3. definition
• Is the process through which living systems
acquire and use free energy to carry out
functions
• It requires highly coordinated cellular activity
4. Metabolism performs 4 functions
• Obtain energy for the cell
• Convert nutrients into macromolecules
• Assemble macromolecules into cellular
structures
• Degrade macromolecules as required for
biological function
5. Classes of living organisms
• Two major classes, autotrophs and
heterotrophs
• Autotrophs-use atmospheric carbondioxide as
a sole source for the synthesis of
macromolecules.
– They use the sun for biosynthetic purposes.
– Can be chemolithotrophs, chemoautotrophs or
photoautotrophs
6. Chemolithotrophs-obtain free energy via the
oxidation of inorganic compounds like Fe,
H2S
Photoautotrophs- these obtain free energy
from light photons through photosynthesis
7. Heterotrophs
• These basically feed on others
• They obtain energy by ingesting complex
carbon containing compound(lipids,
carbohydrates and proteins)
• They are also divided into two,
– Aerobes
– Anaerobes
8. • Aerobes –live in the presence of oxygen which
they use to oxidise organic nutrients
• Anaerobes – live in the absence of oxygen
– Obligate anaerobes are poisoned by oxygen
– Facultative anaerobes can survive in either
conditions e.g. yeast
9. Types of metabolic pathways
• Catabolism-is the degradation pathway
– It generates energy
• Anabolism- biosynthesis of biomolecules such
as nucleotides, proteins, lipids and
polysaccharides from simple precursor
molecule
– This process requires energy
• Biomolecules are composed of predominantly
hydrogen, carbon and nitrogen molecules
10. Principles of metabolic pathways
• Metabolic pathways are irreversible-
metabolic pathways are highly exergonic
which gives the pathway direction.
• Two interconvertible metabolites must take
different pathways which helps the two
pathways to be regulated independently
11. • Every metabolic pathway has a first
committed step-this step or reaction is
irreversible and highly exergonic which
commits the intermediate it produces to
continue down the pathway
12. • All metabolic pathways are regulated
– The control of the metabolic flux of metabolites
through a pathway is accomplished by regulating
the rate determining step of the pathway which
often is the first committed step of the pathway
13. • Metabolic pathways in eukaryotes occur in specific cellular
locations
• Compartmentalization of metabolic pathways
This helps the metabolic pathways to occur in different locations
and simultaneously
– Mitochondrion-citric acid cycle, oxidative phosphorylation,
amino acid catabolism
– Cytosol-glycolysis, pentose phosphate pathway, fatty acid
biosynthesis, gluconeogenesis
– Nucleus-DNA replication, RNA TRANSCRIPTION, RNA processing
– Lysosomes- enzymatic digestion of cellular components
– Golgi apparatus-post translational modification of membrane
and secretory proteins, formation of plasma membranes and
secretory vesicles
14. • Flux - flow of material through a metabolic
pathway which depends upon:
(1) Supply of substrates
(2) Removal of products
(3) Pathway enzyme activities
15. • Levels of Metabolism Regulation
1.Nervous system.
2.Endocrine system.
3.Interaction between organs.
4.Cell (membrane) level.
5.Molecular level
16. Feedback inhibition
• Product of a pathway controls the rate of its own synthesis by
inhibiting an early step (usually the first “committed” step
(unique to the pathway)
Feed-forward activation
Metabolite early in the pathway activates an enzyme further down
the pathway
17. Covalent modification for enzyme regulation
• Interconvertible
enzyme activity can be
rapidly and reversibly
altered by covalent
modification
• Protein kinases
phosphorylate
enzymes (+ ATP)
• Protein phosphatases
remove phosphoryl
groups
18. Stages of metabolism
Catabolism
• Stage I. Breakdown of macromolecules (proteins,
carbohydrates and lipids to respective building blocks.
• Stage II. Amino acids, fatty acids and glucose are
oxidized to common metabolite (acetyl CoA)
• Stage III. Acetyl CoA is oxidized in citric acid cycle to
CO2 and water. As result reduced cofactor, NADH2 and
FADH2, are formed which give up their electrons.
