Citric Acid Cycle

The citric acid cycle — also known as the tricarboxylic acid cycle (TCA cycle), the Krebs cycle, or the Szent-Györgyi–Krebs cycle — is a series of chemical reactions used by all aerobic organisms to generate energy through the oxidization of acetate derived from carbohydrates, fats and proteins into carbon dioxide. In addition, the cycle provides precursors including certain amino acids as well as the reducing agent NADH that is used in numerous biochemical reactions. Its central importance to many biochemical pathways suggests that it was one of the earliest established components of cellular metabolism and may have originated abiogenically.

The name of this metabolic pathway is derived from citric acid (a type of tricarboxylic acid) that is first consumed and then regenerated by this sequence of reactions to complete the cycle. In addition, the cycle consumes acetate (in the form of acetyl-CoA) and water, reduces NAD+ to NADH, and produces carbon dioxide. The NADH generated by the TCA cycle is fed into the oxidative phosphorylation pathway. The net result of these two closely linked pathways is the oxidation of nutrients to produce usable energy in the form of ATP.

In eukaryotic cells, the citric acid cycle occurs in the matrix of the mitochondrion. Bacteria also use the TCA cycle to generate energy, but since they lack mitochondria, the reaction sequence is performed in the cytosol with the proton gradient for ATP production being across the plasma membrane rather than the inner membrane of the mitochondrion.

The components and reactions of the citric acid cycle were established in the 1930s by seminal work from the Nobel laureates Albert Szent-Györgyi and Hans Adolf Krebs.