TCA Cycle topic M.sc Medical biochemistry students

2. About....
Also called Krebs cycle orTCA cycle.
It is a series of reactions that brings catabolism of Acetyl
CoA to CO2
and H2O with ATP generation.
The cycle reactions takes place in mitochondrial matrix.

3. Reactions of the cycle...
Acetyl CoA enters the cycle and get oxidized.
Pyruvate is oxidatively decarboxylated formsAcetyl CoA by
Pyruvate dehydrogenase.
pyruvate Acetyl-CoA
(Glycolysis) (ᵝ-Oxidation of fatty acids)
NAD+ NADH+H+
Pyruvate dehydrogenase
co2C0-ASH

4. First step:Formation of citricacid
Oxaloacetate (4C) condenses wid Acetyl CoA (2C) –
citrate (TCA).
Enzyme – citrate synthase.
Irreversible step.
Enzyme- ATP-citrate lyase can reverse this step.

5. I. Condensation
Oxaloacetate citrate
Acetyl-CoA
H2O
Citratesynthase

6. Secondstep: Formationof isocitrate
Citrate isomerised to isocitrate (6C).
Enzyme- aconitase.
Takes place in 2 steps;
1. Dehydration to cis-aconitate
2. Rehydration to isocitrate

7. II.Isomerisation
1.Dehydration
Citrate Cis-
aconitase
H2O
Aconitase
2. Rehydration
Cis-aconitase Isocitrate
H2O
Aconitase

8. Third step: Formation of α-ketoglutarate
Isocitrate dehydrogenated to form oxalosuccinate.
NADH is generated in this step.
Undergoes decarboxylation to form α-ketoglutarate
(5C).
Enzyme – isocitrate dehydrogenase.
One molecule of CO2 is liberated.

9. III.Dehydrogenation
Isocitrate oxalosuccinate
NAD+ NADH+H+
Isocitrate Dehydrogenase

10. Decarboxylation
Oxalosuccinate α -Ketoglutarate
Co2
Oxalosuccinate dehydrogenase

11. Fourth step: Formation of succinyl-CoA
α- ketoglutarate is oxi.decarboxylated forms succinyl
CoA.
Enzyme – α-ketoglutarate dehydrogenase(multi
enzyme complex- 3 enzyme proteins & 5
coenzymes).
NADH generated &CO2 liberated.
Only irreversible step in the whole cycle.

12. IV.Oxidative decarboxylation
α –Ketoglutarate Succinyl-CoA
α –Ketoglutarate dehydrogenase
Co-A Co2
NAD+ NADH+H+

13. Fifth step: Formation of succinate
Substrate level phosphorylation and cleavage
of thioester bond forming succinate.
Enzyme –succinate thiokinase.
1 molecule of GDP phosphorylated to GTP.
GTP + ADP →GDP +ATP.

14. V. Substrate levelATP/GTP synthesis
Succinyl Co-A Succinate
Succinyl Co-A Synthase
GDP,pi
GTP

15. Sixth step: Formation of fumarate
Succinate dehydrogenated to fumarate,
unsaturated carbo.acid.
Enzyme –succinate dehydrogenase.
FADH2 is generated, by accepting H atom
by FAD.

16. VI. Dehydrogenation
Succinate Fumarate
FAD FADH2
Succinate Dehydrogenase

17. Seventh step: Formation of malate
 Involves formtn of malate from fumarate
by hydrolysis.
 Enzyme – fumarase.
 Involves addition of water molecule.

18. VII. Hydration
Fumarate Malate
H2O
Fumarase

19. Eighth step: Regeneration of
Oxaloacetate
 Malate is oxidized to Oxaloacetate (catalyst).
 Enzyme –malate dehydrogenase.
 NADH is generated,ATPs are produced.
 Oxaloacetate further react wid another Acetyl CoA.
 Total ATP generated during whole reactions is 10.

20. VIII. Dehydrogenation
Malate Oxaloacetate
NAD+ NADH+H+
Malate dehydrogenase

22. 2.5
1.5
1
12
7.5
1.5
20

23. Functions....
Primary function - provide energy in the form of
ATP.
 plays amphibolic role.
Precursors for synthesis of amino acids and
nucleotides.
Final common pathway for the oxidation of
carbohydrates, lipids and proteins.

24. Significance of citric acid cycle
Complete oxidation of acetyl-CoA.
ATP generation.
Fat is burned on the wick of carbohydrates –oxi.of acetyl-CoA
needs help of Oxaloacetate(source –Pyruvate).
Amphibolic pathway –both anabolic and catabolic reactions.
Anabolic reactions;
1. Oxaloacetate is the precursor for the synthesis of
aspartate.
2. Alpha ketoglutarate can be made into glutamate.

25. 3.Succinyl CoA is used for the synthesis of
heme.
4.Mitochondrial citrate is transported to
cytoplasm, where it is cleaved into
acetyl-CoA, which then is the starting
point of fatty acid synthesis.
catabolic reactions
1. Acetyl CoA produced from metabolism of
carbohydrates, proteins & fats gets oxidized viaTCA
cycle to produce energy.

26. Contn....
Anaplerotic role-To counterbalance the
losses & to keep conc. of the 4 c units in the
cell. Filling up reactions or influx reactions.
Some of them;
a. Pyruvate to Oxaloacetate by pyruvate
carboxylase enzyme.
b. Glutamate is transaminated to alpha
ketoglutarate.
c. Aspartate to oxaloacetate
d. Pyruvate can be carboxylated to malate by
NADP dependent malic enzyme.

27. Contn....
Excess carbohydrates are converted as neutral fat
–fat cannot be converted to glucose.
No net synthesis of carbohydrate from fat –
Acetyl–CoA cannot be used for gluconeogenesis.

28. Regulations of TCA cycle
Regulated at 3 steps;
a. Citrate synthase.
a. Isocitrate dehydrogenase.
a. Alpha ketoglutarate dehydrogenase.

29. Metabolic defects of TCA
cycle
Enzymes Reactions catalyzed Abnormalities
Pyruvate dehydrogenase Pyruvate acetyl CoA Lactic acidosis,
neurological disorders
Acetyl CoA dehydrogenase Fatty acetyl CoA
unsatura.fatty ace. CoA
Organic aciduria,glutaric
aciduria,acidosis.
Pyruvate carboxylase Pyruvate
oxaloacetate
Lactic
acidosis,hyperalaninemia

30. Trick to remember TCA cycle in 5 minutes....????

31. ▪ oh – oxaloacetate
▪ A – acetyl-CoA
▪ Citric – Citrate
▪ Acid – Aconitate
▪ Is – Isocitrate
▪ Of course – Oxalosuccinate
▪ A – α-Ketoglutarate
▪ Silly – Succinyl Co-A
▪ Stupid – Succinate
▪ Funny – Fumarate
▪ Molecule - Malate

32. CD RODOD POHO
C – condensation
D – Dehydration
R - Rehydration
O – Oxidation
D – Decarboxylation
OD – Oxidative decarboxylation
P – Phosphorylation
O- Oxidation
H– Hydration
O - Oxidation