The steps of TCA cycle are as follows:
1. Formation of cirate by means of condensation of acetyl CoA with oxaloacetate to form citrate in presence of enzyme citrate synthase.
2. Formation of isocitrate that is the isomeric form of citrate by means of rearrangement of the citrate to in presence of enzyme acotinase.
3. Oxidation of isocitrate to alpha ketoglutarate in presence of enzyme isocitrate dehydrogenase by oxidative decarboxylation in which NAD is reduced to NADH and H+.
4. Oxidation of alpha ketoglutarate succinyl CoA in presence of enzyme alphaketoglutarate dehydrogenase in which NAD is reduced to NADH and H+.
5. Conversion of succinyl CoA to succinate in presence of enzyme succinyl CoA synthase, where the energy is released to form GTP from GDP in which the ATP is formed from GTP.
6. Oxidation of succinate to fumarate in presence of enzyme succinate dehydrogenase, in which FAD is reduced to FADH2.
7. Hydration of fumarate to malate in presence of enzyme fumarase
8. Oxidation of malate to oxaloacetate in presence of enzyme malate dehydrogenase in which NAD+ is reduced to NADH and H+.
So, from the above reaction the total ATP generation is 12 is concluded which is given below:
3NAD+ = 9ATP
1 FAD = 2 ATP
1 ATP = 1 ATP
So from above total ATP generated is 9+2+1= 12.