Mechanism of Unidirectional Rotation of γ Subunit in F1-ATPase

Abstract

In this chapter, the discussion is focused on the functional expression of F1-ATPase, the unidirectional rotation of the γ subunit. In the structure of the α3β3 complex stabilized by the water-entropy effect, the atoms in three β subunits, to which different chemical compounds (i.e., ATP just before the hydrolysis reaction, ATP, ADP + Pi, nothing, or Pi) are bound, are closely, moderately, and loosely packed, respectively. This nonuniformity and the structural asymmetry of the γ subunit play essential roles. The γ subunit takes a particular orientation in accordance with the structure of the α3β3 complex so that the water entropy can be maximized. Due to the occurrence of the ATP binding, ATP hydrolysis, and dissociation of ADP and Pi during each ATP hydrolysis cycle, the chemical compounds bound to the three β subunits successively change, in concert with which the α3β3 complex structure and the orientation of the γ subunit sequentially change to retain the maximized water entropy. In one hydrolysis cycle, the γ subunit exhibits a 120° rotation. It is experimentally known that the ATP synthesis occurs when the γ subunit is forced to rotate in the inverse direction by a sufficiently strong external torque imposed. This can also be explicated on the basis of the water-entropy effect.