The unusual thing about the structure of N-acetylmuramic acid compared to other carbohydrates is the presence of a lactic acid side chain.
Cell walls of plants are cellulosic (polymer of D-glucose); bacterial cell walls consist mainly of polysaccharide crosslinked to peptide through murein bridges ; and fungal cell walls are chitinous ( polymer of N-acetyl- β -D-glucosamine )
Glycogen and starch differ mainly in the degree of chain branching .
Enantiomers are nonsuperimposable, mirror-image stereoisomers differing configuration on all carbons while diastereomers are nonsuperimposable nonmirror-image stereoisomers differing only on two carbons .
Fischer projection of glucose has 4 chiral centers while its Haworth projection has 5 chiral centers .
The enzyme β -amylase is an exoglycosidase degrading polysaccharides from the ends. The enzyme α -amylase is an endoglycosidase cleaving internal glycosidic bonds.
Dietary fibers bind toxic substances in the gut and decreases the transit time , so harmful compounds such as carcinogens are removed from the body more quickly than would be the case with low-fiber diet.
The sugar portions of the blood group glycoproteins are the source of the antigenic difference.
Glycosidic bonds can be formed between the side chain hydroxyls of serine or threonine residues and the sugar hydroxyls . In addition, there is a possibility of ester bonds forming between the side chain carboxyl groups of aspartate or glutamate and the sugar hydroxyls .
In glycolysis , reactions that require ATP are:
1. phosphorylation of glucose ( HK,GK )
2. phosphorylation of fructose-6-phosphate ( PFK )
Aldolase catalyzes the reverse aldol condensation of fructose-1,6-bisphosphate to glyceraldehyde-3-phosphate and DHAP .
The energy released by all the reactions of glycolysis is 184.5 kJ mol glucose/mol . The energy released by glycolysis drives the phosphorylation of two ADP to ATP for each molecule of glucose , trapping 61.0 kJ mol/glucose . The estimate of 33% efficiency comes from the calculation (61.0/184.5) x 100 = 33%.
There is a net gain of two ATP molecules per glucose molecule consumed in glycolysis . The gross yield of 4 ATPs per glucose molecule, but the reactions of glycolysis require two ATP per glucose.
Isozymes allow for subtle control of the enzyme to respond to different cellular needs. For example, in the liver, LDH is most often used to convert lactate to pyruvate, but the reaction is often reversed in the muscles . Having a different isozyme in the liver and the muscle allows for those reactions to be optimized .
Fructose-1,6-bisphosphate can only undergo the reactions of glycolysis . The components of the pathway up to this point can have other metabolic fates.
The physiologically irreversible glycolytic steps are those catalyzed by HK, PFK and PK . Thus, they are controlling points in glycolysis.
The glucokinase has a higher Km for glucose than hexokinase . Thus, under conditions of low glucose , the liver will not convert glucose to glucose-6-phosphate , using a substrate that is needed elsewhere. When the glucose concentration becomes higher , however, glucokinase will function to help phosphorylate glucose so that it can be stored as glycogen .
The net yield of ATP from glycolysis is the same, 2 ATP , when either fructose, mannose, and galactose is used . The energetics of the conversion of hexoses to pyruvate are the same regardless of hexose type.
The net yield of ATP is 3 from glucose derived from glycogen because the starting material is glucose-1-phosphate. One of the priming reactions is no longer used.
ATP is an inhibitor of several steps of glycolysis as well as other catabolic pathways. The purpose of catabolic pathways is to produce energy , and high levels of ATP mean the cell already has sufficient energy. G-6-P inhibits HK and is an example of product inhibition . If G-6-P level is high, it may indicate that sufficient glucose is available from glycogen breakdown or that the subsequent enzymatic steps of glycolysis are going slowly. Either way there is no reason to produce more G-6-P.
Phosphofructokinase is inhibited by a special effector molecule, fructose-2,6-bisphosphate , whose levels are controlled by hormones. It is also inhibited by citrate , which indicates that there is sufficient energy from the TCA cycle probably from fat or amino acid catabolism.
PK is also inhibited by acetylCoA , the presence of which indicates that fatty acids are being used to generate energy for the citric acid cycle.
