A New View on Mechanism of Functional Expression of an ATP-Driven Molecular Motor

Abstract

The ATP hydrolysis reaction can hardly occur in bulk aqueous solution without a catalyst. An ATP-driven molecular motor, which acts as the catalyst, is coupled with the reaction. Consequently, the reaction proceeds as the ATP hydrolysis cycle comprising the following events: the ATP binding to the molecular motor, hydrolysis of ATP into ADP and Pi, and dissociation of ADP and Pi from the molecular motor. Water in which ATP, ADP, and Pi are dissolved as well as the molecular motor (a protein or protein complex) forms the system of interest. In this system, the ATP hydrolysis reaction catalyzed by the molecular motor occurs as an irreversible process (i.e., a spontaneously occurring process). Upon each event in the ATP hydrolysis cycle, the molecular motor exhibits sequential changes in its configuration for minimizing the system free energy. We rationalize the argument that the translational, configurational entropy of water is a principal component of the system free energy. A protein (e.g., myosin) is moved or a protein in the complex (e.g., the γ subunit in the α3β3γ complex of F1-ATPase) is rotated in the direction where the water entropy already maximized can be retained.