There are eight steps in the cycle, all except three are reversible.
Step-wise reactions of the TCA cycle
1. Aldol condensation of oxaloacetate with acetylCoA followed by hydrolysis of CoASH to give citrate. Enzyme: citrate synthase. This is the first irreversible step
2. Dehydration followed by hydration leads to interchange of -H and –OH and isomerization of citrate to isocitrate. Enzyme called aconitase because of formation of intermediate called cis-aconitate.
3. Oxidative decarboxylation of isocitrate is catalyzed by isocitrate dehydrogenase to form α-keto-glutarate. This is the second irreversible step. NAD+ is reduced to NADH and one C is removed as CO2.
4. This is the second oxidative decarboxylaion and 3rd irreversible step catalyzed by a-ketoglutarate dehydrogenase. The enzyme is a complex similar to PDH and the coenzymes TPP, lipoamide and FAD are required. CO2 is removed, NADH is formed from NAD+ and a thioester bond is formed with CoASH to form succinylCoA.
5. Hydrolysis of the thioester of Succinyl CoA releases ~31 kJ/mol which is captured for the synthesis of GTP. The enzyme is succinyl CoA synthetase.
6. Succinate is oxidized to fumarate by succinate dehydrogenase. A double bond is introduced between the 2 central carbons and FAD is reduced to FADH2 Succinate .Dehydrogenase is an electron carrier of the ETC and is the only enzyme of the TCA cycle present on the inner mitochondrial membrane rather than in the matrix.
7. Fumarate is hydrated to malate by the enzyme fumarase.
8. In the final step, malate is oxidized to oxaloacetate with the coupled reduction of NAD+ to NADH by the enzyme malate dehydrogenase
• Overall reaction: AcetylCoA + 3NAD+ + FAD + GDP + Pi + 2H2O Æ 2CO2 + CoASH + 3NADH+ 2H+ + FADH2 + GTP
University lecture notes.
· 8 years ago