Small segmental rearrangements in the myosin head can explain force generation in muscle

Poisson-Boltzmann calculations of the distribution of electrostatic potentials around an actin filament in physiological-strength solutions show that negative isopotential surfaces protrude into the solvent. Each protrusion follows the actin two-start helix and is located on the sites implicated in the formation of the actomyosin complex. Molecular dynamic calculations on the S1 portion of the myosin molecule indicate that in the presence of ATP the crystallographically invisible loops (comprising residues 624-649 and 564-579) remain on the surface, whereas in the absence of ATP they can move toward the actin-binding sites and experience electrostatic forces that range from 1 to 10 pN. The molecular dynamics calculations also suggest that during the ATP cycle there exist at least three states of electrostatic interactions between the loops and actin. Every time a new interaction is formed, the strain in the myosin head increases and the energy of the complex decreases by 2kT to 5kT. This can explain muscular contraction in terms of a Huxley-Simmons-type mechanism, while requiring only rearrangements of small mobile S1 segments rather than the large shape changes in the myosin molecule postulated by the conventional tilting head model.