The citric acid cycle, also known as the tricarboxylic acid cycle or the Krebs cycle, is the second phase of cellular respiration that oxidizes biomolecules to produce energy.
It occurs in the mitochondrial matrix in eukaryotes and within the cytosol in prokaryotes.
Pyruvate, the end product of glycolysis, combines with coenzyme A, generating acetyl coenzyme A or acetyl-CoA.
Then, the enzyme citrate synthase initiates the cycle by condensing acetyl-CoA and oxaloacetate to form the first product, citric acid.
In the second step, aconitase rearranges citric acid to its easily oxidizable isomer, isocitrate.
The third step involves oxidation of isocitrate to α-ketoglutarate by isocitrate dehydrogenase.
Next, α-ketoglutarate dehydrogenase decarboxylates and oxidizes α-ketoglutarate in the presence of coenzyme A to form succinyl-CoA.
In the fifth step, succinyl-CoA synthetase converts succinyl-CoA to succinate, releasing coenzyme A.
Further, succinate is oxidized to fumarate by succinate dehydrogenase.
Fumarase hydrates the double bond of fumarate in the seventh step to yield malate.
Finally, the malate dehydrogenase enzyme oxidizes malate, regenerating oxaloacetate for the next round of the cycle.