Temperature dependence of the decay of the UV absorption difference spectrum of heavy meromyosin induced by adenosine triphosphate and inosine triphosphate.

The UV absorption difference spectrum of heavy meromyosin induced by ATP was measured at various temperatures. At higher temperatures, the difference spectrum formed rapidly after adding ATP and continued steadily during the steady state which we have called the ATP-form of difference spectrum. At lower temperatures, the ATP-form of difference spectrum decayed into the other form before the steady state was attained. This was identical to the difference spectrum obtained by adding ADP and has been called the ADP-form of difference spectrum. At intermediate temperatures, biphasic decay was observed. The results indicate that the dominant intermediate at the steady state is altered from the one showing the ATP-form of difference spectrum at higher temperatures to that showing the ADP-form at lower temperatures. The population of the two intermediates depends on the temperature between the two extremes. This temperature-induced transition was observed in the presence of any divalent cation such as Mg2+, Mn2+, or Ca2+. A similar transition was observed with the difference spectrum induced by ITP in the presence of MgCl2. The pH dependence of the single early decay of the ATP-induced difference spectrum was measured in the presence of MnCl2 at 1 degree. The apparent rate constant of the decay showed a biphasic pH dependence, having the same shape as the pH activity curve of ATPase [EC 3.6.1.3] observed at higher temperatures. The rate determining step for the steady state ATPase at higher temperatures is thought to be the step of changing from the intermediate complex showing the ATP-form of difference spectrum to that showing the ADP-form. This is inconsistent with our previous mechanism (Yazawa, M. et al. (1973) J. Biochem. 74, 1107-1117). The rate determining step at lower temperatures was assigned as a step of ADP dissociation.