![]() For example, the motor protein kinesin generates about 5-pN force in its 8-nm walking step along the microtubule (MT), while hydrolyzing 1 ATP molecule. With advances in modern statistical mechanics, it became feasible to ask about the physical mechanisms by which motor proteins operate on energy scales not much greater than that of thermal energy ( 2). Study of these molecular “machines” dates back to the late 19th century, when the name “myosin” was coined for the molecular constituents responsible for muscle contraction ( 1). ![]() The function of motor proteins is to generate unidirectional motion, such as translation or rotation, making use of molecules, primarily ATP (adenosine 5′-triphosphate), as the energy source. As examples, we discuss linear motor proteins Kinesin-1 and myosin-V, and the rotary motor F 1-ATPase, all of which involve a power stroke as the essential element of their stepping mechanism. ![]() ![]() The 2 mechanisms are not mutually exclusive, and various motor proteins employ them to different extents to perform their biological function. Here, we compare the various models of the power stroke and the Brownian ratchet that have been proposed. Recent advances in experiments that reveal the details of the stepping motion of motor proteins, together with computer simulations of atomistic structures, have provided greater insights into the mechanisms. Both mechanisms require input of free energy, which generally involves the processing of an ATP (adenosine 5′-triphosphate) molecule. The former refers to generation of a large downhill free energy gradient over which the motor protein moves nearly irreversibly in making a step, whereas the latter refers to biasing or rectifying the diffusive motion of the motor. Two mechanisms have been proposed for the function of motor proteins: The power stroke and the Brownian ratchet.
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