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 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.[ref]
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
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.[ref]
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
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