On the functioning mechanism of an ATP-driven molecular motor.

Abstract

A molecular motor, an ATP-driven protein or protein complex, fulfills its function by utilizing the ATP hydrolysis cycle comprising the following elementary processes: (1) the binding of ATP to the motor, (2) ATP hydrolysis (ATP+H2O→ADP+Pi), and (3) dissociation of ADP and Pi from the motor. Paradigmatic examples which have received extensive investigations are myosin achieving unidirectional movement along filamentous actin (F-actin) and F1-ATPase whose γ subunit accomplishes unidirectional rotation within the α3β3 complex (see Fig. 1). Though they are simpler than the sarcomere and the FoF1-ATP synthase, the investigations provide physical insights into and much useful information on the functioning mechanism of the motor. In the prevailing view, the motor converts the free energy of ATP hydrolysis to mechanical work that must be performed against the viscous resistance force by water during the unidirectional movement or rotation. The motor is highlighted as the system of interest and the aqueous solution surrounding it is regarded as the external system. In this article, the author suggests a new view by pointing out that the prevailing view is problematic and inconsistent with some of experi­mental observations. Detailed discussions are given in recent books written by the author [1,2] and references therein, and only the physical essence is recapitulated here. Open in a separate window Figure 1 Functional expression of a molecular motor utilizing ATP hydrolysis cycle. (a) Unidirectional movement of myosin, an ATP-driven protein, along F-actin. The ATP hydrolysis cycle occurs for myosin. In the real system, F-actin possesses a double helical structure. (b) Unidirectional rotation of γ subunit within α3β3 complex of F1-ATPase, an ATP-driven protein complex. The ATP hydrolysis cycle occurs for the α3β3 complex. The γ subunit rotates in the counterclockwise direction when F1-ATPase is viewed from the Fo side (θ denotes the rotation angle arbitrarily defined for the γ subunit).