Reaction mechanism of Mn2+-ATPase of acto-H-meromyosin in 0.1 M KCl at 5 degrees C: evidence for the Lymn-Taylor mechanism.

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 0.1 M KCl at 5 degrees 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 X 10(5) M-1.s-1, and that for the Pi burst in the acto-HMM ATPase reaction was 1.7 X 10(5) M-1.s-1. The second-order rate constant for the binding of HMM.ADP 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 HMMPADP into HMM.ADP plus Pi, when the F-actin concentration was high.