Electrons are transported via the tissue respiration
chain and released energy is coupled directly to ATP
synthesis
19.
20. • Anabolism can also be divided into stages, however the
anabolic pathways are characterized by divergence.
• Monosaccharide synthesis begin with CO2,
oxaloacetate, pyruvate or lactate.
Amino acids are synthesized from acetyl CoA, pyruvate
or keto acids of Krebs cycle.
Fatty acids are constructed from acetyl CoA.
• On the next stage monosaccharides, amino acids and
fatty acids are used for the synthesis of
polysaccharides, proteins and fats.
Catabolism is characterized by convergence of three major routs toward a final
common pathway.
Different proteins, fats and carbohydrates enter the same pathway –
tricarboxylicacid cycle.
21. The chemistry of metabolism
• There are about 3000 reactions in human cell.
•
All these reactions are divided into six categories:
1. Oxidation-reduction reactions
2. Group transfer reactions
3. Hydrolysis reactions
4. Nonhydrolytic cleavage reactions
5. Isomerization and rearrangement reactions
6. Bond formation reactions using energy from ATP
24. • Learning Objectives
• Answer questions about carbohydrate digestion
• Demonstrate understanding of glucose transport
• Solve problems concerning glycolysis
• Interpret scenarios about galactose metabolism
• Explain information related to fructose
metabolism
• Answer questions about pyruvate dehydrogenase
25. • All cells can carry out glycolysis
• In a few tissues, most importantly red blood cells,
glycolysis represents the only energy-yielding
pathway available
• Glucose is the major monosaccharide that enters
the pathway, but others such as galactose and
fructose can also be used
• The first steps in glucose metabolism in any cell
are transport across the membrane and
phosphorylation by kinase enzymes inside the
cell to prevent it from leaving via the transporter
26. CARBOHYDRATE DIGESTION
• Most of the carbohydrates in foods are in complex forms, such as
starch (amylose and amylopectin) and the disaccharides sucrose
and lactose
• Salivary amaylase randomly hydrolises the starch to dextrins
• Dextrins are hydrolysed in the stomach to the disaccharides
maltose and isomaltose
• Disaccharides in the intestinal brush border complete the digestion
process:
– Maltase cleaves maltose to 2 glucoses
– Isomaltase cleaves isomaltose to 2 glucoses
– Lactase cleaves lactose to glucose and galactose
– Sucrase cleaves sucrose to glucose and fructose
• Uptake of glucose into the mucosal cells is performed by the
sodium/glucose transporter, an active transport system
27. GLUCOSE TRANSPORT
• Glucose entry into most cells is concentration
driven and independent of sodium
• Four glucose transporters (GLUT) are known
• They have different affinities for glucose
coinciding with their respective physiologic
roles
28. Normal glucose concentration in peripheral blood is 4–6 mM (70–110 mg/dL)
GLUT 1 and GLUT 3 mediate basal glucose uptake in most tissues,
including brain, nerves, and red blood cells. Their high affinities for
glucose ensure glucose entry even during periods of relative
hypoglycemia
GLUT 2, a low-affinity transporter, is in hepatocytes. After a meal, portal
blood from the intestine is rich in glucose. GLUT 2 captures the excess
glucose primarily for storage. When the glucose concentration drops
below the Km for the transporter, much of the remainder leaves the liver
and enters the peripheral circulation. In the β-islet cells of the pancreas.
GLUT-2, along with glucokinase, serves as the glucose sensor for insulin
release.
GLUT 4 is in adipose tissue and muscle and responds to the glucose
concentration in peripheral blood. The rate of glucose transport in these
two tissues is increased by insulin, which stimulates the movement of
additional GLUT 4 transporters to the membrane by a mechanism
involving exocytosis
29.
30.
31. glycolysis
• Glycolysis is a cytoplasmic pathway that converts
glucose into two pyruvates, releasing a modest
amount of energy captured in two substrate-level
phosphorylations and one oxidation reaction
• If a cell has mitochondria and oxygen, glycolysis is
aerobic. If either mitochondria or oxygen is
lacking, glycolysis may occur anaerobically
(erythrocytes, exercising skeletal muscle),
although some of the available energy is lost
32.
