Is myosin a "back door" enzyme?

ATP has been modeled into the active site of chicken skeletal myosin subfragment-1 using the adenylate kinase.Ap5A structure as a starting reference. The resulting docked ATP.S1 structure is justified in that it rationalizes the photolabeling data from several ATP analogs. The gamma-phosphate of ATP sits at the bottom of the active site pocket and is partially visible via a view along the prominent 50-kDa cleft of S1 but not when viewed from above the active site. It is postulated that actin binding promotes the movement of the P-loop and Arg-245 to allow Pi from ATP to leave via a "back-door" in the 50-kDa fragment while ADP is still bound at the active site. Such a mechanism can explain a number of experimental observations, including the kinetics of ATP hydrolysis, the nucleotide dependence of Pi exchange into ATP, and the formation of stable myosin.ADP.vanadate complexes in muscle fibers.