
may be an additional protein that binds NRase to the
cytoplasmic membrane, NRase of H. marismortui, evi-
dently, differs from the rest enzymes because it is a
homotetramer consisting of 63 kD subunits. All NRases
of halophilic archaebacteria, except H. denitrificans,
require an increased salt concentration for revealing
activity and structural stabilization, but the increase in
salt concentration resulted in a shift of temperature opti-
mum of the H. mediterranei enzyme activity towards
higher temperatures.
Nitrate reductase of hyperthermophilic archaebac-
terium P. aerophilum was isolated from cells grown under
anaerobic conditions in the presence of nitrate and low
amounts of tungsten and molybdenum. The isolated
enzyme has been characterized [107, 110]. The enzyme
contained molybdenum (0.8 mol/mol protein) in its
8. 1158 MOROZKINA, ZVYAGILSKAYA
BIOCHEMISTRY (Moscow) Vol. 72 No. 10 2007
active center and was structurally similar to NRases of
mesophilic organisms [30], but differed from them by
unusually high specific activity (326 U/mg), measured at
75°C with benzyl viologen as an electron donor; the activ-
ity measured at 95°C was even higher (526 U/mg). Like
other dissimilatory NRases of mesophilic organisms, the
enzyme of P. aerophilum was also able to utilize chlorate
as a substrate at a rate exceeding that of nitrate utilization.
The NRase affinity to nitrate and chlorate 4-60 times
higher than this is described for dissimilatory NRases of
mesophilic organisms for which the Km value is 2-3 mM
[30].
It should be noted that anaerobic growth of this bac-
terium on the nitrate-containing medium required the
presence of 0.7-1.5 µM tungstate, which could not be
replaced by molybdate [107]. In most prokaryotes and
eukaryotes, tungsten is an antagonist of molybdenum and
easily replaces the latter within inactive analogs of molyb-
denum-containing enzymes [111]. Owing to this, tung-
sten is often used as a tool for studying structure and prop-
erties of molybdenum enzymes [112]. The addition of
100 µM molybdate to culture medium completely inhib-
ited bacterial growth. At low tungstate concentrations (0.3
µM and lower) nitrite accumulation in the growth medi-
um was observed, which was prevented by tungsten addi-
tion in higher concentrations. The dependence of P.
aerophilum growth on tungsten suggests the incorporation
of tungsten-containing enzymes into metabolic pathways,
but the key enzyme of nitrate dissimilation NRase con-
tained molybdenum in its active center [110].
A similar tungsten effect was also observed in the
salt-tolerant yeast Rhodotorula glutinis on a medium con-
taining molybdenum and tungsten in equimolecular
amounts (1 mM). Tungsten stimulated Mo-co synthesis
and following increase in NRase activity, but tungsten was
not found in NRase; its activity was revealed only at the
level of Mo-co expression [113, 114].
During the past decades a number of tungsten-con-
taining enzymes have been isolated from cells of ther-
mophilic archaebacteria. Among them there are formate
dehydrogenase, aldehyde:ferredoxin oxidoreductase,
formaldehyde:ferredoxin oxidoreductase, etc. containing
tungsten in their active center and not able to replace it
with molybdenum. At the same time a number of
enzymes were found whose catalytic activity can be
exhibited both with molybdenum and tungsten (formyl
methanofuran dehydrogenase, trimethylamine-N-oxide
reductase, etc.). Molybdopterin, able to coordinate both
molybdenum and tungsten, is the active center of these
enzymes [112].
Very recently two NRases, free of molybdenum and
molybdenum cofactor, have been isolated from the cells
of vanadate-reducing bacterium Pseudomonas isachen-
hovii growing under high content of vanadium
(0.5 g/liter) [115]. The periplasmic NRase was a 55-kD
monomer, containing vanadium, whereas the membrane-
bound NRase was a heterodimer of 130 and 67 kD sub-
units and no metal was found in the active center [115].
The reductase complex isolated from the dissimila-
tory iron-reducing bacterium Geobacter metallireducens
did not contain molybdenum and molybdenum cofactor
in the active center, but contained cytochrome c and was
able to reduce both nitrate and nitrite [116].
In our laboratory a highly active NRase of 130-
140 kD was isolated from cells of a halophilic denitrifying
bacterium of the Halomonas genus, strain AGJ 1-3. The
enzyme was purified to homogeneity, and it contained no
molybdenum or molybdo-cofactor in the active center
[117]. The enzyme was characterized by thermal stability
and relative insensitivity to NaCl in high concentrations,
had temperature optimum at 70-80°C, and exhibited
polyreductase activity by reducing nitrates and other oxo-
anions.
The above-mentioned data from the literature allow
us to conclude unambiguously that stress factors in cul-
ture medium have a substantial effect on structural and
functional characteristics of NRases. Attempts to system-
atize leading mechanisms of the pro- and eukaryotic
organisms’ “responses” to the environmental stress con-
ditions were not successful, probably due to not enough
experimental data. However, analysis of data from the lit-
erature indicates that, as a rule, alterations take place at
the structural level of the active center, which results in a
shift of the enzyme activity optimum to the region of
higher temperatures, an increase in thermal stability, and
changes in substrate specificity towards polyfunctionality;
changes also happen at the level of synthesis of specific
molecules that prevent or decrease the effect of stress fac-
tors on the enzyme.
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