2. Oxidative Decarboxylation of
Pyruvate
Oxidative decarboxylation occurs by a series of reactions
catalysed by three enzymes in the presence of five
coenzymes
The enzymes are:
• Pyruvate decarboxylase
• Dihydrolipoyl acetyltransferase
• Dihydrolipoyl dehydrogenase
These are tightly bound together to form the
pyruvate dehydrogenase complex (PDH Complex)
3. The coenzymes required are:
• Thiamin pyrophosphate (TPP)
• Lipoic acid
• CoA
• FAD
• NAD+
The reactions occur in mitochondria
6. PDH REGULATION:
PYRUVATE ACETYL CoA + CO2
NADH+ H+
PDH
- -
(A) END PRODUCT INHIBITION
PDH inhibited by its products NADH+H+
& ACETYL CoA
NAD
CoA SH
7. (B) COVALENT MODIFICATION:
H2O
KINASE
PDH
INACTIVE
PDH
ACTIVEATP
ADP
Pi
PDH exists in active (dephosphorylated) & inactive( phosphorylated) forms.
These two forms are interconverted by separate enzymes – protein kinase &
phosphatase.
The kinase is activated by in NADH+H+
/NAD+
& acetyl CoA / CoA.
The in the ADP/ATP signals for energy production & inhibits the kinase.
ARSENITE & MERCURIC ion complexes – SH group of lipoate & inhibit PDH.
+
NADH+
H+
Acetyl
CoA
PHOSPHATASE
8. Alcohol ingestion leads to hypoglycemia??
Inhibition of PDH by alcohol:
• Many alcoholics are thiamine deficient due to poor diet &
also because alcohol inhibits thiamine absorption .
• Thiamine is converted to TPP, an impt coenzyme of PDH
complex as well as transketolase enzyme in the PPPathway.
• Lack of TPP inhibits PDH, as a result pyruvate lactate,
resulting in lactic acidosis & neurologic manifestations.
9. SIGNIFICANCE OF PDH REACTION:
• It is the first reaction in which carbon atom from original
glucose is lost as CO2.
• The conversion of pyruvate to acetyl CoA is a central step
linking the glycolytic pathway with citric acid cycle.
• Acetyl CoA is also an impt intermediate in lipid metabolism,
cholesterol biosynthesis & acetylation reactions.
12. “ Citric acid cycle is the final common pathway for the
complete oxidation of acetyl CoA to CO2 & generating
energy.”
Also called as common metabolic pathway because it
acts as the final common pathway for the oxidation of
carbohydrate, fat & proteins.
The acetyl CoA, the product of CHO, lipid & protein
catabolism is taken into the cycle, together with H2O &
oxidized to CO2 with release of reducing equivalents.
SITE : All tissues except mature RBC’s.
Mitochondrial matrix.
13. The cycle begins ....
With the condensation of:
Acetyl CoA, a two-carbon compound
with
Oxaloacetate, a four-carbon compound
to form
Citrate, a six-carbon compound
14. By a series of reactions ….
Citrate is reconverted into oxaloacetate
Two carbon atoms are removed as carbon dioxide
in these reactions
A number of reducing equivalents are also
removed which are oxidised to water in the respiratory
chain
16. • “ Our City Is Kept Safe & Sound From
Malice Officer”
17. • “ Citrate Is Kreb’s Starting Substrate For
Making Oxaloacetate”
18. Stages:
I. Condensation of oxaloacetate & acetyl CoA.
C COOH
CH2 COOH
O
+ CH3CO ~ S.CoA C COOH
CH2 COOH
CH2 COOH
HO
CITRATE
SYNTHASE
CoA
H2O
CITRIC ACID
Or
TCA [6C]
[2C][4C]
Irreversible reaction. OA + A C
19. II. Isomerisation of citrate to isocitrate via cis-aconitate.
C COOH
CH2 COOH
CH2 COOH
HO
CH2 COOH
C COOH
CH COOH
ACONITASE
H2O
Fe++
H2O
ACONITASE
Fe++
CH2 COOH
C COOH
CH COOH
CITRATE CIS - ACONITATE ISOCITRATE
H
HO
Aconitase contains a iron sulphur protein ( Fe: S).
