Rapid regeneration of the actin-myosin power stroke in contracting muscle.

At the molecular level, muscle contraction is the result of cyclic interaction between myosin crossbridges, which extend from the thick filament, and the thin filament, which consists mainly of actin. The energy for work done by a single crossbridge during a cycle of attachment, generation of force, shortening and detachment is believed to be coupled to the hydrolysis of one molecule of ATP. The distance the actin filament slides relative to the myosin filament in one crossbridge cycle has been estimated as 12 nm by step-length perturbation studies on single fibres from frog muscle. The 'mechanical' power stroke of the attached crossbridge can therefore be defined as 12-nm shortening with a force profile like that shown by the quick recovery of force following a length perturbation. According to this definition, power strokes cannot be repeated faster than the overall ATPase rate. Here, however, we show that the power stroke can be regenerated much faster than expected from the ATPase rate. This contradiction can be resolved if, in the shortening muscle, the free energy of ATP hydrolysis is used in several actin-myosin interactions consisting of elementary power strokes each of 5-10 nm.