A Mass Spectrometric Study of Metal Binding to Osteocalcin Original Research Article

Electrospray ionization Fourier transform ion cyclotron resonance mass spectrometry was used to investigate Ca2+, Mg2+, and La3+ binding to bovine bone osteocalcin (OCN). OCN was shown to bind 3 mol Ca2+ per mol protein. There was also evidence for the presence of four additional metal binding sites. Ca2+ increased the formation of the OCN dimer. Mg2+ bound to OCN to the same extent as Ca2+ but did not induce the dimerization of OCN. La3+ bound to a lesser extent than either Ca2+ or Mg2+ to OCN and, like Mg2+, did not influence dimerization. Each Gla residue of OCN participates in Ca2+ binding, whereas Mg2+ binding may occur preferentially at sites other than Gla residues. This implies that the different natures of Ca2+- and Mg2+-containing OCN complexes influence the tendency of OCN to form a dimer.