Osteoblasts secrete osteocalcin in a form that cannot grab calcium, because its 3 glutamate residues must first be modified.The bone matrix contains a protein called osteocalcin — secreted by osteoblasts, but useless at birth: it carries 3 glutamate (Glu) residues that must first be modified before they can grab calcium. Surrounding Ca²⁺ ions drift freely, with no one directing where they go.
2 · K2 fires γ-carboxylation
(menaquinone) acts as a cofactor for γ-glutamyl carboxylase, which converts the 3 Glu residues into Gla (γ-carboxyglutamate).Each Gla residue carries two negative charges, so the protein binds calcium better — it shifts from a gray under-carboxylated form to active sage green. Without , osteocalcin stays under-carboxylated and binds calcium poorly; it is not bone's only door.
3 · Bind Ca²⁺
Active osteocalcin uses its 3 Gla calcium claws to clamp nearby Ca²⁺ by electrostatic attraction, so calcium that drifted everywhere is now precisely locked onto the protein.No is consumed; this step is pure charge affinity.
4 · Dock into hydroxyapatite
Carboxylated osteocalcin can dock onto hydroxyapatite and help place calcium in the lattice.That does not mean calcium cannot enter bone without : Ducy 1996 osteocalcin-deficient mice still mineralise (they form more bone). K2's job here is to carboxylate osteocalcin, not to be bone's only door.