How occasional backstepping can speed up a processive motor protein.

Fueled by the hydrolysis of ATP, the motor protein kinesin literally walks on two legs along the biopolymer microtubule. The number of accidental backsteps that kinesin takes appears to be much larger than what one would expect given the amount of free energy that ATP hydrolysis makes available. This indicates that backsteps are not simply the forward stepping cycle run backwards. We propose here a simple effective model that consistently includes the backstep transition. Using this model, we show how more backstepping increases the entropy of the final state, and probably also the activation state, thus reducing their free energy. This free energy reduction of the activation state (related to backstepping) speeds up the catalytic cycle of the kinesin, making both forward and backward steps more frequent. As a consequence, maximal net forward speed is achieved at nonzero backstep percentage. In addition, the optimal backstep percentage coincides with the backstep percentage measured for kinesin. This result suggests that, through natural selection, kinesin could have evolved to maximal speed.