The main function of glycolysis is to feed carbon units to the TCA cycle . When these carbon skeletons can come from other sources, glycolysis is inhibited to spare glucose for other purposes.
Thiamine pyrophosphate (TPP) is a coenzyme in the transfer of 2-carbon units . It is required for catalysis by pyruvate decarboxylase in alcoholic fermentation . The important part of TPP is the five-membered ring where a C is found between an S and N . This carbon forms a carbanion and is extremely reactive, making it able to perform nucleophilic attack on carbonyl groups leading to decarboxylation of several compounds in different pathways.
TPP is a coenzyme required in the reaction catalyzed by pyruvate carboxylase . Because this reaction is a part of the metabolism of ethanol , less will be available to serve as a coenzyme in the reactions of other enzymes that require it.
Animals that have been run to death have accumulated large amounts of lactic acid in their muscle tissue, accounting for the sour taste of their meat.
Conversion of glucose to lactate rather than pyruvate recycles NADH .
The formation of fructose-1,6-bisphosphate is the committed step in glycolysis . It is also one of the energy-requiring steps of the said pathway.
A positive Δ G o does not necessarily mean that the reaction has a positive Δ G . Substrate concentrations can make a negative Δ G out of a positive Δ G o .
The entire pathway can be looked at as a large coupled reaction . Thus, if the overall pathway has a negative Δ G, an individual step may be able to have a positive Δ G and the pathway can still continue.
In making equal amounts of NADPH and pentose phosphates , it only involves oxidative reactions . In making mostly or purely NADPH , the use of oxidative reactions, transketolase and transaldolase reactions, and gluconeogenesis are required. In making mostly or only pentose phosphates , needed reactions are transketolase, transaldolase, and glycolysis in reverse .
Transketolase catalyzes the transfer of 2-carbon unit , whereas transaldolase catalyzes the transfer of a 3-carbon unit .
It is essential that the mechanisms that activate glycogen synthesis also deactivate glycogen phosphorylase because they both occur in the same cell compartment. If both are on at the same time, a futile ATP hydrolysis results . On/off mechanism is highly efficient in its control.
UDPG , in glycogen biosynthesis, transfers glucose to the growing glycogen molecule .
Glycogen synthase is subject to covalent modification and to allosteric control . The enzyme is active in its phosphorylated form and inactive when dephosphorylated .
AMP is an allosteric inhibitor of glycogen synthase , whereas ATP and glucose-6-phosphate are allosteric activators .
In gluconeogenesis , biotin is the molecule to which carbon dioxide is attached to the process of being transferred to pyruvate . The reaction produces oxaloacetate , which then undergoes further reactions of gluconeogenesis. Biotin is not used in glycogenesis and PPP.
In gluconeogenesis, glucose-6-phosphate is dephosphorylated to glucose (last step); in glycolysis, G-6-P isomerizes to fructose-6-phosphate (early step).
The Cori cycle is a pathway in which there is cycling of glucose due to glycolysis in muscle and gluconeogenesis in liver . The blood transports lactate from muscle to liver and glucose from liver to muscle .
There is a net gain of 3 , rather than 2, ATP when glycogen , not glucose, is the starting material of glycolysis .
Control mechanisms are important in metabolism. They are:
Allosteric control (takes place in msec)
Covalent control (takes place from s to min)
Genetic control ( longer time scale)
Enzymes, like all catalysts, speed up the forward and reverse reaction to the same extent. Having different catalysts is the only way to ensure independent control over the rates of the forward and the reverse process.
The glycogen synthase is an exergonic reaction overall because it is coupled to phosphate ester hydrolysis.
Increasing the level of ATP is favorable to both gluconeogenesis and glycogen synthesis.
Decreasing the level of fructose-1,6-bisphosphate would tend to stimulate glycolysis, rather than gluconeogenesis and glycogen synthesis.
If a cell needs NADPH, all the reactions of the PPP take place. If a cell needs ribose-5-phosphate, the oxidative portion of the pathway can be bypassed and only the nonoxidative reshuffling reactions take place. The PPP does not have a significant effect on the ATP supply of a cell.
Glucose-6-phosphate is expectedly oxidized to a lactone rather than an open-chain ester because the latter is easy to hydrolyze.