An analytical theory is presented for the dynamics of myosin-V molecular motor, where both ATP-dependent and ATP-independent steppings are taken into account. Specifically, the dependences of velocity, run length and unbinding rate upon both forward and backward loads and ATP concentration are studied, explaining quantitatively the diverse available single-molecule data and providing predicted results. The results show that the unbinding rate increases with the increase of ATP concentration and

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... P concentration and levels off at both low and high ATP concentrations. More interestingly, at the ATP concentration that is not very low, the unbinding rate exhibits characteristic of a catch-slip bond under the backward load, with the unbinding rate decreasing rapidly with the increase of the backward load in the range smaller than about 2.5 pN and then increasing slowly with the further increase of the backward load. By contrast, under the forward load the unbinding rate exhibits a slip-bond characteristic.