Molecular motors and nonuniform ratchets

Dimeric kinesin presumably moves in a "hand-over-hand" fashion via alternat ing steps of its two heads, which can cooperate in various ways. This motio n is discussed in the framework of nonuniform ratchet models in which the m olecular motor is described by M internal states and undergoes transitions at K spatial locations within the period of the molecular force potentials. Two subclasses of models with (M, K) = (3, 2) and (M, K) = (2, 2) are stud ied which correspond to weakly and strongly cooperative heads, respectively . Both subclasses lead to the same universal relationship between the motor velocity and the unbinding rate constant of the motor heads which is remin iscent of, but distinct from, Michaelis-Menten kinetics.