Crossbridge Movements Monitored by Extrinsic Probes

Abstract

An appealing model of the molecular mechanism of muscle contraction proposes that active protein elements of the mechanism move in a cyclical manner in synchrony with the hydrolysis of ATP to produce muscle shortening against a load. In a muscle fibre the proteins myosin and actin make up about 55 per cent and 15 per cent of the fibre weight, respectively, and are spatially arranged into filaments. The filaments are interdigitated so that each myosin filament is equidistant from six actin filaments, as in Figure 7.1. The globular head region of the myosin molecule projects from the myosin filament backbone and is free to move across the interfilament space and bind to actin. The timely conversion of the free energy of ATP hydrolysis to useful mechanical work, in accordance with a moving protein model, requires that energy transduction take place inside the myosin molecule where the ATPase site is located. Actin is also of interest in this class of models, since the actin filament has a specific binding site for myosin. The possibility that the cyclical interaction of myosin and actin during muscle contraction produces muscle shortening against a load is suggested by the observation that the specific actomyosin affinity in a fibre varies over several orders of magnitude, depending on the substrate intermediates that occupy the myosin ATPase site.