33. a. This is the first intracellular reaction of glycolysis (remember all reactions are in the cytoplasm).
b. Requires an ATP (Mg). This is one of the investment reactions.
c. The phosphorylation of glucose traps the glucose inside the cell.
d. The reaction is considered irreversible.
e. Hexokinase has a Km for glucose of less than 0.1 mM (high affinity). It is also inhibited by the product glucose-6-phosphate
f. Liver hepatocytes and pancreatic β cells contain another enzyme Glucokinase.
Glucokinase has a Km for glucose of about 10 mM (low affinity).
Glucokinase is not inhibited by its product glucose-6-phosphate.
Glucokinase is induced by insulin.
The levels of glucokinase in the liver of untreated Type 1 diabetics are lower than normal.
Hexokinase and glucokinase are isoenzymes. Therefore, irrespective of the isoenzyme catalyzing the reaction, the Keq, ΔG, and ΔGo for the
reaction remain the same.
34. • a. This reaction is readily reversible (not a controlling step) and functions in both
glycolysis and gluconeogenesis.
• b. Conversion of an aldose to a ketose.
35. Reaction #3: 6-Phosphofructo-1-kinase (PFK-1) or Phosphofructokinase-1.
a.
a. Reaction is the rate-limiting step of glycolysis.
b. It is irreversible, and the committed step. It is an allosteric enzyme and also a
major regulatory enzyme.
c. We have invested our second ATP molecule.
36. Reaction #4: Aldolase.
a. We now have two phosphorylated trioses.
b. Only glyceraldehyde-3-phosphate is used in glycolysis. Therefore, dihydroxyacetone
phosphate has to be converted into glyceraldehyde-3-phosphate. This occurs in the next
step.
37. Reaction #5: Triose phosphate isomerase.
a. Catalyzes the interconversion of dihydroxyacetone phosphate and glyceraldehyde-3-
phosphate.
b. Because of the interconversion, one glucose molecule can be converted to two
glyceraldehyde-3-phosphate molecules.
38. Reaction # 6: Glyceraldehyde-3-phosphate dehydrogenase
a. The enzyme oxidizes the number one carbon aldehyde and then adds a phosphate
group. We have an acid anhydride in the product 1,3-bisphophoglycerate. Remember
from bioenergetics that acid anhydrides are high-energy bonds.
b. The phosphate on the number 3 carbon is not an high-energy bond
c. We have used an NAD+ for the oxidation reaction. The cell has limited amounts of
NAD+, so somewhere along the line we have to regenerate it or glycolysis will stop.
d. This reaction is a target for Arsenate (AsO43-). The arsenate resembles inorganic
phosphate (Pi). In the presence of arsenate, the product of the reaction is 1-arseno-3-
phosphoglycerate. This product is unstable and decomposes into arsenate and 3-
phosphoglycerate with no ATP formation. After this step, glycolysis continues.
e. The enzyme contains an essential thiol (cysteine-SH) group at the active site.
Iodoacetic acid (ICH2COOH) is also an inhibitor of this reaction. It reacts with the active
site SH group and inhibits the enzyme.
39. Reaction #7: Phosphoglycerate kinase
a. This is the first step of energy production.
b. This is referred to as substrate-level phosphorylation as opposed to oxidative
phosphorylation that occurs in mitochondrial ATP production.
d. We have recovered both ATP that were invested. Remember that each glucose gives 2
phosphoglycerate molecules.
41. a. Catalyzes the dehydration of 2-phosphoglycerate to form phosphoenolpyruvate
(PEP).
c. Recall from the bioenergetics lecture that PEP contains an high-energy bond.
d. This reaction is inhibited by Fluoride.
Reaction#9: Enolase
42. Reaction # 10: Pyruvate Kinase
a. Catalyzes the transfer of the phosphate from PEP to ADP to generate ATP and 9
pyruvate.
b. Again, this is substrate-level phosphorylation.
c. This reaction completes that part of glycolysis that is common to both anaerobic and
aerobic metabolism.
d. Under aerobic conditions and the presence of mitochondria, pyruvate can enter the
citric acid cycle. NAD+ is regenerated by malate/aspartate shuttle or by α-
glycerophosphate shuttle
e. Under anaerobic conditions or in the absence of mitochondria, pyruvate is reduced to
lactate via the following reaction and regenerate NAD+.
