  The diffusion of a molecular motor in the presence of a constant external
force is considered on the basis of a simple theoretical model. The motor is
represented by a Brownian particle moving in a series of parabolic potentials
placed periodically on a line, and the potential is switched stochastically
from one parabola to another by a chemical reaction, which corresponds to the
ATP hydrolysis or synthesis in motor proteins. It is found that the diffusion
coefficient as a function of the force exhibits peaks. The mechanism of this
diffusion enhancement and the possibility of observing it in F1-ATPase, a
biological rotary motor, are discussed.
