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Oxidative decarboxylation of pyruvate
and Krebs cycle
OXIDATIVE DECARBOXYLATION
OF PYRUVATE
Matrix of the mitochondria
contains pyruvate
dehydrogenase complex
The fate of glucose molecule in the cell
Synthesis of
glycogen
Glucose
Glucose-6phosphate
Glycogen
Pentose phosphate
pathway
Ribose, NADPH
Degradation of
glycogen
Gluconeogenesis
Glycolysis
Ethanol
Pyruvate
Acetyl Co A
Lactate
OXIDATIVE DECARBOXYLATION OF PYRUVATE
Only about 7 % of the total potential energy
present in glucose is released in glycolysis.
Glycolysis is preliminary phase, preparing glucose
for entry into aerobic metabolism.
Pyruvate formed in the aerobic conditions
undergoes conversion to acetyl CoA by pyruvate
dehydrogenase complex.
OXIDATIVE DECARBOXYLATION OF PYRUVATE
Pyruvate dehydrogenase complex is a bridge
between glycolysis and aerobic metabolism –
citric acid cycle
Pyruvate dehydrogenase complex and
enzymes of cytric acid cycle are located in
the matrix of mitochondria
Entry of Pyruvate into the Mitochondrion
Pyruvate freely diffuses through the outer membrane of mitochondria through the channels formed by transmembrane proteins porins.
Pyruvate translocase, protein embedded into the inner
membrane, transports pyruvate from the intermembrane space
into the matrix in symport with H+ and exchange (antiport) for
OH-.
Conversion of Pyruvate to Acetyl CoA
• Pyruvate dehydrogenase complex (PDH complex) is
a multienzyme complex containing 3 enzymes, 5
coenzymes and other proteins.
Pyruvate
dehydrogenase
complex is giant,
with molecular
mass ranging
from 4 to 10
million daltons.
Electron micrograph of the
pyruvate dehydrogenase
complex from E. coli.
Pyruvate dehydrogenase complex
Enzymes:
E1 = pyruvate dehydrogenase
E2 = dihydrolipoyl acetyltransferase
E3 = dihydrolipoyl dehydrogenase
Pyruvate dehydrogenase complex
Coenzymes: TPP (thiamine pyrophosphate),
lipoamide, HS-CoA, FAD+, NAD+.
TPP is a prosthetic group of E1;
lipoamide is a prosthetic group of E2; and
FAD is a prosthetic group of E3.
The building block of
TPP is vitamin B1 (thiamin);
NAD – vitamin B5 (nicotinamide);
FAD – vitamin B2 (riboflavin),
HS-CoA – vitamin B3 (pantothenic acid),
lipoamide – lipoic acid
Pyruvate dehydrogenase complex is a classic example of
multienzyme complex
Overall reaction of pyruvate dehydrogenase complex
The oxidative decarboxylation of pyruvate catalized by
pyruvate dehydrogenase complex occurs in five steps.
Aerobic cells
use a
metabolic
wheel – the
citric acid
cycle – to
generate
energy by
acetyl CoA
oxidation
The Citric
Acid
Cycle
Synthesis of
glycogen
Glucose
Pentose phosphate
pathway
Glucose-6phosphate
Glycogen
Ribose, NADPH
Degradation of
glycogen
Gluconeogenesis
Glycolysis
Ethanol
Fatty Acids
The citric acid
cycle is the
final common
pathway for the
oxidation of fuel
molecules —
amino acids,
fatty acids, and
carbohydrates.
Pyruvate
Lactate
Acetyl Co A
Amino Acids
Most fuel
molecules
enter the
cycle as
acetyl
coenzyme A.
Names:
The Citric Acid
Cycle
Tricarboxylic
Acid Cycle
Krebs Cycle
In
eukaryotes
the reactions
of the citric
acid cycle
take place
inside
mitochondria
Hans Adolf Krebs.
Biochemist; born in Germany.
Worked in Britain. His
discovery in 1937 of the
‘Krebs cycle’ of chemical
reactions was critical to the
understanding of cell
metabolism and earned him
the 1953 Nobel Prize for
Physiology or Medicine.
An Overview of the Citric Acid Cycle
A four-carbon oxaloacetate
condenses with a two-carbon
acetyl unit to yield a six-carbon
citrate.
An isomer of citrate is
oxidatively decarboxylated and
five-carbon -ketoglutarate is
formed.
-ketoglutarate is oxidatively
decarboxylated to yield a fourcarbon succinate.
Oxaloacetate is then
regenerated from succinate.
An Overview of the Citric Acid Cycle
Two carbon atoms (acetyl
CoA) enter the cycle and
two carbon atoms leave the
cycle in the form of two
molecules of carbon dioxide.