43. Anerobic Glycolysis
When pyruvate cannot be oxidized within mitochondria for some reason (e.g., hypoxia,
genetic defects in pyruvate dehydrogenase or citric acid cycle enzymes, genetic defects
in electron transport chain), pyruvate is reduced to lactate by lactate dehydrogenase.
44. Clinical considerations:
• A. Lactic acidosis
• Characterized by high blood levels of lactate (generally greater than 5 mM, while normal
levels are usually less than 1.2 mM). Blood pH can be less than 7.1 in severe cases.
• Most tissues can convert lactate to CO2 and H2O through the TCA cycle.
• If the oxygen supply is inadequate, cells must rely on glycolysis for ATP production.
– A decrease in ATP enhances glycolysis at the level of PFK-1 and produces more
pyruvate and hence lactate.
– Accumulation of plasma lactate may be secondary to tissue hypoxia (circulatory
insufficiency due to shock, heart failure), severe anemia, mitochondrial enzyme defects
and inhibitors of oxygen transport (carbon monoxide, cyanide), liver disease, and
ethanol.
• Bicarbonate is usually administered to control acidosis associated with lactate
overproduction. Dichloroacetate is used as a drug to treat lactic acidosis because it
activates pyruvate dehydrogenase in mitochondria, which facilitates the conversion
of pyruvate to acetyl CoA so that lactate production is decreased.
• Strenuous exercise leads to anerobic glycolysis. This causes overproduction of lactate.
45. Diabetes
• In diabetes, insulin activity is low (type 1 – no insulin; type 2 – insulin resistance).
• Consequently, glucagon levels increase. Thus, glucagon/insulin ratio is higher in
diabetes.
• There is no change in glucose uptake in liver because this tissue does not express
GLUT4,
• the insulin-responsive facilitative glucose transporter. In skeletal muscle and
adipocytes,
• glucose uptake decreases because of the absence of insulin-dependent recruitment
of GLUT4
• into the plasma membrane. In liver, glucose is not metabolized effectively because:
46. • This also results in an increase in gluconeogenesis, producing more glucose from
gluconeogenic
• precursors alanine and glycerol. These precursers come from muscle and adipocytes
as a result
• of increased protein and aminoacid breakdown and lipolysis to provide energy
because of the
• decreased utility of glucose as the energy source.
47. • Glucokinase deficiency: Mutations in glucokinase gene leading to inactivation of the
enzyme are associated with a form of non-insulin-dependent diabetes mellitus (type 2)
called Maturity Onset diabetes of the Young (MODY). Complete absence of the enzyme
activity will lead to type 1 diabetes because of the lack of insulin secretion in response to
blood glucose. This condition is associated with severe hyperglycemia. Mutations in the
gene leading to increased activity of the enzyme will cause hyperinsulinemic
hypoglycemia.
• Pyruvate kinase deficiency and hemolytic anemia
• 1. Genetic defect causing a 5-20% reduction in red cell pyruvate kinase levels.
• a. It's rare but the most common genetic disease associated with the glycolytic pathway.
• 2. Results in markedly lower ATP concentrations in erythrocytes.
• 3. Cells swell and lyse.
• 4. Reticulocytes are unaffected because these "immature" red cells contain
• mitochondria and are able to generate ATP through oxidative phosphorylation.
• 5. The levels of 2, 3-bisphosphoglycerate are high in erythrocytes.
• 6. No Heinz bodies
• 7. The 2nd most common genetic cause of hemolytic anemia, only next to glucose
• 6-phosphate dehydrogenase deficiency.
48. • Other glycolytic enzyme defects
• Deficiencies in the activities of phosphofructokinase, phopshoglycerate kinase,
phosphoglycerate mutase, and lactate dehydrogenase represent important genetic
defects in glycolysis. All of these have certain common clinical features: exercise
intolerance, myoglobinuria, hemolytic anemia, absence of lactate increase in forearm
exercise test, and increased glycogen deposition in the liver and skeletal muscle.
49.
