Figure 37.1: Different Reactions of Kreb Cycle.

STEP 4: Oxidation of α-keto glutarate to succinyl CoA-This is the second oxidative decarboxylation to produce succinyl CoA and CO₂ in the presence of α-ketoglutarate dehydrogenase complex. One molecule of NADH is generated which will give 3 ATP molecule after oxidative phosphorylation. α-ketoglutarate dehydrogenase is a multimeric enzyme complex comprised of 3 enzymes, E1, E2 and E3.  

STEP 5: Conversion of Succinyl CoA to Succinate- This is the first step where thio ester linkage containing high energy compound is converted into a low energy product with the help of succinyl CoA synthetase. The energy of thio ester bond is utilized by the enzyme to produce GTP from condensation of GDP+Pi.