title: The kinetics of bivalent metal ion dissociation from myosin subfragments.
abstract: Bivalent metal ions have multiple roles in subunit association and ATPase regulation in scallop adductor-muscle myosin. To help elucidate these functions, the rates of Ca2+ and Mg2+ dissociation from the non-specific high-affinity sites on the regulatory light chains were measured and compared with those of rabbit skeletal-muscle myosin subfragments. Ca2+ dissociation had a rate constant of about 0.7 s-1 in both species, as measured by the time course of the pH change on EDTA addition. Mg2+ dissociation had a rate constant of 0.05 s-1, as monitored by its displacement with the paramagnetic Mn2+ ion. It is concluded that the exchange between Ca2+ and Mg2+ at the non-specific site, on excitation of both skeletal and adductor muscles, is too slow to contribute to the activation itself. The release of bivalent metal ions from the non-specific site is, however, the first step in release of the scallop regulatory light chain (Bennett & Bagshaw (1986) Biochem. J. 233, 179-186). In scallop myosin additional specific sites are present, which can bind Ca2+ rapidly, to effect activation of the ATPase. In the course of this work, Ca2+ dissociation from EGTA was studied as a model system. This gave rates of 1 s-1 and 0.3 s-1 at pH 7.0 and pH 8.0 respectively.
mesh_terms: D000818:Animals; D001665:Binding Sites; D002118:Calcium / Q000378:metabolism*; D004533:Egtazic Acid / Q000378:metabolism; D004578:Electron Spin Resonance Spectroscopy; D006863:Hydrogen-Ion Concentration; D007700:Kinetics; D008274:Magnesium / Q000378:metabolism*; D008974:Mollusca; D015879:Myosin Subfragments; D009218:Myosins / Q000378:metabolism*; D010446:Peptide Fragments / Q000378:metabolism*; D011817:Rabbits
chemical_list: D015879:Myosin Subfragments; D010446:Peptide Fragments; D004533:Egtazic Acid; D009218:Myosins; D008274:Magnesium; D002118:Calcium