50. • Glycolysis also provides intermediates for
other pathways
• In the liver, glycolysis is part of the process by
which excess glucose is converted to fatty
acids for storage
51. Important enzymes in glycolysis
• Hexokinase/glucokinase: glucose entering the cell is
trapped by phosphorylation using ATP. Hexokinase is
widely distributed in tissues, whereas glucokinase is
found only in hepatocytes and pancreatic β-islet cells
• Phosphofructokinases (PFK-1 and PFK-2): PFK-1 is the
rate-limiting enzyme and main control point in
glycolysis. In this reaction, fructose 6-phosphate is
phosphorylated to fructose 1,6-bisphosphate using
ATP.
– PFK-1 is inhibited by ATP and citrate, and activated by AMP.
– Insulin stimulates and glucagon inhibits PFK-1 in
hepatocytes by an indirect mechanism involving PFK-2 and
fructose 2,6-bisphosphate
52. • Insulin activates PFK-2 (via the tyrosine kinase
receptor and activation of protein
phosphatases), which converts a tiny amount
of fructose 6-phosphate to fructose 2,6-
bisphosphate (F2,6-BP). F2,6-BP activates PFK-
1. Glucagon inhibits PFK-2 (via cAMP-
dependent protein kinase A), lowering F2,6-BP
and thereby inhibiting PFK-1
53. • Glyceraldehyde 3-phosphate dehydrogenase:
catalyzes an oxidation and addition of
inorganic phosphate (Pi) to its substrate. This
results in the production of a high-energy
intermediate 1,3-bisphosphoglycerate and the
reduction of NAD to NADH. If glycolysis is
aerobic, the NADH can be reoxidized
(indirectly) by the mitochondrial electron
transport chain, providing energy for ATP
synthesis by oxidative phosphorylation.
54. • 3-Phosphoglycerate kinase: transfers the high-energy
phosphate from 1,3-bisphosphoglycerate to ADP, forming
ATP and 3-phosphoglycerate. This type of reaction in which
ADP is directly phosphorylated to ATP using a high-energy
intermediate is referred to as a substrate-level
phosphorylation. In contrast to oxidative phosphorylation
in mitochondria, substrate level phosphorylations are not
dependent on oxygen, and are the only means of ATP
generation in an anaerobic tissue.
• Pyruvate kinase: the last enzyme in aerobic glycolysis, it
catalyzes a substrate-level phosphorylation of ADP using
the high-energy substrate phosphoenolpyruvate (PEP).
Pyruvate kinase is activated by fructose 1,6-bisphosphate
from the PFK-1 reaction (feed-forward activation)
55. • Lactate dehydrogenase: is used only in anaerobic
glycolysis. It reoxidizes NADH to NAD, replenishing the
oxidized coenzyme for glyceraldehyde 3-phosphate
dehydrogenase. Without mitochondria and oxygen,
glycolysis would stop when all the available NAD had been
reduced to NADH. By reducing pyruvate to lactate and
oxidizing NADH to NAD, lactate dehydrogenase prevents
this potential problem from developing. In aerobic tissues,
lactate does not normally form in significant amounts.
However, when oxygenation is poor (skeletal muscle during
strenuous exercise, myocardial infarction), most cellular
ATP is generated by anaerobic glycolysis, and lactate
production increases
56. • Important Intermediates of Glycolysis
– Dihydroxyacetone phosphate (DHAP) is used in liver and adipose
tissue for triglyceride synthesis.
