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Department of Biochemistry, NGMC, Nepal
Thursday, August 18,
2016
Rajesh Chaudhary
1
TCA
Cycle
Krebs
Cycle
Citric
acid
Cycle
Citric acid cycle
Final pathway for metabolism of carbohydrate, proteins
and fat.
The citric acid cycle occur totally in mitochondria.
Has central role in gluconeogenesis, lipogenesis, and
interconversion of amino acids.
Occurs in most of the tissues but in liver its activity is
significant.
Defects in enzymes of TCA cycle has been linked to the
severe neurological diseases because of the severe
impairment in ATP production. Thursday, August 18,
2016
Rajesh Chaudhary
2
Thursday, August 18,
2016
Rajesh Chaudhary
3
Final metabolic pathway
for carbohydrate, protein
and lipids.
Citric acid cycle
 Hyperammonemia in advance liver disease leads to loss of
consciousness, coma and convulsions as a result of impaired TCA
cycle.
 Ammonia depletes TCA cycle intermediates by withdrawing
a-ketoglutarate for the formation of glutamate and glutamine
and also inhibits the oxidative decarboxylation of a-
ketoglutarate.
Thursday, August 18,
2016
Rajesh Chaudhary
4
Catalytic role of
Oxaloacetate in TCA
cycle
Thursday, August 18,
2016
Rajesh Chaudhary
5
Only small amount of
oxaloacetate is required
for the oxidation of large
quantity of Acetyl-CoA.
Thursday, August 18,
2016
Rajesh Chaudhary
6
Role of poison
Fluoroacetate
3 isoenzymes of isocitrate
dehydrogenase:
1. One that uses NAD+:
found only in
mitochondria.
2. Other two uses NADP+:
found in mitochondria
+ Cytoplasm.
Poison
Arsenite
Succinate
Thiokinase /
2 isoenzymes are present
in liver and kidney: one
specific for GDP and
another for ADP
Requires
NAD+
7
Role of poison
Fluoroacetate
Fluroacetate + acetyl-CoA  Fluroacetyl-CoA, then condenses
with oxaloacetate to form flurocitrate. Flurocitrate inhibits
aconitase resulting in accumulation of Citrate.
Thursday, August 18,
2016
Rajesh Chaudhary
8
3 isoenzymes of
isocitrate
dehydrogenase:
1. One that uses NAD+:
found only in
mitochondria.
2. Other two uses NADP+:
found in mitochondria
+ Cytoplasm.
Thursday, August 18,
2016
Rajesh Chaudhary
9
Requires same cofactors as
that of pyruvate
dehydrogenase. Eg.
NAD+, FAD, CoA
Poison
Arsenite
Thursday, August 18,
2016
Rajesh Chaudhary
10
Same sequence of chemical
reactions that occurs in b-
oxidation of Fattyacids.
Thursday, August 18,
2016
Rajesh Chaudhary
11
Total reactions of TCA cycle
 Total no. of reactions: 8
 Total carbon dioxide released: 2
 Total no. of reversible reactions: 5 reactions (Reaction no.: 2,
5, 6, 7, 8)
 Types of reactions: 1. Condensation, 2 (a). Dehydration, 2 (b).
Hydration, 3. Oxidative decarboxyation, 4. Oxidative
decarboxyation, 5. Substrate level phosphorylation, 6.
Dehydrogenation, 7. Hydration, 8. Dehydrogenation.
Thursday, August 18,
2016
Rajesh Chaudhary
12
Product of one turn of TCA cycle
Thursday, August 18,
2016
Rajesh Chaudhary
13
3 NADH
1 FADH2
1 GTP or ATP
2 CO2
Energetics of TCA cycle
No. of ATP generated by oxidation of 3 NADH = 9
No. of ATP generated by oxidation of FADH2 = 2
No. of ATP generated from GTP = 1
Total ATP generated from oxidation of 1 molecule of
Acetyl-CoA = 12
Thursday, August 18,
2016
Rajesh Chaudhary
14
Vitamins play key role in TCA cycle
Riboflavin in the form of FAD.
Niacin in the form of NAD.
Thiamin / Vitamin B1
Pantothenic acid
Thursday, August 18,
2016
Rajesh Chaudhary
15
Role of TCA cycle in anabolic pathways
Thursday, August 18,
2016
Rajesh Chaudhary
16
Anaplerotic reactions
Reactions that make sure that intermediates of TCA
cycle are replenished when consumed during anabolic
phase.
