Many proteins are dependent on vitamin K for the carboxylation of 7-carboxyglutamyl (Gla) residues. Several of these vitamin K-dependent proteins play integral roles in the bone matrix. Osteocalcin, one of the vitamin K-dependent proteins, is the most abundant noncollagenous protein in bone. Osteocalcin contains three Gla residues that require vitamin K for carboxylation. The ability of osteocalcin to bind to the hydroxyapatite fraction of bone is dependent on its degree of carboxylation. Deficiency of vitamin K increases the fraction of undercarboxy-lated osteocalcin in the circulation. In addition to osteocalcin, other vitamin K-dependent proteins (including matrix Gla protein and protein S) are found in bone and cartilage. Research is needed to elucidate the impact of vitamin K deficiency on risk of osteoporosis and fracture. Because of the known relationship between vitamin K and several crucial bone proteins, optimal status of this vitamin should be achieved to promote skeletal health.
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