– 1,3-Bisphosphoglycerate and phosphoenolpyruvate (PEP) are
high-energy intermediates used to generate ATP by substrate-
level phosphorylation
Glycolysis Is Irreversible
– Three enzymes in the pathway catalyze reactions that are
irreversible. When the liver produces glucose, different
reactions and therefore different enzymes must be used at
these three points:
– ● Glucokinase/hexokinase
– ● PFK-1
– ● Pyruvate kinase
57. ATP Production and Electron Shuttles
• Anaerobic glycolysis yields 2 ATP/glucose by substrate-level
phosphorylation Aerobic glycolysis yields these 2 ATP/glucose plus 2
NADH/glucose that can be utilized for ATP production in the
mitochondria; however, the inner membrane is impermeable to
NADH. Cytoplasmic NADH is reoxidized to NAD and delivers its
electrons to one of two electron shuttles in the inner membrane
• In the malate shuttle, electrons are passed to mitochondrial NADH
and then to the electron transport chain. In the glycerol phosphate
shuttle, electrons are passed to mitochondrial FADH
• Important points;
– Cytoplasmic NADH oxidized using the malate shuttle produces a
mitochondrial NADH and yields approximately 3 ATP by oxidative
phosphorylation
– Cytoplasmic NADH oxidized by the glycerol phosphate shuttle
produces a mitochondrial FADH2 and yields approximately 2 ATP by
oxidative phosphorylation
58. Glycolysis in the Erythrocyte
• In red blood cells, anaerobic glycolysis represents the only
pathway for ATP production, yielding a net 2 ATP/glucose
• Erythrocytes have bisphosphoglycerate mutase, which
produces 2,3-bisphosphoglycerate(BPG) from 1,3-BPG in
glycolysis. 2,3-BPG binds to the β-chains of hemoglobinA
(HbA) and decreases its affinity for oxygen. The rightward
shift in the curve is sufficient to allow unloading of oxygen
in tissues, but still allows 100% saturation in the lungs. An
abnormal increase in erythrocyte 2,3-BPG might shift the
curve far enough so HbA is not fully saturated in the lungs.
• Although 2,3-BPG binds to HbA, it does not bind well to
HbF (α2γ2), with the result that HbF has a higher affinity for
oxygen than maternal HbA, allowing transplacental passage
of oxygen from mother to fetus
59.
60. We need to remember that galactose is a
metabolite of lactose
Lactose is broken down to one molecule of
glucose and galactose
GALACTOSE METABOLISM
61. • There are no catabolic pathways to metabolize
galactose, so the strategy is to convert
galactose into a metabolite of glucose
• The first reaction in the galactose–glucose
interconversion pathway is the
phosphorylation of galactose to galactose 1-
phosphate by galactokinase in the liver
62. • Galactose 1-phosphate then acquires a uridyl
group from uridine diphosphate glucose (UDP-
glucose)
63. • The products of this reaction, which is catalyzed by
galactose 1-phosphate uridyl transferase, are UDP-
galactose and glucose 1-phosphate
• The galactose moiety of UDP-galactose is then epimerized
to glucose.
• The configuration of the hydroxyl group at carbon 4 is
inverted by UDP-galactose 4-epimerase
• Note that UDP-glucose is not consumed in the conversion
of galactose into glucose, because it is regenerated from
UDP-galactose by the epimerase
• Finally, glucose 1-phosphate, formed from galactose, is
isomerized to glucose 6-phosphate by
phosphoglucomutase
64. Lactase deficiency
• Causes a condition known as Lactose intolerance, or hypolactasia
• Lactose is fermented in the colon to lactic acid, methane and
hydrogen gas
• The gas produced creates the uncomfortable feeling of gut
distention and the annoying problem of flatulence(bloating)
• Lactic acid and lactose are osmotically active, they draw water into
the intestines which leads to diarrhea, dehydration
• Diagnosis is based on a positive hydrogen breath test after an oral
lactose load.
• Best treatment is to avoid foods containing lactose, milk and milk
products (except unpasteurized yogurt, which contains active
Lactobacillus) or by lactase pills.
65. Galactosemia
• important enzymes in galactose metabolism
are:
• Galactokinase
• Galactose 1-phosphate uridyltransferase
• The disruption of galactose metabolism is
referred to as galactosemia
• The most common form is called clasic
galactosemia which is an inherited deficiency
of Galactose 1-phosphate uridyltransferase
66. Cont……
• symptoms manifest on day three of life and
affected infants fail to thrive
• Vomiting and diarrhea occur after milk
ingestion, liver cirrhosis, jaundice which does
not resolve on phototherapy, severe bacterial
infections, Failure to thrive, lethargy,
hypotonia, and mental retardation are other
common and apparent features
• Babies will also develop cataracts
67. mgt
• mandatory screening of newborns for
galactosemia is recommended through a
newborn heel prick test in the early weeks of
life
• formulas containing galactose-free
carbohydrates are given
• The life expectancy will then be normal with
an appropriate diet.
