PDC mechanism intermediate of TCA cycle

1. Pyruvate Dehydrogenase
Complex
SUBMITTED BY:- SUBMITTED TO:-
Anikesh Kr. Singh Dr. Prem Chandra
M.Sc. FMT (1stSem)
DEM - BBAU

2.  Inside the mitochondria, pyruvate is
oxidatively decarboxylated to acetyl CoA
by pyruvate dehydrogenase (PDH).
 It is a multi-enzyme complex with 5co-
enzymes &3apo-enzymes.
Pyruvate Dehydrogenase Complex

3.  Thiamine pyrophosphate (TPP)
 Co-enzyme A (CoA)
 FAD
 NAD+
 Lipoamide
 Thelipoic acid, also called thioctic acid has two
sulphur atoms &8carbonatoms.
 It can accept or donate hydrogen atoms

4. Coenzyme Functions
Thiamine Pyro Phosphate (TPP) Decarboxylation and aldehyde group
transfer
Lipoic acid Carrier of hydrogen or acetyl groups.
NADH Electron carrier
FADH2 Electron carrier
Coenzyme A (CoASH) Acetyl group carrier.
Enzyme activity Function Coenzymes
Pyruvate dehydrogenase
(E1)
Decarboxylates pyruvate TPP
Dihydrolipoil
transacetylase (E2)
Catalyze transfer of acetyl
group to CoASH
Lipoic acid, CoASH
Dihydrolipoyl
dehydrogenase (E3)
Reoxidizes
dihydrolipoamide
NAD
+
, FAD

5. 1) Pyruvate Dehydrogenase (Enzyme 1)
2) Dihydro Lipoyl TransAcetylase
(Enzyme 2)
3) Dihydro Lipoyl Dehydrogenase
(Enzyme 3)

6. Pyruvate Dehydrogenase Subunits
Enzyme Abbreviated Prosthetic Group
Pyruvate
Dehydrogenase
E1
Thiamine
pyrophosphate (TPP)
Dihydrolipoyl
Transacetylase
E2
Lipoamide
Dihydrolipoyl
Dehydrogenase
E3
FAD, NAD+

7.  It catalyses oxidative decarboxylation.
 It requires TPP.
 Thiamine (B1),a B-complex group vitamin
is essential for utilization of pyruvate.
 The two carbon unit remains attached to
the enzyme, as hydroxyethyl-TPP.

8.  Hydroxyethyl group isoxidized to formanacetyl
group and then transferred from TPP to lipoamide
to form acetyl lipoamide.
Catalyzes transfer of acetyl group to CoASH

9.  Thelast step isthe oxidation of lipoamide.
 At the end of the reaction the cofactors,
namely TPP,Lipoamide &FADare
regenerated.
 FADH2transfers the reducing equivalents to
NAD+ to give NADH + H+, which can pass
through the ETCto give 3ATP(6ATPfrom 2moles
of pyruvate formed from glucose) by oxidative
phosphorylation.

11. Pyruvate Dehydrogenase, catalyzes oxidative
decarboxylation of pyruvate, to form acetyl-CoA.
O O
H3C C C
HSCoA
O
NAD+
+ CO2
Pyruvate Dehydrogenase
pyruvate
O
H3C C S CoA
NADH
acetyl-CoA

12. Sequence of reactions catalyzed by Pyruvate
Dehydrogenase complex:
1. The keto C of pyruvate reacts with the carbanion of
TPP on E1to yield an addition compound.
The electron-pulling (+) charged N of the thiazole ring
promotes CO2loss. Hydroxyethyl-TPPremains.
2. The hydroxyethyl carbanion on TPP of E1reacts with
the disulfide of lipoamide on E2.What was the keto C
of pyruvate is oxidized to a carboxylic acid, as the
lipoamide disulfide is reduced to a dithiol.
The acetate formed by oxidation of the hydroxyethyl
is linked to one of the thiols of the reduced lipoamide
as a thioester (~).

13. Sequence of reactions (continued)
3. Acetate is transferred from the thiol of lipoamide
to the thiol of coenzyme A, yielding acetyl CoA.
4. The reduced lipoamide, swings over to the E3
active site.
Dihydrolipoamide is reoxidized to the disulfide, as
2 e+ 2 H+are transferred to a disulfide on E3
(disulfide interchange).
3. The dithiol on E3is reoxidized as 2 e-+ 2 H+are
transferred to FAD.
The resulting FADH2 is reoxidized by electron
transfer to NAD+, to yield NADH + H+.

14. O
H3C C S CoA
acetyl-coenzyme A
Acetyl CoA, a product of the Pyruvate Dehydrogenase
reaction, is a central compound in metabolism.
The "high energy" thioester linkage makes it an
excellent donor of the acetate moiety.

15. glucose-6-P
acetyl CoA
Glycolysis
pyruvate
fatty acids
ketone bodies
cholesterol
oxaloacetate citrate
Krebs Cycle
Acetyl CoA functions as:
 input to Krebs Cycle, where the acetate moiety
is further degraded to CO2.
 donor of acetate for synthesis of fatty acids,
ketone bodies, & cholesterol.

16. Regulation of Pyruvate Dehydrogenase Complex:
 Product inhibition by NADH & acetyl CoA:
 NADH competes with NAD+for binding to E3.
 Acetyl CoA competes with CoA for binding to E2.

17. Regulation by E1 phosphorylation/dephosphorylation:
 Specific regulatory Kinases & Phosphatases associated
with Pyruvate Dehydrogenase in the mitochondrial
matrix:
 Pyruvate Dehydrogenase Kinases catalyze
phosphorylation of serine residues of E1,
inhibiting the complex.
 Pyruvate Dehydrogenase Phosphatases reverse
this inhibition.

18. o Pyruvate Dehydrogenase Kinases are
activated by NADH & acetyl-CoA,
providing another way the 2 major
products of Pyruvate Dehydrogenase
reaction inhibit the complex.
o Kinase activation involves interaction
with E2subunits to sense changes in
oxidation state & acetylation of
lipoamide caused by NADH & acetyl-
CoA.

19. During starvation:
• Pyruvate Dehydrogenase Kinase increases in
amount in most tissues, including skeletal muscle, via
increased gene transcription.
• Under the same conditions, the amount of Pyruvate
Dehydrogenase Phosphatase decreases.
The resulting inhibition of Pyruvate Dehydrogenase
prevents muscle and other tissues from catabolizing
glucose & gluconeogenesis precursors.
• Metabolism shifts toward fat utilization.
• Muscle protein breakdown to supply gluconeogenesis
precursors is minimized.
• Available glucose is spared for use by the brain.

20. The increased cytosolic Ca++that occurs during
activation of muscle contraction can lead to Ca++uptake
by mitochondria.
The higher Ca++stimulates the phosphatase, &
dephosphorylation activates Pyruvate Dehydrogenase.
Thus mitochondrial metabolism may be stimulated
during exercise.
Pyruvate
Dehydro-
genase
in matrix
mitochondrion Ca++
A Ca++-sensitive isoform of
the phosphatase that
removes Pifrom E1is
expressed in muscle.