Three hydride ions (six
electrons) are transferred to
three molecules of NAD+,
one pair of hydrogen atoms
(two electrons) is
transferred to one molecule
of FAD.
The function of the citric acid cycle is the harvesting of
high-energy electrons from acetyl CoA.
1. Citrate Synthase
• Citrate formed from acetyl CoA and oxaloacetate
• Only cycle reaction with C-C bond formation
• Addition of C2 unit (acetyl) to the keto double bond
of C4 acid, oxaloacetate, to produce C6 compound,
citrate
citrate synthase
2. Aconitase
• Elimination of H2O from citrate to form C=C bond
of cis-aconitate
• Stereospecific addition of H2O to cis-aconitate to
form isocitrate
aconitase
aconitase
3. Isocitrate Dehydrogenase
• Oxidative decarboxylation of isocitrate to
a-ketoglutarate (a metabolically irreversible reaction)
• One of four oxidation-reduction reactions of the cycle
• Hydride ion from the C-2 of isocitrate is transferred to
NAD+ to form NADH
• Oxalosuccinate is decarboxylated to a-ketoglutarate
isocitrate dehydrogenase
isocitrate dehydrogenase
4. The -Ketoglutarate Dehydrogenase Complex
• Similar to pyruvate dehydrogenase complex
• Same coenzymes, identical mechanisms
E1 - a-ketoglutarate dehydrogenase (with TPP)
E2 – dihydrolipoyl succinyltransferase (with flexible
lipoamide prosthetic group)
E3 - dihydrolipoyl dehydrogenase (with FAD)
-ketoglutarate
dehydrogenase
5. Succinyl-CoA Synthetase
• Free energy in thioester bond of succinyl CoA is
conserved as GTP or ATP in higher animals (or ATP
in plants, some bacteria)
• Substrate level phosphorylation reaction
+
Succinyl-CoA
Synthetase
GTP + ADP
GDP + ATP
HS-
6. The Succinate Dehydrogenase Complex
• Complex of several polypeptides, an FAD prosthetic group and
iron-sulfur clusters
• Embedded in the inner mitochondrial membrane
• Electrons are transferred from succinate to FAD and then to
ubiquinone (Q) in electron transport chain
• Dehydrogenation is stereospecific; only the trans isomer is
formed
Succinate
Dehydrogenase
7. Fumarase
• Stereospecific trans addition of water to the
double bond of fumarate to form L-malate
• Only the L isomer of malate is formed
Fumarase
8. Malate Dehydrogenase
Malate is oxidized to form oxaloacetate.
Malate
Dehydrogenase
Stoichiometry of the Citric Acid Cycle
 Two carbon atoms
enter the cycle in the
form of acetyl CoA.
 Two carbon atoms
leave the cycle in the
form of CO2 .
 Four pairs of hydrogen
atoms leave the cycle in
four oxidation reactions
(three molecules of
NAD+ one molecule of
FAD are reduced).
 One molecule of GTP,
is formed.
 Two molecules of
water are consumed.
Stoichiometry of the Citric Acid Cycle
 9 ATP (2.5 ATP per
NADH, and 1.5 ATP
per FADH2) are
produced during
oxidative
phosphorylation
 1 ATP is directly
formed in the
citric acid cycle
 1 acetyl CoA
generates
approximately 10
molecules of ATP
Functions of the Citric Acid Cycle
• Integration of metabolism. The citric acid cycle is
amphibolic (both catabolic and anabolic).
The cycle is involved in
the aerobic catabolism
of carbohydrates, lipids
and amino acids.
Intermediates of the
cycle are starting points
for many anabolic
reactions.
• Yields energy in the form of GTP (ATP).
• Yields reducing power in the form of NADH2 and
FADH2.
Regulation of the Citric Acid Cycle
• Pathway controlled by:
(1) Allosteric modulators
(2) Covalent modification of cycle enzymes
(3) Supply of acetyl CoA (pyruvate
dehydrogenase complex)
Regulation of the Citric Acid Cycle
Three enzymes have regulatory properties
- citrate synthase (is allosterically inhibited by
NADH, ATP, succinyl CoA, citrate – feedback
inhibition)
- isocitrate dehydrogenase
(allosteric effectors: (+) ADP; (-) NADH, ATP.
Bacterial ICDH can be covalently modified by
kinase/phosphatase)
--ketoglutarate dehydrogenase complex
(inhibition by ATP, succinyl CoA and NADH
Regulation of the citric acid cycle
-
NADH, ATP, succinyl
CoA, citrate
Krebs Cycle is a Source of Biosynthetic Precursors
Glucose
Phosphoenolpyruvate
The citric acid
cycle provides
intermediates for
biosyntheses
Role of the citric cycle in anabolism
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