68. cataracts
• A cataract is the clouding of the normally clear
lens of the eye
• The eye lens has high concentrations of aldose
reductase enzyme.
• In absence of galactose metabolizing enzymes,
aldose reductase reduces galactose to
galactitol
69. • Galactitol is osmotically active leading to
osmotic damage of the lens
71. • Fructose is found in honey and fruit and as part
of the disaccharide sucrose (common table sugar)
• Sucrose is hydrolyzed by intestinal brush border
sucrase, and the resulting monosaccharides,
glucose and fructose, are absorbed into the
portal blood
• The liver phosphorylates fructose and cleaves it
into glyceraldehyde and DHAP
• The important enzymes are,
– Fructokinase
– Fructose 1-P aldolase (aldolase B)
72. • Fructose is found in honey and fruit and as part
of the disaccharide sucrose (common table sugar)
• Sucrose is hydrolyzed by intestinal brush border
sucrase, and the resulting monosaccharides,
glucose and fructose, are absorbed into the
portal blood
• The liver phosphorylates fructose and cleaves it
into glyceraldehyde and DHAP
• The important enzymes are,
– Fructokinase
– Fructose 1-P aldolase (aldolase B)
73. Hereditary Fructose Intolerance
• Hereditary fructose intolerance is an autosomal
recessive disease (incidence of 1/20,000) due to a
defect in the gene that encodes aldolase B in
fructose metabolism
• There is accumulation of fructose 1-phosphate in
hepatocytes and thereby sequestering of
inorganic phosphate in this substance
• Eventually, the liver becomes damaged due to the
accumulation of trapped fructose 1-phosphate
• Signs include lethargy, jaundice, vomiting,
diarrhoea
74. Glycolysis
The fate of pyruvate depends on oxygen availabi
lity.
When oxygen is present, pyruvate is oxidized to
acetyl-CoA which enters the Krebs cycle
Without oxygen, pyruvate is reduced in order to
oxidize NADH back to NAD+
77. • also called the Krebs cycle or the tricarboxylic
acid (TCA) cycle
• Takes place is in the mitochondria
• Although oxygen is not directly required in the
cycle, the pathway will not occur anaerobically
because NADH and FADH2 will accumulate if
oxygen is not available for the electron
transport chain
78. • The primary function of the citric acid cycle is
oxidation of acetyl-CoA to carbondioxide
• The energy released from this oxidation is saved
as NADH, FADH2, and guanosine triphosphate
(GTP)
• It does not represent a pathway for the net
conversion of acetyl-CoA to citrate, to malate, or
to any other intermediate of the cycle.
• The only fate of acetyl-CoA in this pathway is its
oxidation to CO2
79. • The cycle is central to the oxidation of any fuel
that yields acetyl-CoA, including glucose, fatty
acids, ketone bodies, ketogenic amino acids, and
alcohol
• There is no hormonal control of the cycle, as
activity is necessary irrespective of the fed or
fasting state
• Control is exerted by the energy status of the cell
through allosteric activation or deactivation.
• Many enzymes are subject to negative feedback
80.
81. • All the enzymes are in the matrix of the
mitochondria except succinate
dehydrogenase, which is in the inner
membrane.
82. Key points:
• Isocitrate dehydrogenase, the major control enzyme, is
inhibited by NADH and ATP and activated by ADP.
• α-Ketoglutarate dehydrogenase, like pyruvate dehydrogenase,
is a multienzyme complex. It requires thiamine, lipoic acid,
CoA, FAD, and NAD. Lack of thiamine slows oxidation of
acetyl-CoA in the citric acid cycle.
• Succinyl-CoA synthetase (succinate thiokinase) catalyzes a
substrate-level phosphorylation of GDP to GTP.
• Succinate dehydrogenase is on the inner mitochondrial
membrane, where it also functions as complex II of the
electron transport chain.
• Citrate synthase condenses the incoming acetyl group with
oxaloacetate to form citrate
83. • Several intermediates of the cycle may serve
other functions
– Citrate may leave the mitochondria (citrate
shuttle) to deliver acetyl- CoA into the cytoplasm
for fatty acid synthesis.
