citric acid

A six-carbon tricarboxylic acid with the formula HOOCCH₂C(OH) (COOH)CH₂COOH. Citric acid is the first intermediate in the TCA cycle, being formed by condensation of acetyl CoA and oxaloacetate. The reaction is the rate-limiting step in the TCA cycle. Citrate is itself an allosteric modulator for several reactions, e.g. the formation of fructose bisphosphate in glycolysis is inhibited by citrate. High concentrations of citrate are found in the fruits of some plants, notably lemons.