SIMULATED ROTATIONAL DIFFUSION OF F_O MOLECULAR MOTOR

F_O is a membrane-embedded part of ATP synthase, transforming proton current running within this molecule into a rotational motion taking place between two subunits, c-subunit and a-subunit. How the proton current could be transformed into the rotational motion remains unclear. In this paper, by means of molecular dynamics simulation, we studied basic properties of the rotational motion. In equilibrium, the c-ring, which consists of 10-12 c-subunits and interacts with the a-subunit, showed stepwise rotational motion. The rotational motions resulted in a free rotational diffusion in a longer period. The diffusion constant calculated as a function of temperature showed a glass-transition-like behavior: at the lower temperatures, diffusional motion was significantly suppressed, deviating from the Einstein's relation. Under a non-equilibrium condition where different heat baths with different temperatures are applied, respectively, to the c-ring and the a-subunit, we found that directionality arises in the rotational diffusion. We finally point out how the structural flexibility (i.e., softness) of protein molecules pertains to our results.