* Flouroacetate inhibits this reaction
20. III. Isocitrate dehydrogenation with oxidative
decarboxylation.
CH2 COOH
H C COOH
HO CH COOH
NAD NADHH+
Mg++
or
Mn++
Mg++
or
Mn++
CH2 COOH
H C COOH
O C COOH
CO2
CH2 COOH
CH2
O C COOH
ICDH ICDH
ISOCITRATE OXALOSUCCINATE α-KETOGLUTARATE
• Mg++
or Mn++
is an important component of
decarboxylation
21. IV. α-ketoglutarate undergoes dehydrogenation &
decarboxylation.
CH2 COOH
CH2
O C COOH
α-KETOGLUTARATE
α-KGDH
COMPLEX
CoA SH CO2
NAD NADHH+
FAD, TPP, LIPOATE
CH2 C ~ S.CoA
CH2 COOH
O
SUCCINYL CoA
Irreversible reaction.
α-KGDH similar to PDH complex - 3 enzymes &
5 coenzymes.
* Arsenite inhibits this reaction.
22. V. Succinyl Co A reduced to succinate.
CH2 COOH
CH2 C ~ S CoA
O
SUCCINYL CoA
SUCCINIC THIOKINASE
GDP+ Pi GTP
CoA SH
CH2 COOH
CH2 COOH
SUCCINATE
• SLP
• Succinyl CoA used for heme synthesis.
• Succinyl CoA also formed from odd chain fatty acids.
Mg++
23. Succinic thiokinase cleaves the high energy thioester
bond of succinyl CoA.
The reaction is coupled to the phosphorylation of
GDP to GTP.
The energy content is the same as that of ATP &
the two nucleotides are interconvertible by the
nucleotide diphosphate kinase reaction.
GTP + ADP ATP + GDP
24. VI. Succinate undergoes dehydrogenation (FAD)
CH2 COOH
CH2 COOH SUCCINATE
DEHYDROGENASE
FAD FADH2 CH - COOH
HOOC - CH
SUCCINATE FUMARATE
Malonate – dicarboxylic acid, a structural analog of
succinate competitively inhibits SDH.
25. VII. Fumarate undergoes hydration to malate.
CH – COOH
HOOC - CH
FUMARATE
H2O
FUMARASE
HO - CH - COOH
CH2 COOH
MALATE
26. VIII. Malate is dehydrogenated to oxaloacetate.
HO - CH – COOH
CH2 COOH
MALATE
NAD NADHH+
MALATE
DEHYDROGENASE
C - COOH
CH2 COOH
O
OXALOACETATE
Oxaloacetate is regenerated.
29. Reaction Change in Energy
coenzyme captured
Isocitrate to oxaloacetate NAD+
→ NADH 2.5 ATP equivalents
α-Ketoglutarate to succinyl CoA NAD+
→ NADH 2.5 ATP equivalents
Malate to oxaloacetate NAD+
→ NADH 2.5 ATP equivalents
Succinate to fumarate FAD → FADH2 1.5 ATP equivalents
Succinyl CoA to succinate GDP → GTP 1 ATP equivalent
Net gain 10 ATP equivalents
30. ATP produced by One Glucose
REACTION ATP s Generated
GLYCOLYSIS 7
Pyruvate to Acetyl CoA 5
TCA Cycle 20
Total 32
31. Pyruvate
PDH
Acetyl CoA Other sources
Citrate synthase
Citrate
CoA
cis-Aconitate
Aconitase
Aconitase
Isocitrate
Isocitrate dehydrogenase
Oxalosuccinate
Isocitratedehydrogenase
α-ketoglutarate
α-ketoglutarate dehydrogenase
complex
Succinyl CoA
Succinate dehydrogenase
Succinate thiokinase
Succinate
Fumarate
Fumarase
Malate
Oxaloacetate
Malate dehydrogenase
MALONATE
FLOUROACETATE
ARSENITE
33. Anaplerotic Reactions
• TCA is a precursor for many pathways
• To counterbalance such loses anaplerotic
reactions are useful
• Filling up reactions
• Influx reactions
• Replenishing reactions
