BINDING OF MYOSIN SUBFRAGMENT-1 TO ACTIN

The dissociation constant for the binding of myosin subfragment 1 (S1) and chromatographed actin in the presence and absence of nucleotide w as measured at various ionic strengths and various temperatures, The d issociation constant was of nM order in the absence of nucleotide and increased by similar to 100- and similar to 100,000-fold in the presen ce of ADP and ATP, respectively, The dissociation constant also increa sed with increasing ionic strength, irrespective of the presence of nu cleotide, and its dependence on the ionic strength was increased by th e presence of ATP but decreased by the presence of ADP, The standard e nthalpy change and entropy change for the binding of S1 to actin were both positive, irrespective of the presence of nucleotide, indicating that the binding was entropy-driven, The standard entropy change was e ssentially unaffected by the presence of ADP but was greatly decreased by ATP, suggesting that the large increase in the dissociation consta nt in the presence of ATP was due to the decrease of hydrophobic inter actions. On the other hand, the increase in the dissociation constant for acto-S1 in the presence of ADP might be induced by the decrease of electrostatic interactions.