A Mass Spectrometric Study of Metal Binding to Osteocalcin

Abstract

Electrospray ionization Fourier transform ion cyclotron resonance mass spectrometry was used to investigate Ca 2+, Mg 2+, and La 3+ binding to bovine bone osteocalcin (OCN). OCN was shown to bind 3 mol Ca 2+ per mol protein. There was also evidence for the presence of four additional metal binding sites. Ca 2+ increased the formation of the OCN dimer. Mg 2+ bound to OCN to the same extent as Ca 2+ but did not induce the dimerization of OCN. La 3+ bound to a lesser extent than either Ca 2+ or Mg 2+ to OCN and, like Mg 2+, did not influence dimerization. Each Gla residue of OCN participates in Ca 2+ binding, whereas Mg 2+ binding may occur preferentially at sites other than Gla residues. This implies that the different natures of Ca 2+- and Mg 2+-containing OCN complexes influence the tendency of OCN to form a dimer.