The rate constants of a series of elementary steps in the H-meromyosin (HMM) Mn2+-ATPase [EC 3. 6. 1. 3] and acto-HMM Mn2+-ATPase reactions were determined in 0.1M KCl at 5°C. We found that the rate-limiting step in the HMM Mn2+-ATPase reaction was the liberation of ADP from HMM•ADP, of which the rate constant was estimated to be 0.17 s-1. All the results obtained with the acto-HMM Mn2+-ATPase reaction could be quantitatively explained by a modified Lymn-Taylor mechanism (see Fig. 15). The second-order rate constant for the dissociation of acto-HMM induced by ATP was 3.0×105M-1•s-1, and that for the P1, burst in the acto-HMM ATPase reaction was 1.7×105M-1•s-1. The second-order rate constant for the binding of HMMADP with F-actin was 0.25 s-1•mg-1•ml, and the rate-limiting step in the acto-HMM Mn2+-ATPase reaction was the conversion of HMMADPP into HMM•ADP plus P1, when the F-actin concentration was high.
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