Acetoacetic acid

Acetoacetic acid (3-oxobutanoic acid) is a weak oxoacid. Its constitutional formula is CH3COCH2COOH. In practice, we encounter and use rather the name of its salt - acetoacetate. Acetic acid undergoes spontaneous decarboxylation to acetone or can be converted to β-hydroxybutyrate. The conversion to β-hydroxybutyrate is catalysed by the enzyme β-hydroxybutyrate dehydrogenase. Acetoacetate is a product of ketogenesis or can be also formed during the metabolism of selected biogenic amino acids. Excess acetyl-CoA is converted into ketone bodies (acetoacetate, acetone and β-hydroxybutyrate) in the liver. The liver is unable to metabolize these substances and their utilization takes place in extrahepatic tissues. Under the influence of the enzyme succinyl-CoA-acetoacetate transferase, CoA is transferred to acetoacetate to form succinate. Acetoacetate-CoA enters the β-oxidation and succinate enters the citrate cycle.

Acetoacetate can also be a product of the metabolism of amino acids - leucine, tryptophan and tyrosine.

Synthesis of ketone bodies

 * 1. Acetoacetyl-CoA is an intermediate in fat breakdown. Acetoacetyl-CoA can be formed by the condensation of two acetyl-CoA.
 * 2. Condensation of acetoacetyl-CoA with the acetyl-CoA molecule gives β-hydroxy-β-methylglutaryl-CoA = HMG-CoA. It is used in the physiological state to produce steroid substances such as cholesterol. In plants, it is used to synthesize terpenes and carotenes.
 * 3. HMG-CoA can be further cleaved by lyase to acetoacetate and acetyl-CoA.
 * Acetoacetate is formed by non-enzymatic (spontaneous) decarboxylation to form acetone.
 * Another possibility is the reduction of acetoacetate by liver dehydrogenase to β-hydroxybutyric acid (β-hydroxybutyrate).

Related articles

 * Citrate cycle
 * Amino acids
 * Ketones