A self-induced translation model of myosin head motion in contracting muscle. I. Force-velocity relation and energy liberation.

In our previous model, it was assumed that the two heads of myosin act co-operatively in producing force for the sliding of actin filaments relative to myosin filaments. We eliminate the assumption of co-operativity in the present model, following the conclusion by Harada and co-workers that a co-operative interaction between the two heads of myosin is not essential in producing actin filament movement. We assume that (1) a myosin head activated by ATP hydrolysis binds to the thin filament at a definite angle and does not do the power stroke, i.e. does not change its orientation during attachment, (2) a potential of force acting on the myosin head is induced around the thin filament when an ATP-activated myosin head binds to an actin molecule in the thin filament, and (3) the potential remains for a while after detachment of the myosin head and statistically controls the direction of thermal motion of the myosin head, so that the myosin head translates toward the Z-line as a statistical average. We did calculations on these assumptions with a mean tension approximation and got the following results. (a) The calculated force-velocity relation in muscle contraction is in fairly good agreement with experimental observation, including the give phenomenon that lengthening velocity becomes very large for a force about twice the isometric tension. (b) The calculated rate of energy liberation during muscle contraction as a function of load on muscle is in good agreement with experimental results. (c) The calculated distance over which a myosin molecule moves along the thin filament during one ATP hydrolysis can be more than 60 nm under unloaded conditions.