  We model a processive linear molecular motor as a particle diffusing in a
one-dimensional periodic lattice with arbitrary transition rates between its
sites. We present a relatively simple proof of a theorem which states that the
ratio of the drift velocity V to the diffusion coefficient D has the upper
bound 2N/d, where N is the number of nodes in an elementary cell and d denotes
its length. This relation can be used to estimate the minimal value of internal
states of the motor and the maximal value of the so called Einstein force,
which approximately equals to the maximal force exerted by a molecular motor.
