Figure 2 - Models for the motor stepping mechanism.

Two different classes of cycle models describe the stepping mechanism of molecular motors. Most models assume a powerstroke that is associated with one of the transitions^{89, 79, 20}. The powerstroke is seen as a visco-elastic relaxation process of a strained molecule when the nucleotide state is changed. During this power stroke, a mechanical element of the motor is moved into a certain direction, determining the directionality of the motor. Only a few models explicitly include geometrical features, by associating different molecular geometries with different nucleotide states⁹⁰. In these models, the attribution of certain transitions to ATP binding, or the assumption that one or several of the transitions are force-dependent, explains how the steady-state velocity of a motor depends on the ATP concentration, or on the applied force. A very different approach is used in ratchet models^{91, 92}. Molecular fluctuations due to thermal motion are not overcome by the motion of the powerstroke, but instead are a central element of the stepping mechanism. The interaction between the motor and the microtubule is described as a periodic potential representing the binding sites for the motor, in which the effect of external forces can be included. Specific interaction potentials are attributed to each nucleotide state of the motor. They alternate as ATP is consumed and this results in directed movement, if at least one of the potentials is asymmetric. D, ADP; T, ATP.