Kinesin motors have been studied extensively both experimentally and theoretically. However, the microscopic mechanism of the processive movement of kinesin is still an open question. In this paper, we propose a hand-over-hand model for the processivity of kinesin, which is based on chemical, mechanical, and electrical couplings. In the model the ATPase rates of the two kinesin heads are regulated by forces, both from internal elasticity and external load, exerted on their necks. At a low external load, the ATPase rate of the trailing head is much higher than the leading head and the two heads are coordinated in their ATP hydrolysis and mechanical cycles. The motor walks processively with one ATP being hydrolyzed per step. At a higher forward external load, the ATPase rates of the two heads become comparable and thus the two heads are no longer well coordinated in their ATP hydrolysis and mechanical cycles. The model is consistent with the structural study of kinesin and the measured pathway of the kinesin ATPase. Using the model we have estimated the driving force to be -5.8pN, which is in agreement with the experimental results (5 7.5pN).The estimated time for moving one step (-10μs) is also consistent with the measured values of 50μs. The previous observation of substeps within the 8nm step is explained. The shapes of velocity versus load (both positive and negative)curves show close resemblance to previous experimental results.
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