Thursday, August 18,
2016
Rajesh Chaudhary
17
Regulation of TCA
cycle
Thursday, August 18,
2016
Rajesh Chaudhary
18
TCA cycle is regulated at 3
exergonic steps:
1. Substrate availability
2. Inhibition by accumulating
product
3. Allosteric feedback
inhibition of the enzymes
that catalyze early steps of
the reactions
Feeder pathways
Thursday, August 18,
2016
Rajesh Chaudhary
19
Feeder pathways
Thursday, August 18,
2016
Rajesh Chaudhary
20
References
Thursday, August 18,
2016
Rajesh Chaudhary
21

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Citric acid cycle

  • 1. Department of Biochemistry, NGMC, Nepal Thursday, August 18, 2016 Rajesh Chaudhary 1 TCA Cycle Krebs Cycle Citric acid Cycle
  • 2. Citric acid cycle Final pathway for metabolism of carbohydrate, proteins and fat. The citric acid cycle occur totally in mitochondria. Has central role in gluconeogenesis, lipogenesis, and interconversion of amino acids. Occurs in most of the tissues but in liver its activity is significant. Defects in enzymes of TCA cycle has been linked to the severe neurological diseases because of the severe impairment in ATP production. Thursday, August 18, 2016 Rajesh Chaudhary 2
  • 3. Thursday, August 18, 2016 Rajesh Chaudhary 3 Final metabolic pathway for carbohydrate, protein and lipids.
  • 4. Citric acid cycle  Hyperammonemia in advance liver disease leads to loss of consciousness, coma and convulsions as a result of impaired TCA cycle.  Ammonia depletes TCA cycle intermediates by withdrawing a-ketoglutarate for the formation of glutamate and glutamine and also inhibits the oxidative decarboxylation of a- ketoglutarate. Thursday, August 18, 2016 Rajesh Chaudhary 4
  • 5. Catalytic role of Oxaloacetate in TCA cycle Thursday, August 18, 2016 Rajesh Chaudhary 5 Only small amount of oxaloacetate is required for the oxidation of large quantity of Acetyl-CoA.
  • 6. Thursday, August 18, 2016 Rajesh Chaudhary 6 Role of poison Fluoroacetate 3 isoenzymes of isocitrate dehydrogenase: 1. One that uses NAD+: found only in mitochondria. 2. Other two uses NADP+: found in mitochondria + Cytoplasm. Poison Arsenite Succinate Thiokinase / 2 isoenzymes are present in liver and kidney: one specific for GDP and another for ADP Requires NAD+
  • 7. 7 Role of poison Fluoroacetate Fluroacetate + acetyl-CoA  Fluroacetyl-CoA, then condenses with oxaloacetate to form flurocitrate. Flurocitrate inhibits aconitase resulting in accumulation of Citrate.
  • 8. Thursday, August 18, 2016 Rajesh Chaudhary 8 3 isoenzymes of isocitrate dehydrogenase: 1. One that uses NAD+: found only in mitochondria. 2. Other two uses NADP+: found in mitochondria + Cytoplasm.
  • 9. Thursday, August 18, 2016 Rajesh Chaudhary 9 Requires same cofactors as that of pyruvate dehydrogenase. Eg. NAD+, FAD, CoA Poison Arsenite
  • 10. Thursday, August 18, 2016 Rajesh Chaudhary 10 Same sequence of chemical reactions that occurs in b- oxidation of Fattyacids.
  • 12. Total reactions of TCA cycle  Total no. of reactions: 8  Total carbon dioxide released: 2  Total no. of reversible reactions: 5 reactions (Reaction no.: 2, 5, 6, 7, 8)  Types of reactions: 1. Condensation, 2 (a). Dehydration, 2 (b). Hydration, 3. Oxidative decarboxyation, 4. Oxidative decarboxyation, 5. Substrate level phosphorylation, 6. Dehydrogenation, 7. Hydration, 8. Dehydrogenation. Thursday, August 18, 2016 Rajesh Chaudhary 12
  • 13. Product of one turn of TCA cycle Thursday, August 18, 2016 Rajesh Chaudhary 13 3 NADH 1 FADH2 1 GTP or ATP 2 CO2
  • 14. Energetics of TCA cycle No. of ATP generated by oxidation of 3 NADH = 9 No. of ATP generated by oxidation of FADH2 = 2 No. of ATP generated from GTP = 1 Total ATP generated from oxidation of 1 molecule of Acetyl-CoA = 12 Thursday, August 18, 2016 Rajesh Chaudhary 14
  • 15. Vitamins play key role in TCA cycle Riboflavin in the form of FAD. Niacin in the form of NAD. Thiamin / Vitamin B1 Pantothenic acid Thursday, August 18, 2016 Rajesh Chaudhary 15
  • 16. Role of TCA cycle in anabolic pathways Thursday, August 18, 2016 Rajesh Chaudhary 16
  • 17. Anaplerotic reactions Reactions that make sure that intermediates of TCA cycle are replenished when consumed during anabolic phase. Thursday, August 18, 2016 Rajesh Chaudhary 17
  • 18. Regulation of TCA cycle Thursday, August 18, 2016 Rajesh Chaudhary 18 TCA cycle is regulated at 3 exergonic steps: 1. Substrate availability 2. Inhibition by accumulating product 3. Allosteric feedback inhibition of the enzymes that catalyze early steps of the reactions
  • 19. Feeder pathways Thursday, August 18, 2016 Rajesh Chaudhary 19
  • 20. Feeder pathways Thursday, August 18, 2016 Rajesh Chaudhary 20