  A simple model of the two-state ratchet type is proposed for molecular
chemical engines that convert chemical free energy into mechanical work and
vice versa. The engine works by catalyzing a chemical reaction and turning a
rotor. Analytical expressions are obtained for the dependences of rotation and
reaction rates on the concentrations of reactant and product molecules, from
which the performance of the engine is analyzed. In particular, the efficiency
of energy transduction is discussed in some detail.
