Carboxylation-dependent conformational changes of human osteocalcin

Osteocalcin (OCN) is a small noncollagenous protein mainly produced by osteoblasts and is highly represented in bones of most vertebrates. Human OCN contains up to three gamma-carboxyglutamic acid (Gla-OCN) residues at positions 17, 21 and 24 which are thought to increase calcium binding strength, improving mechanical properties of the bone matrix. Recent studies revealed that OCN exerts also important endocrine functions, affecting energy metabolism and male fertility. The latter effect seems to be mediated by the uncarboxylated form of OCN (Glu-OCN). We employed human and mouse OCN as models of fully carboxylated and uncarboxylated OCN forms to investigate, by the use of circular dichroism and molecular dynamics simulations, the respective conformational properties and Ca2+ affinity. Ca2+ binding was found to trigger a similar conformational transition in both Glu-OCN and Gla-OCN, from a disordered structure to a more compact/stable form. Notably, gamma-carboxylation increases the affinity of OCN for Ca2+ by > 30 fold suggesting that, in physiological conditions, Gla-OCN is essentially Ca2+-bound, whereas Glu-OCN circulates mainly in the Ca2+-free form.