MOTOR PROTEIN MECHANICS - A STOCHASTIC-MODEL WITH MINIMAL MECHANOCHEMICAL COUPLING

A stochastic model for the action of motor proteins such as kinesin is presented. The mechanical components of the enzyme are 1) two identic al head domains that bind to discrete sites on a microtubule and that are capable of undergoing a conformational change; and 2) an elastic e lement that connects each head to the rest of the molecule, We investi gate the situation in which the strain dependence of the chemical reac tion rates is minimal and the heads have independent biochemical cycle s. The enzyme advances stochastically along a filament when one head d etaches and diffuses to a new binding site, while the other head remai ns bound to the microtubule. We also investigate the case in which the chemical cycles of the heads are correlated so that the molecule shif ts each head alternately. The predictions of the model are found to be in agreement with experimentally measured force-velocity relationship s for kinesin-both when the force is applied externally and when the e nzyme is loaded by a viscous drag. For reasonable values of the parame ters, this agreement is quantitative. The molecular stepping character istics observed in recent motility assays are also reproduced, A numbe r of experiments are suggested that would provide a more stringent tes t of the model and help determine whether this simple picture is an ap propriate description of motor proteins or whether models that include strain-dependent reaction rates or more complicated types of cooperat ion of the two heads need be considered.