title: Osteocalcin: the vitamin K-dependent Ca2+-binding protein of bone matrix.
abstract: Osteocalcin is an abundant Ca2+-binding protein indigenous to the organic matrix of bone, dentin, and possibly other mineralized tissues. This protein contains 47-50 amino acid residues (molecular weight 5,200-5,900) depending on the species. Osteocalcin is distinguished by its content of three gamma-carboxyglutamic (Gla) residues. The vitamin-K-dependent biosynthesis of osteocalcin occurs in bone, and the protein is not homologous to the Gla-containing regions of known vitamin-K-dependent blood coagulation proteins. The two major structural features of osteocalcin which appear to control its function include: the 'Gla helix', a compact Ca2+-dependent alpha-helical conformation, in which the three Gla residues are aligned to facilitate adsorption to hydroxyapatite, and the 'COOH-terminal beta-sheet' which exhibits chemoattractant activity toward mononuclear leukocytes, specifically monocytes, the putative precursors of osteoclasts. While the biological function of osteocalcin is unknown, it appears to be a highly specific osteoblastic marker produced during bone formation, and is rapidly becoming a clinically important diagnostic parameter of bone pathology. This article reviews recent advances in the understanding of osteocalcin.
mesh_terms: D000595:Amino Acid Sequence; D000818:Animals; D001857:Bone Matrix / Q000378:metabolism*; D002118:Calcium / Q000378:metabolism; D002130:Calcium Phosphates / Q000378:metabolism; D002135:Calcium-Binding Proteins / Q000378:metabolism*; D006801:Humans; D015675:Osteocalcin; D011487:Protein Conformation; D014807:Vitamin D / Q000378:metabolism; D014812:Vitamin K / Q000378:metabolism
chemical_list: D002130:Calcium Phosphates; D002135:Calcium-Binding Proteins; C485828:alpha-tricalcium phosphate; C485830:tetracalcium phosphate; D015675:Osteocalcin; D014812:Vitamin K; D014807:Vitamin D; C485838:calcium phosphate, monobasic, anhydrous; C020243:calcium phosphate; C485829:calcium phosphate, dibasic, anhydrous; D002118:Calcium