Download Tricarboxylic Acid Cycle (TCA), Krebs Cycle

Tricarboxylic Acid Cycle (TCA),
Krebs Cycle
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Occurs totally in mitochondria
Pyruvate (actually acetate) from glycolysis is
degraded to CO2
Some ATP is produced
More NADH is made
NADH goes on to make more ATP in electron
transport and oxidative phosphorylation
Traffic circle, comp. entering & leaving
Tricarboxylic Acid Cycle (TCA),
Oxidative Decarboxylation of Pyruvate
Pyr. from aerobic glycolysis
is transported to cross inner
mitochondrial membrane
by specific transporter.
 In the matrix, pyr. is
irreversibly decarboxylated
by a multienzyme complex
 Five coenzyme’re needed
See figure
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Oxidative Decarboxylation of Pyruvate
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Pyr is converted to acetyl CoA by pyr
dehydrogenase (pyr DH) complex , which is a
multienzyme complex.
pyr dehydrogenase complex is not part of
TCA cycle proper, but is a mojor source of
acetyl CoA.
The irreversibility of the reaction explains why
glucose can not be formed from acetyl CoA in
gluconeogenesis.
Oxidative Decarboxylation of Pyruvate
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pyr dehydrogenase complex is composed of
three enzymes
– pyr decarboxylase (E1)
- dihydrolipoyl transacylase (E2)
- dihydrolipoyl dehydrogenase (E3)
Each catalyzed a part of the overall reaction
In addition to two regulatory enzymes protein
kinase and phosphoprotein phosphatase.
Oxidative Decarboxylation of Pyruvate
Coenzymes: Pyr DH complex contains 5
coenzyme which act as a carriers or
oxidant for intermediates.
(1) Thiamine pyrophosphate
(2)Lipoic acid
(3) CoA
(4) FAD
(5) NAD
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Mechanism of Pyr. decarboxylase
Regulation of Pyr. Dehydrogenase Complex
Allosteric activation of kinase & Phosphatase:
- Cyclic AMP-independent protein kinase ( activated)activates
phosphorylated E1 ( inactive ) & inhibits dephosphorylated (
active )  inhibit Pyr DH.
protein kinase allosterically activated by ATP, acetyl CoA, NADH
( high energy signals) inhibit Pyr DH (turned off).
protein kinase allosterically inactivated by NAD+ CoA, ( low
energy signals) activate Pyr DH (turned ).
Pyr is a potent inhibitor of kinase, if pyr concentration is
elevated so E1 is active
Ca+ is strong activator of Phosphatase, stimulating E1 activity
( skeletat muscle contraction)
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Regulation of Pyr. Dehydrogenase Complex
Reactions of TCA
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Synthesis of citrate from acetyl CoA and
oxaloacetate (OAA):
Irreversible, catalyzed by citrate synthase.
Aldol condensation reaction.
citrate synthase is inhibited by ATP, NADH,
succinyl CoA & fatty acyle CoA.
Function of citrate: It provides a source of
acetyl CoA for fatty acid synthesis & it inhibits
PFK1
Reactions of TCA
(3) Isomerisation of citrate: to isocitrate by aconitase
( reversible reaction), It is inhibited by fluroacetate, a
compound used for rat poisoning(fluroacetate is
converted to flurocitrate which is a potent inhibitor
for aconitase)
 (4) Oxidative Decarboxylation of isocitrate:
irreversible oxidative phosphorylation, by isocitrate
DH to give  -Ketoglutarate, NADH & CO2
-It is rate limiting step
-isocitrate DH is activated by ADP and Ca +2 & inhibited
by ATP, NADH
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Reactions of TCA
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(5) Oxidative Decarboxylation of  -Ketoglutarate: by
 -Ketoglutarate DH to give succinyle CoA (similar to
pyr DH),
Release of 2nd NADH & CO2
 -Ketoglutarate DH need coenzymes
TPP,NAD,FAD,CoA& lipoic acid.
 -Ketoglutarate DH is inhibited by ATP,NADH, GTP&
succinyle CoA. And activated by Ca +2 .
However it is not regulated by the phosphorylation
and de phosphorylation reaction that describe in Pyr
DH
Reactions of TCA
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(5) Cleavage of succinyle CoA: Cleavage of
(high-energy thioester dound) succinyle CoA
to succinate by succinate thiokinase.
It is coupled by release of GTPwhich interconverted by nucleoside diphosphate kinase
reaction
Substrate –level phosphorylation.
succinyle CoA can be produced from
Proponyle CoA ( metabolism of fatty acids)
Reactions of TCA
(6) Oxidation of succinate: to fumarate
by succinate DH, producing FADH2
 (7) Hydration of fumarate: to malate by
fumarase
 (8)Oxidation of malate: By malate DH
To OAA & 3nd NADH.
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Regulation of TCA Cycle
Intermediates for Biosynthesis
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 -Ketoglutarate is transaminated to make
glutamate, which can be used to make purine
nucleotides, Arg and Pro
Succinyl-CoA can be used to make porphyrins
Fumarate and oxaloacetate can be used to
make several amino acids and also pyrimidine
nucleotides
mitochondrial citrate can be exported to be a
cytoplasmic source of acetyl-CoA (F.A in
fed state) and oxaloacetate glucose in fast
state
Biosynthetic & Anaplerotic reactions
Anaplerotic Reactions (filling up reactions)
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PEP carboxylase - converts PEP to
oxaloacetate
Pyruvate carboxylase - converts
pyruvate to oxaloacetate
Malic enzyme converts pyruvate to
malate
See fig. Reactions from1-5 is
anaplerotic i.e. filling up reactions
Membrane Transport System
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The inner mitochondrial membrane is
impermeable to the most charged and
hydrophilic substances. However it contains
numerous transport proteins that permit the
passage of specific molecules.
1- ATP-ADP transport, see oxid-phospho,
Transporter for ADP & Pi from cytosol into
mitochondria by specialized carriers ( adenine
nucleotide carrier) which transport ADP from
cytosol into mitochondria, while exporting
ATP from matrix back into the cytosol .
Membrane Transport System
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Transport of reducing equivalents from cytosol into
mitochondria using: The inner mitochondrial
membrane lacks an NADH transport proteins, NADH
produced in cytosol cannot directly penetrate into
mitochondria. However two electron of NADH (
called reducing equivalents) are transported by using
shuttle.
1. glycerophosphate shuttle ( results in synthesis of 2
ATP for each cytosolic NADH oxidized )
2. malate-aspartate shuttle ( results in synthesis of 3
ATP in the mitochondrial matrix for each cytosolic
NADH oxidized )
Membrane Transport System
Pyruvate DH deficiency.
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Pyruvate DH deficiency is the most common
biochemical cause of congenital lactic
acidosis.
Pyruvate  cannot to acetyl CoA but to
lactate
The most sever form cause neonatal death.
The moderate form cause psychomotor
retardation with damage in cerebral cortx,
basal ganglia and brain stem and death.
The third form cause episodic ataxia.
Energy produced from TCA