Vitamin K is the cofactor for the carboxylation of glutamate residues in the postsynthetic modification of proteins to form the unusual amino acid gamma carboxyglutamate (Gla). Prothrombin and several other proteins of the blood-clotting system (Factors VII, IX, and X, and proteins C and S, each contain 4-6 gamma-carboxyglutamate residues. Gamma-carboxyglutamate chelates calcium ions, and so permits the binding of the blood-clotting proteins to membranes. Ref: Bender D.A. (2011). Chapter 44. Micronutrients: Vitamins & Minerals. In D.A. Bender, K.M. Botham, P.A. Weil, P.J. Kennelly, R.K. Murray, V.W. Rodwell (Eds), Harper's Illustrated Biochemistry, 29e.