Citric Acid Cycle

The citric acid cycle operates in the mitochondria of eukaryotes and in the cytosol of prokaryotes. The cycle oxidizes pyruvate to CO₂ and H₂O, with the concomitant production of energy. Acetyl CoA from fatty acid breakdown and amino acid degradation products are also oxidized. The cycle has a role in producing precursors for biosynthetic pathways. The cycle forms the central part of a three-step process which oxidizes organic fuel molecules into CO₂ with the concomitant production of adenosine triphosphate. Regulation of the cycle is governed by substrate availability, inhibition by accumulating products, and allosteric feedback inhibition by subsequent intermediates in the cycle. The cycle has eight stages including production of citrate from oxaloacetate and acetyl CoA, isomerization of citrate to isocitrate, oxidation of isocitrate to α-ketoglutarate and oxidation of a-ketoglutarate to succinyl CoA. They also include conversion of succinyl CoA to succinate, oxidation of succinate to fumarat, hydration of fumarate to malate and oxidation of malate to oxaloacetate.