COUPLING OF PROTEIN SURFACE HYDROPHOBICITY CHANGE TO ATP HYDROLYSIS BY MYOSIN MOTOR DOMAIN

Dielectric spectroscopy with microwaves in the frequency range between 0.2 and 20 GHz was used to study the hydration of myosin subfragment 1 (S1). The data were analyzed by a method recently devised, which can resolve the total amount of water restrained by proteins into two com ponents, one with a rotational relaxation frequency (f(c)) in the giga hertz region (weakly restrained water) and the other with lower f(c) ( strongly restrained water), The weight ratio of total restrained water to S1 protein thus obtained (0.35), equivalent to 2100 water molecule s per S1 molecule, is not much different from the values (0.3-0.4) for other proteins. The weakly restrained component accounts for about tw o-thirds of the total restrained water, which is in accord with the nu mber of water molecules estimated from the solvent-accessible surface area of alkyl groups on the surface of the atomic model of S1. The num ber of strongly restrained water molecules coincides with the number o f solvent-accessible charged or polar atoms. The dynamic behavior of t he S1-restrained water during the ATP hydrolysis was also examined in a time-resolved mode, The result indicates that when S1 changes from t he S1 ADP state into the S1 . ADP . P-i state (ADP release followed by ATP binding and cleavage), about 9% of the weakly restrained waters a re released, which are restrained again on slow P-i release, By contra st, there is no net mobilization of strongly restrained component, The observed changes in S1 hydration are quantitatively consistent with t he accompanying large entropy and heat capacity changes estimated by c alorimetry (Kodama, 1985), indicating that the protein surface hydroph obicity change plays a crucial role in the enthalpy-entropy compensati on effects observed in the steps of S1 ATP hydrolysis.