Citric acid cycle

Article By:

Plaut, Gerhard W. E. Department of Biochemistry, Temple University School of Medicine, Philadelphia, Pennsylvania.

Last reviewed:February 2020

DOI:https://doi.org/10.1036/1097-8542.366100

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• Citric acid cycle, published June 2014:Download PDF Get Adobe Acrobat Reader

The major pathway for the complete oxidation of acetyl coenzyme A (the thioester of acetic acid with coenzyme A) in aerobic cells from animal and certain other species; also known as the Krebs cycle or tricarboxylic acid cycle. The citric acid cycle (see illustration) is a sequence of enzymatic reactions involving oxidation of a two-carbon acetyl unit to carbon dioxide (CO₂) and water (H₂O) to provide energy for storage in the form of high-energy phosphate bonds. As its name suggests, citric acid (C₆H₈O₇) is an integral part of this biochemical cycle. Reduced electron carriers generated in the cycle are reoxidized by oxygen via the electron transport system; water is formed, and the energy liberated is conserved by the phosphorylation of adenosine diphosphate (ADP) to adenosine triphosphate (ATP). Reactions of the cycle also function in metabolic processes other than energy generation, including the metabolic conversion of carbohydrates, fats (lipids), and proteins in most living organisms. See also: Adenosine triphosphate (ATP); Biological oxidation; Carbohydrate; Carbon dioxide; Cell (biology); Citric acid; Coenzyme; Energy metabolism; Enzyme; Lipid metabolism; Protein

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