Figure 11.

Cooperative action of multiple heads undergoing stochastic steps. (A) Schematic diagrams of actin and myosin filaments in skeletal muscle78. The actin filament has a helical structure with a half pitch of 36 nm. The myosin filament also has a helical structure with a pitch of 43 nm and a subunit repeat of 14.3 nm. Myosin heads on a myosin filament project toward an actin filament at 43 nm intervals. In skeletal muscle, the actin and myosin filaments are arranged in a hexagonal lattice and one actin is surrounded by three myosin filaments. Therefore, the number of myosin molecules project toward one actin filament 0.7 µm long (length when fully overlapped with myosin filaments) is approximately 50. When the actin filament is rotated 90°64, the relative position between the actin helical pitches and the myosin heads shifts by approximately 3 actin monomers. The actin slopes along the actin helical pitches are represented by a color gradient. (B) Qualitative explanation of the cooperative action of myosin heads on a thick filament. The myosin filament is equivalently represented by a row of myosin heads connected with springs at intervals of 43 nm. The actin filament is represented by straight, periodic, saw-tooth shape potentials along the half helical pitches as shown in Fig. 10. Cooperative action of the myosin heads causes a long (>60 nm) sliding distance of an actin filament per ATP (see text for detail).