= Gla domain
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A \b[Gla domain] is a <protein domain> containing several <gamma-carboxyglutamate residues>. These are <glutamate residues> changed by a <Vitamin K>-dependent <post-translational modification>: an additional <carboxyl group> gives the modified residue extra capacity to bind <calcium ions>. In proteins such as <prothrombin>, the resulting <calcium ion> interactions help organize the domain's <protein folding> and its association with a <phospholipid> membrane. The residue chemistry, the folded domain and its membrane interaction are related but distinct levels of explanation.
The unusual residue was identified experimentally in 1974. <J. Stenflo> and colleagues found that a <prothrombin> fragment's <electrophoretic mobility> could not be explained by its ordinary <amino acid> composition. <Nuclear magnetic resonance> and <mass spectrometry> identified <gamma-carboxyglutamic acid>. The extra chemical group helped explain the fragment's <calcium ion>-binding behaviour.<Vitamin K dependent modifications of glutamic acid residues in prothrombin by J. Stenflo and others>[abstract, electrophoretic anomaly and structural identification]{ref}
Later, <M. Soriano-Garcia> and colleagues used <X-ray crystallography> to resolve a calcium-bound <prothrombin> fragment at $2.2\,\text{Å}$ resolution. Much of the Gla domain was disordered in the calcium-free crystal, whereas the calcium-bound structure was well defined and contained seven bound <calcium ions>. Their coordination by <carboxylate> groups and water connected residue chemistry to a three-dimensional structure. Seven is the count for that reported structure, rather than a universal definition of every Gla domain.<The Ca2+ ion and membrane binding structure of the Gla domain of Ca-prothrombin fragment 1 by M. Soriano-Garcia and others>[abstract, apo/calcium-bound comparison and calcium coordination]{ref}
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