Acetoacetic acid

Acetoacetic acid is a four-carbon beta-keto acid and ketone body produced during fatty-acid metabolism. It can be reduced to beta-hydroxybutyrate or spontaneously decarboxylate into acetone and carbon dioxide.

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Ketogenesis: Ketogenesis is the production of ketone bodies from fatty acids, mainly in liver mitochondria. This pathway produces acetoacetic acid when carbohydrate availability is low.
Beta-keto acid: A beta-keto acid is a carboxylic acid with a ketone group on the beta carbon. Acetoacetic acid is the simplest widely encountered biological example of this structural class.
Ketone bodies: Ketone bodies are water-soluble metabolites produced from fatty acids and used as fuels by many tissues. Acetoacetic acid is one of the three principal ketone bodies.
Beta-hydroxybutyrate: Beta-hydroxybutyrate is a major ketone body formed by reduction of acetoacetic acid. Despite its name, it is a hydroxy acid rather than a ketone.
Acetoacetate decarboxylase: Acetoacetate decarboxylase is an enzyme that converts acetoacetate into acetone and carbon dioxide. This enzyme-catalyzed route parallels acetoacetic acid's spontaneous breakdown.
Keto–enol tautomerism: Keto–enol tautomerism is the reversible shift of a hydrogen and double bond between keto and enol forms. Acetoacetic acid exists in keto and enol forms, affecting its reactions.
Ketonuria: Ketonuria is the presence of elevated ketone bodies in urine. Urine tests can detect acetoacetate during substantial ketone production.
Acetone: Acetone is a volatile ketone formed largely by spontaneous decarboxylation of acetoacetic acid. Unlike acetoacetic acid, it is not substantially used as an energy fuel.
HMG-CoA lyase: HMG-CoA lyase is a mitochondrial enzyme that cleaves HMG-CoA to form acetoacetate and acetyl-CoA. It catalyzes the pathway's final step in ketone-body synthesis.
Decarboxylation: Decarboxylation is the removal of a carboxyl group as carbon dioxide from an organic molecule. The beta-keto-acid structure makes acetoacetic acid particularly prone to this reaction.