Oxaloacetic acid

Oxaloacetic acid is an organic compound with the chemical formula HO2CC(O)CH2CO2H. It also has other names (see Table) and its conjugate base is called "oxaloacetate." This four-carbon dicarboxylic acid is a protonated variant of oxaloacetate, which is an intermediate of the citric acid cycle and gluconeogenesis. Oxaloacetate forms upon oxidation of L-malate, catalysed by malate dehydrogenase, and reacts with Acetyl-CoA to form citrate, catalysed by citrate synthase. It also forms in the mesophyll of plants by the condensation of CO2 with phosphoenolpyruvate, catalysed by PEP Carboxykinase. It can arise from pyruvate via an anaplerotic reaction.

The enol forms of oxaloacetic acid are particularly stable, so much so that the two isomers have different melting points (152 °C cis, 184 °C trans). The enol proton has a pKa value of 13.02. The enzyme fumarase A from E. coli catalyses the conversion between the keto and enol forms.

Oxaloacetic acid is unstable in solution, decomposing to pyruvic acid by decarboxylation over a period of hours (room temperature) or days (0 °C). Refrigerated storage of the solid is therefore recommended.