Osteocalcin:

Vitamin K-dependent calcium-binding protein synthesized by osteoblasts and found primarily in bone. Serum osteocalcin measurements provide a noninvasive specific marker of bone metabolism. The protein contains three residues of the amino acid gamma-carboxyglutamic acid (Gla), which, in the presence of calcium, promotes binding to hydroxyapatite and subsequent accumulation in bone matrix.