decarboxylation

Decarboxylation is a chemical reaction in which a carboxyl group (-COOH) is removed from a molecule as carbon dioxide (CO2). The loss of CO2 often converts the substrate into a smaller molecule, such as a hydrocarbon or an altered carbon framework, depending on the remaining structure. In biochemistry, decarboxylation is a common step in metabolism and is frequently coupled to energy release.

Chemical mechanisms and scope: Decarboxylation can occur non-enzymatically, typically requiring heat, acid, or base. Certain carboxylic acids, notably β-keto acids, decarboxylate readily upon heating to yield ketones or aldehydes after tautomerization. Malonic esters and related compounds can undergo decarboxylation under thermal or catalytic conditions, a principle used in malonic ester synthesis to form substituted ketones or alkanes.

Enzymatic decarboxylation: Enzymes known as decarboxylases catalyze the removal of CO2 from substrates such as amino acids, carboxylic acids, or other intermediates. Examples include pyruvate decarboxylase, which converts pyruvate to acetaldehyde and CO2 in fermentation; and the pyruvate dehydrogenase complex, which decarboxylates pyruvate to form CO2 and acetyl-CoA in energy metabolism. In the citric acid cycle, isocitrate dehydrogenase releases CO2 in a separate decarboxylation step. Amino-acid decarboxylases also generate biogenic amines, such as GABA from glutamate, histamine from histidine, and tyramine from tyrosine.

Applications and significance: Decarboxylation is important for energy production, metabolism, and the biosynthesis of certain compounds. In organic synthesis, it provides routes to transform carboxyl-containing substrates into ketones, alkanes, or amines. In analytical contexts, decarboxylation pathways can influence mass spectrometry fragmentation patterns used to identify molecules.