Fluctuation theorem for a single enzym or molecular motor

Cyclically operating enzyms and molecular motors are shown to be restricted non-linearly by a fluctuation theorem that basically relates the number of backward steps to that of forward steps. Only if the rates obey a quasi-equilibrium form in terms of chemical potentials and mechanical load, this fluctuation theorem becomes the usual one for entropy fluctuations. Boundary terms can be subsumed under an entropy change if one defines a trajectory-dependent entropy of the enzym or motor. Explicit expressions are derived for a three-state motor with and without an intermediate state and an enzym with Michaelis-Menten kinetics.