The Effects of Ions and Water on Actin-Myosin Binding Energetics and Mechanics

Myosin's discrete lever arm rotation upon strong actin binding is capable of generating large forces, but in ensemble myosin systems the average force generated by this transition is limited by the free energy for actin-myosin binding. Yet when looking for insights into mechanisms of muscle force generation, myosin structures are studied while factors that affect actin-myosin binding energetics are seldom considered. Here we use in vitro motility assays, AM binding assays, and stopped flow kinetics to study the effects of water and KCl on actin-myosin binding. First, we show that sucrose dramatically inhibits actin-myosin binding kinetics through a mechanism other than crowding or viscous damping, implying that sucrose has a desolvation effect. Second, we show that the effects of sucrose are ionic strength dependent, having little effect at low ionic strength. Our data support a simple model in which the strength of weak actin-myosin bonds is reduced by the shielding effect of ions, and sucrose enhances this effect possibly by displacing ordered waters around actin and myosin. Our data suggest that water solvation and ionic shielding may have a significant influence on muscle mechanics.