cycle , steps and energy calculation

1. 7. Formation of succinate: Succinyl CoA is converted to succinate by succinate thiokinase. This reaction is
coupled with the phosphorylation of GDP to ATP. This is a substrate level phosphorylation. GTP is converted to
ATP by the enzyme nucleoside diphosphate kinase.
8.Conversion of succinate to fumarate: Succinate is oxidized by succinate dehydrogenase to fumarate. This reaction results in
the production of FADH2 and not NADH.
9.Formation of malate: The enzyme fumarase catalyses the conversion of fumarate to malate with the addition of H2O.
10. Conversion of malate to oxaloacetate: Malate is then oxidized to oxaloacetate by malate dehydrogenase. The third and
final synthesis of NADH occurs at this stage. The oxaloacetate is regenerated which can combine with another molecule of
acetyl CoA, and continue the cycle.

2. Summary of TCA cycle
Acetyl CoA + 3 NAD+ + FAD + GDP + Pi + 2H2O 2CO2 + 3NADH + 3H+ + FADH2 + GTP +
CoA
REACTION NO. OF ATP PRODUCED
1. ISOCITRATE OXALOSUCCINATE 3
2. α-KETOGLUTRATE SUCCINYL-COA 3
3. SUCCINYL CO-A SUCCINATE 1
4. SUCCINATE FUMARATE 2
5. MALATE OXALOACETATE 3
TOTAL 12
Since two molecules of Acetyl co-A are used , so total ATP 2×12=24

3. NOTE
NOTE-1
1. The citric acid cycle is the final common oxidative pathway for carbohydrates, fats and amino acids.
2. Krebs cycle is the most important central pathway connecting almost all the individual metabolic pathway.
3. The enzymes of TCA cycle are located in mitochondrial matrix, in close proximity to the electron transport chain
(enables the synthesis of ATP by oxidative phosphorylation).

4. Aconitase Fluoroacetate
α-Ketoglutarate
dehydrogenase
Arsenic
Succinate
dehydrogenase
Malonate
NOTE-2 Inhibitors of Krebs cycle