Temperature induced analog reaction of adenylyl imidodiphosphate to an intermediate step of heavy meromyosin adenosine triphosphatase.

The UV absorption difference spectrum of heavy meromyosin induced by adenylyl imidodiphosphate (AMP-PNP) was found to be changed by temperature. At higher temperatures, the shape of the difference spectrum resembled the ATP-form of difference spectrum induced by ATP. At lower temperatures, a different shape was observed, resembling that induced by ADP. This temperature transition was found in the presence of both MgCl2 and MnCl2. The transition temperatures, were 21 degrees and 9 degrees in the presence of MnCl2 and MgCl2, respectively. A similar temperature dependence was observed with the difference spectrum induced by ATP at the steady state. The transition temperatures in this case were 11 degrees and 4.5 degrees in the presence of MnCl2 and MgCl2, respectively. The similarity of the effects of the two kinds of divalent cation on both transitions indicates that the temperature induced transition between two species of heavy meromyosin-AMP-PNP complex mimics the step in APTase [EC 3.6.1.3] reaction in which the intermediate complex showing the ATP-form of difference spectrum changes to that showing the ADP-form. The equilibrium constant of the decay step of the ATP-form of difference spectrum to the ADP-form in ATPase is, therefore, thought to be highly temperature dependent. Thermodynamic parameters were calculated for the transition between the two species of heavy meromyosin AMP-PNP complex. Large decreases in enthalpy and entropy were observed, while the standard free energy change was small. The results suggest that the intermediate showing the ATP-form of difference spectrum hardly changes to the forward direction in the ATPase reaction at higher temperature. The complex appears to be so stable in the steady state that almost all the myosin is present as this complex. The decay step in ATPase of the difference spectrum from the ATP-form to to the ADP-form may be coupled to muscular contraction. The temperature induced transition of heavy meromyosin AMP-PNP complex may, therefore, provide information concerning the state of myosin in active muscles.