Alphacarboxyl

Alphacarboxyl is a term used in biochemistry and organic chemistry to designate the alpha-carboxyl functional group, the carboxyl group (-COOH) attached to the first carbon (the alpha carbon) of an amino acid or other alpha-substituted molecule. In standard α-amino acids, the alphacarboxyl group sits adjacent to the amino group on the same carbon and is a key contributor to the molecule's acidity and reactivity.

Chemistry and properties: The alphacarboxyl group behaves as a carboxylic acid, capable of losing a proton to form a carboxylate (-COO−). Its typical pKa is around 2.0–2.5, so at physiological pH it is largely deprotonated. In physiological conditions, amino acids commonly exist as zwitterions, with the amino group protonated to NH3+ and the alphacarboxyl group as COO−. The alpha-carboxyl group participates in peptide bond formation, reacting with the amino group of another amino acid to form a covalent amide linkage and releasing water.

Biological role and terminology: The alphacarboxyl group is essential for protein synthesis and determines charge distribution, solubility, and conformation of amino acids and proteins. The term alphacarboxyl is often used interchangeably with “alpha-carboxyl group,” and is distinct from the carboxyl groups found on amino acid side chains (beta- or gamma-carboxyl groups in some residues). The carboxyl terminus of a polypeptide contains an alphacarboxyl group as part of its end.