The magnesium ion-dependent adenosine triphosphatase of myosin. Two-step processes of adenosine triphosphate association and adenosine diphosphate dissociation

摘要

The kinetics of protein-fluorescence change when rabbit skeletal myosin subfragment 1 is mixed with ATP or adenosine 5′-(3-thiotriphosphate) in the presence of Mg²⁺ are incompatible with a simple bimolecular association process. A substrate-induced conformation change with ΔG⁰<−24kJ·mol⁻¹ (i.e. ΔG⁰ could be more negative) at pH8 and 21°C is proposed as the additional step in the binding of ATP. The postulated binding mechanism is M+ATP⇌M·ATP⇌M*·ATP, where the association constant for the first step, K1, is 4.5×10³m⁻¹ at I 0.14m and the rate of isomerization is 400s⁻¹. In the presence of Mg²⁺, ADP binds in a similar fashion to ATP, the rate of the conformation change also being 400s⁻¹, but with ΔG⁰ for that process being −14kJ·mol⁻¹. The effect of increasing ionic strength is to decrease K1, the kinetics of the conformation change being essentially unaltered. Alternative schemes involving a two-step binding process for ATP to subfragment 1 are possible. These are not excluded by the experimental results, although they are perhaps less likely because they imply uncharacteristically slow bimolecular association rate constants.