  The dynamics of motor protein molecules that have two subunits is
investigated using simple discrete stochastic models. Exact steady-state
analytical expressions are obtained for velocities and dispersions for any
number of intermediate states and conformations between the corresponding
binding states of proteins. These models enabled a detailed description and
comparison of two possible mechanisms of the motion of motor proteins along the
linear tracks: the hand-over-hand mechanism when the motion of subunits
alternate, and the inchworm mechanism when one subunit is always trailing
another one. It is shown that particles in the hand-over-hand mechanism move
faster and fluctuate more than the molecules in the inchworm mechanism. The
effect of external forces on dynamic properties of motor proteins is discussed.
Finally, a method is proposed for distinguishing between these two mechanisms
based on experimental observations.
