Theoretical considerations in the equilibrium binding of myosin fragments on F-actin.

In a previous paper, equilibrium ocnstants for the binding of myosin fragments onto F-actin were assumed known and the statistical problems encountered when the actin sites are occupied to an arbitrary fractional extent were analyzed. The object of the present paper is to attempt to understand the observed order of magnitude of these equilibrium constants in terms of the statistical mechanical degrees of freedom involved. That is, we examine here the equilibrium constants themselves rather than the statistical consequences of the equilibrium constants. The treatment given amounts to a semi-quantitative sketch or outline of the problem. Structural details are much too uncertain to warrant a careful and rigorous treatment at this time. But the discussion suffices to establish the essential qualitative features of the problem. The procedure used is to examine the important equilibrium constants, one at a time, in terms of the factors (partition functions) that contribute to each constant, together with numerical estimates for these factors.