= Gamma-carboxyglutamic acid
<Gamma-carboxyglutamic acid> is a derivative of <glutamic acid> with an additional <carboxyl group> attached at the gamma <carbon> of the side chain. Its side chain can be written $\mathrm{-CH_2-CH(COOH)_2}$ in the fully protonated representation. The extra <carboxyl group> changes the ion-binding properties of the corresponding <gamma-carboxyglutamate residue> within a <protein>.
In 1974, <J. Stenflo> and colleagues identified this unusual <amino acid> in <prothrombin>. Anomalous <electrophoresis> behaviour led to analysis by <nuclear magnetic resonance> and <mass spectrometry>, connecting a chemical modification with the special properties of this clotting <protein>.<Vitamin K dependent modifications of glutamic acid residues in prothrombin by J. Stenflo and others>[PNAS 71, pp. 2730–2733; abstract and identification of gamma-carboxyglutamic acid]{ref}
An illuminating comparison is to count the side-chain acid groups: <glutamic acid> has one, while <gamma-carboxyglutamic acid> has two. Their ionized forms provide additional negatively charged oxygen donors for <calcium ions>. The arrangement of several modified residues in a folded <Gla domain> gives the <protein> its calcium-binding architecture; an isolated modified <amino acid> is only one component of that structure.<The Ca2+ ion and membrane binding structure of the Gla domain of Ca-prothrombin fragment 1 by M. Soriano-Garcia and others>[Biochemistry 31, pp. 2554–2566; calcium-bound Gla-domain structure]{ref}
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