– Succinyl-CoA is a high-energy intermediate that
can be used for heme synthesis and to activate
ketone bodies in extrahepatic tissues.
– Malate can leave the mitochondria (malate
shuttle) for gluconeogenesis
84. • When intermediates are drawn out of the
citric acid cycle, the cycle slows. Therefore
when intermediates leave the cycle they must
be replaced to ensure sufficient energy for the
cell
85.
86. • The cycle involves a sequence of compounds interrelated by
oxidation-reduction and other reactions which finally
produces CO2 and H2O.
• It is the final common pathway of break down/catabolism
of carbohydrates, fats and proteins. (Phase III of metabolism).
• Acetyl-CoA derived mainly from oxidation of either glucose
or β-oxidation of FA and partly from certain amino acids
combines with oxaloacetic acid (OAA) to form citrate the first
reaction of citric acid cycle.
87. • In this reaction acetyl-CoA transfers its ‘acetyl-group’ (2-C)
to OAA.
• By stepwise dehydrogenations and loss of two molecules of
CO2, accompanied by internal re-arrangements,
• the citric acid is reconverted to OAA , which again starts
the cycle by taking up another acetyl group from acetyl-
CoA.
• A very small catalytic amount of OAA can bring about the
complete oxidation of active-acetate
• Enzymes are located in mitochondrial matrix,
• either free or attached to the inner surface of the inner
mitochondrial membrane,
• which facilitates the transfer of reducing equivalents to the
adjacent enzymes of the respiratory chain.
88. • The whole process is aerobic ,
• requiring O2 as the final oxidant of the reducing
equivalents.
• Absence of O2 (anoxia) or partial deficiency of O2 (hypoxia)
causes total or partial inhibition of the cycle.
• The H atoms removed in the successive dehydro- genations
are accepted by corresponding coenzymes.
• Reduced coenzymes transfer the reducing equivalents to
electron-transport system,
• where oxidative phos- phorylation produces ATP
molecules.
89. BIOMEDICAL IMPORTANCE OF CITRIC ACID CYCLE
• Final common pathway for carbohydrates, proteins and
fats,
• through formation of 2-carbon unit acetyl-CoA.
• Acetyl-CoA is oxidised to CO2 and H2O giving out energy
(CATABOLIC ROLE)
• Intermediates of TCA cycle play a major role in synthesis
• also like heme formation,
• formation of non-essential amino acids,
• FA synthesis,
• cholesterol and steroid synthesis- anabolic role.
90. ELECTRON TRANSPORT CHAIN
AND OXIDATIVE PHOSPHORYLATION
• Although the value of ΔG should not be
memorized, it does indicate the large amount
of energy released by both reactions.
• The electron transport chain is a device to
capture this energy in a form useful for doing
work.
91. Electron Transport Chain
The electron transport chain (ETC) is a series of
membrane-bound electron carriers.
-embedded in the mitochondrial inner membran
e
-electrons from NADH and FADH2 are transferred
to complexes of the ETC
-each complex transfers the electrons to the nex
t complex in the chain
92. Electron Transport Chain
As the electrons are transferred, some electron
energy is lost with each transfer.
This energy is used to pump protons (H+) across
the membrane from the matrix to the inner m
embrane space.
A proton gradient is established.
94. Electron Transport Chain
The higher negative charge in the matrix attracts
the protons (H+) back from the intermembran
e space to the matrix.
The accumulation of protons in the intermembr
ane space drives protons into the matrix via di
ffusion.
95. Electron Transport Chain
Most protons move back to the matrix through
ATP synthase.
ATP synthase is a membrane-bound enzyme that
uses the energy of the proton gradient to synt
hesize ATP from ADP + Pi.
98. Energy Yield of Respiration
theoretical energy yields
- 38 ATP per glucose for bacteria
- 36 ATP per glucose for eukaryotes
actual energy yield
- 30 ATP per glucose for eukaryotes
- reduced yield is due to “leaky” inner membran
e and use of the proton gradient for purposes
other than ATP synthesis
