Citric acid cycle - Krebs cycle

The citric acid cycle (CAC), also known as the TCA cycle (tricarboxylic acid cycle) or the Krebs cycle, is the primary metabolic pathway by which all aerobic organisms generate energy and is the final common oxidative pathway for carbohydrates, fats and amino acids. The CAC cycle begins with an enzymatic aldol addition reaction of acetyl CoA to oxaloacetate, forming citrate; citrate is then isomerized by a dehydration-hydration sequence to yield isocitrate. Further enzymatic oxidation and decarboxylation gives 2-ketoglutarate that after another enzymatic decarboxylation and oxidation is transformed into succinyl-CoA. The hydrolysis of this metabolite to succinate is coupled to the phosphorylation of ADP or GDP to ATP or GTP. Enzymatic desaturation by flavin adenine dinucleotide (FAD)-dependent succinate dehydrogenase forms fumarate. A stereospecific hydration catalyzed by fumarase transforms fumarate to L-malate.The last step of NAD-coupled oxidation of L-malate to oxaloacetate is catalyzed by malate dehydrogenase and closes the cycle.