The carboxylated form is one of the main organic components of the bone matrix, which constitutes 1-2% of the total bone protein (PubMed:3019668, PubMed:6967872). Acts as a negative regulator of bone formation and is required to limit bone formation without impairing bone resorption or mineralization (By similarity). Binds strongly to apatite and calcium upon gamma-carboxylation; this modification is essential for bone metabolism (PubMed:6967872, PubMed:39880952). {ECO:0000250|UniProtKB:P86546, ECO:0000269|PubMed:3019668, ECO:0000269|PubMed:39880952, ECO:0000269|PubMed:6967872}. The uncarboxylated form acts as a hormone secreted by osteoblasts, which regulates different cellular processes, such as energy metabolism, male fertility and brain development. Regulates of energy metabolism by acting as a hormone favoring pancreatic beta-cell proliferation, insulin secretion and sensitivity and energy expenditure. Uncarboxylated osteocalcin hormone also promotes testosterone production in the testes: acts as a ligand for G protein- coupled receptor GPRC6A at the surface of Leydig cells, initiating a signaling response that promotes the expression of enzymes required for testosterone synthesis in a CREB-dependent manner. Also acts as a regulator of brain development: osteocalcin hormone crosses the blood- brain barrier and acts as a ligand for GPR158 on neurons, initiating a signaling response that prevents neuronal apoptosis in the hippocampus, favors the synthesis of all monoamine neurotransmitters and inhibits that of gamma-aminobutyric acid (GABA). Osteocalcin also crosses the placenta during pregnancy and maternal osteocalcin is required for fetal brain development. . This is the function of ENSG00000242252 (BGLAP, bone gamma-carboxyglutamate protein).