The calcium-carboxylate networks of the factor IX Gla-domain and conantokin G. One role of γ-carboxyglutamic acid is to stabilize a protein structure by formation of an extended calcium-carboxylate network. This property of γ-carboxyglutamic acid is used to stabilize the factor IX Gla-domain and conantokin G. (A) The calcium ligands for the Gla-domain of factor IX determined by NMR and molecular dynamics simulation64,65 are shown. In addition to the γ-carboxyglutamic acid residues, Tyr 1 and Ser 3 provide ligands to calcium 5 and Asn 2 provides a ligand to calcium 3. Tyr 1, Ser 3, and Asn 2 are shown in light grey. Calcium ions 3 through 5 are buried within the protein and not exposed to water. (B) The calcium ligands for conantokin G determined by NMR and molecular dynamics simulation78 are shown. Each calcium ion is coordinated by three or four carboxylate oxygens contributed by two γ-carboxyglutamic acid residues. The γ-carboxyglutamic acid residues and the bound calcium ions are solvent exposed.