title: Carboxylation-dependent conformational changes of human osteocalcin.
abstract: 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.
authors.affiliation: CRS4, Biomedicine sector, Parco Polaris, 09010 Pula (CA), Italy.
authors.forename: Andrea
authors.initials: A
authors.lastname: Cristiani
authors.affiliation: CRS4, Biomedicine sector, Parco Polaris, 09010 Pula (CA), Italy.
authors.forename: Fabio
authors.initials: F
authors.lastname: Maset
authors.affiliation: CRS4, Biomedicine sector, Parco Polaris, 09010 Pula (CA), Italy.
authors.forename: Luca
authors.initials: L
authors.lastname: De Toni
authors.affiliation: CRS4, Biomedicine sector, Parco Polaris, 09010 Pula (CA), Italy.
authors.forename: Diego
authors.initials: D
authors.lastname: Guidolin
authors.affiliation: CRS4, Biomedicine sector, Parco Polaris, 09010 Pula (CA), Italy.
authors.forename: Davide
authors.initials: D
authors.lastname: Sabbadin
authors.affiliation: CRS4, Biomedicine sector, Parco Polaris, 09010 Pula (CA), Italy.
authors.forename: Giacomo
authors.initials: G
authors.lastname: Strapazzon
authors.affiliation: CRS4, Biomedicine sector, Parco Polaris, 09010 Pula (CA), Italy.
authors.forename: Stefano
authors.initials: S
authors.lastname: Moro
authors.affiliation: CRS4, Biomedicine sector, Parco Polaris, 09010 Pula (CA), Italy.
authors.forename: Vincenzo
authors.initials: V
authors.lastname: De Filippis
authors.affiliation: CRS4, Biomedicine sector, Parco Polaris, 09010 Pula (CA), Italy.
authors.forename: Carlo
authors.initials: C
authors.lastname: Foresta
mesh_terms: D015055:1-Carboxyglutamic Acid / Q000737:chemistry / Q000235:genetics / Q000378:metabolism; D000595:Amino Acid Sequence; D000818:Animals; D001667:Binding, Competitive; D002118:Calcium / Q000737:chemistry / Q000378:metabolism; D002264:Carboxylic Acids / Q000378:metabolism*; D002942:Circular Dichroism; D018698:Glutamic Acid / Q000737:chemistry / Q000235:genetics / Q000378:metabolism; D006801:Humans; D007700:Kinetics; D051379:Mice; D056004:Molecular Dynamics Simulation; D008969:Molecular Sequence Data; D015675:Osteocalcin / Q000737:chemistry* / Q000235:genetics / Q000378:metabolism*; D011485:Protein Binding; D011487:Protein Conformation; D055550:Protein Stability; D017386:Sequence Homology, Amino Acid; D013816:Thermodynamics
chemical_list: D002264:Carboxylic Acids; D015675:Osteocalcin; D018698:Glutamic Acid; D015055:1-Carboxyglutamic Acid; D002118:Calcium