citric acid cycle [Gk, kitron, citron; L, acidus, sour; Gk, kyklos, circle] , a sequence of enzymatic reactions involving the metabolism of carbon chains of sugars, fatty acids, and amino acids to yield carbon dioxide, water, and high-energy phosphate bonds. The cycle is initiated when pyruvate combines with coenzyme A (CoA) to form a two-carbon unit, acetyl-CoA, which enters the cycle by combining with four-carbon oxaloacetic acid to form six-carbon citric acid. In subsequent steps, isocitric acid, produced from citric acid, is oxidized to oxalosuccinic acid, which loses carbon dioxide to form alpha-ketoglutaric acid. Succinic acid, resulting from the oxidative decarboxylation of alpha-ketoglutaric acid, is oxidized to fumaric acid, and its oxidation regenerates oxaloacetic acid, which condenses with acetyl-CoA, closing the cycle. The citric acid cycle provides a major source of adenosine triphosphate energy and also produces intermediate molecules that are starting points for a number of vital metabolic pathways including amino acid synthesis. Also called Krebs cycle, tricarboxylic acid cycle. See also acetylcoenzyme A.
