= Gamma-carboxyglutamate residue
= Gla residue
{c}
{synonym}
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.<Vitamin K dependent modifications of glutamic acid residues in prothrombin by J. Stenflo and others>[PNAS 71, pp. 2730–2733; identification of modified glutamic acid in prothrombin]{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.<The Ca2+ ion and membrane binding structure of the Gla domain of Ca-prothrombin fragment 1 by M. Soriano-Garcia and others>[Abstract; 2.2-angstrom structure, seven calcium sites and comparison with apo-fragment 1]{ref}
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