The TCA cycle (tricarboxylic acid cycle) is also known as the citric acid cycle and the Krebs cycle. This pathway is the catabolism of aerobic respiration which produces energy and reducing power. It also initiates the production of precursors necessary for biosynthesis. If the carbon source for the cycle is acetate then citrate synthase becomes rate-limiting. Respiration produces ATP through a process of compounds acting as electron donors transferring electrons to electron acceptors. During this electron transport chain, a proton motive force is generated by the transport of protons outside the cytoplasmic membrane. As protons return to the cytoplasm, a multisubunit ATPase catalyzes the production of ATP from the proton motive force energy. During aerobic respiration, the final electron acceptor is oxygen. During anaerobic respiration, several organic compounds act as acceptors such as hydrogen, fumarate and nitrate. The cycle can start from Acetyl-CoA interacting with Oxalacetic acid and water through a citrate synthase monomer resulting in a hydrogen ion, CoA and a Citric Acid. The latter compound is dehydrated by a Citrate hydro-lyase resulting in the release of water and a cis-Aconitic acid. This compound is then hydrated through a Citrate hydro-lyase resulting in a D-threo-Isocitric acid. This compound is decarboxylated by an NADP dependent Citrate dehydrogenase, resulting in a release of carbon dioxide and NADPH and Oxoglutaric acid. The oxoglutaric acid interacts with a Coenzyme A through a NAD driven 2-oxoglutarate dehydrogenase resulting in a release of carbon dioxide, an NADH and succinyl-CoA. The succinyl-CoA interacts with a phosphate and an ADP through a 2-oxoglutarate dehydrogenase resulting in a CoA, an ATP and Succinic Acid. Succinic acid interacts with a ubiquinone, in this case a ubiquinone 1 through a succinate:quinone oxidoreductase resulting in an ubiquinol, in this case a ubiquinol-1 and a fumaric acid. The fumaric acid interacts with water through a fumarase hydratase resulting in a L-Malic acid. Malic acid can either react with a NAD dependent dehydrogenase resulting in the release of pyruvate. Malic Acid acid can also react with a malate dehydrogenase resulting in the release of oxalacetic acid

TCA Cycle References