Oxaloacetic acid, also known as oxosuccinic acid or oxalacetic acid, is a four-carbon dicarboxylic acid appearing as an intermediate of the citric acid cycle. In vivo, oxaloacetate (the ionised form of oxaloacetic acid) is formed by the oxidation of L-malate, catalysed by malate dehydrogenase, and reacts with Acetyl-CoA to form citrate, catalysed by citrate synthase. It is also formed in the mesophyll of plants by the condensation of CO₂ with phosphoenolpyruvate, catalysed by oxaloacetate decarboxylase. It can be formed from pyruvate as 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 pK_a 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). Refrigerator storage of the solid is recommended for long periods.

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Citric acid cycle compounds | Dicarboxylic acids | Ketones

Oxalessigsäure | オキサロ酢酸