A gamma-carboxyglutamate residue, or Gla residue, is a glutamate residue within a protein whose side chain carries an additional carboxyl group at its gamma carbon. It is the protein-bound form associated with gamma-carboxyglutamic acid. The amino acid residue remains part of the polypeptide backbone; the modification changes its side chain.
The distinction from the free amino acid matters experimentally. Identifying gamma-carboxyglutamic acid after breaking a protein into smaller components reveals chemical composition, while determining which residues are modified and how they are arranged requires structural or sequence evidence.[ref]
In a Gla domain, multiple Gla residues help coordinate calcium ions. A 1992 X-ray crystallography study of a prothrombin fragment resolved seven bound calcium ions in its calcium-bound structure and found substantial disorder in the corresponding calcium-free region. This illustrates how a local chemical modification can contribute to an ion-dependent folded arrangement.[ref]

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  1. Amino acid residue
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