Principles of Biochemistry

The citric acid cycle (Kreb’s cycle, Tricarboxylic acid cycle) is a series of reactions in mitochondria that bring about the catabolism of acetyl residues to CO₂ and water in aerobic condition, lead to the release of most of the free energy which is captured as ATP . The acetyl residues are in the form of acety1- CoA (CH₃-CO~S-CoA, active acetate), an ester of coenzyme A. Coenzyme A contains the vitamin pantothenic acid.

The major function of the citric acid is to act as the final common pathway for the oxidation of carbohydrate, lipids, and protein.This is because glucose, fatty acids, and many amino acids are all metabolized to acetyl-CoA or intermediates of the cycle. It also plays a major role in gluconeogenesis, transamination, deamination, and lipogenesis.

Several of these processes are carried out in many tissues but the liver is the only tissue in which all occur to a significant extent. Reactions of the citric acid cycle liberate reducing equivalents and CO2 in eight steps .