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.

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