Fig. 6.

Plausible backward step mechanisms for kinesin. Upper panel corresponds to pathway I explained in Discussion. In this case the [T] dependence is identical to forward stepping. Lower panel is pathway II in which [T] dependence could be different from the forward stepping. T, D, DP, and $\varphi$ stand for ATP, ADP, (ADP + phosphate), and no nucleotide state, respectively. In Upper panel, after ATP binds to the leading head (shown in red), the neck linker docks. Consequently, the backward step along this pathway would be possible only at higher loads.