The effect of divalent metal ions on the ATPase activity and ADP binding of H-meromyosin

The effects of Mg2+, Mn2+, Zn2+, Co2+, Ni2+, Ca2+, and Sr2+ upon the equilibrium constant (K) and formation rate constant (k1) of H-meromyosin-ADP complex were studied. The affinity constant of ADP binding decreases in the following order Mn > Mg > Zn > Ni > Co > Sr ∼- Ca. The calculated dissociation rate constant (k2) of the enzyme-ADP complex was similar to the rate of ATP hydrolysis with all divalent metals tested, except Sr2+. With Mg as predominant metal ion the following dissociation constants were obtained for the binding of various nucleoside diphosphates to H-meromyosin: ADP 1–2 × 10−6 m , UDP 2 × 10−5 m , CDP 3 × 10−5 m , IDP 10−4 m . The results are compatible with the suggestion that at 6 °C the dissociation of ADP from the active site limits the rate of ATP hydrolysis with Mg2+, Mn2+, Co2+, and Ca2+.