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Osteocalcin (BGLAP)
Osteocalcin is a small (~49 aa) vitamin K-dependent Gla protein secreted by osteoblasts into bone matrix, where it binds calcium in hydroxyapatite and regulates bone mineral deposition. Its activity depends on gamma-carboxylation by ggcx (gamma-glutamyl carboxylase) via the vitamin-k-cycle; uncarboxylated osteocalcin (ucOC) is biologically inert for calcium binding. Osteocalcin is a central substrate in the D3+K2 calcium-partitioning hypothesis: 1,25(OH)₂D₃ (calcitriol) induces BGLAP transcription via vitamin D response elements (VDREs) in the osteocalcin gene promoter; K2 (MK-7) then activates the secreted protein through carboxylation. Circulating osteocalcin is used clinically as a bone-formation biomarker; undercarboxylated osteocalcin (ucOC) is a biomarker of vitamin K insufficiency in bone tissue. A disputed endocrine role for osteocalcin in insulin secretion and glucose metabolism has been proposed in mouse models; translation to humans is contested.
Key relationships
- ggcx — carboxylase enzyme; gamma-carboxylates osteocalcin’s Glu residues to Gla
- vitamin-k-cycle — provides vitamin K hydroquinone cofactor for GGCX
- matrix-gla-protein — sibling Gla protein; vascular calcification inhibitor; parallel substrate of GGCX
- d3-k2-calcium-partitioning — hypothesis page where the D3→osteocalcin transcription + K2→carboxylation combined mechanism is synthesized
Gaps requiring full seeding
- Full protein-structure section (Gla domain, propeptide, Ca²⁺ binding geometry)
- Loss-of-function phenotypes (Bglap-/- mouse and human correlates)
- Endocrine osteocalcin controversy (mouse insulin-secretion claim; human translation gap)
- GTEx age-stratified expression data
- Quantitative D3 dose-response for BGLAP transcription in human osteoblasts
- MR evidence for causal role in aging outcomes