This brief discusses the mechanism of functional expression of a protein or protein complex utilizing the ATP hydrolysis cycle or proton-motive force from a unique point of view focused on the roles of water. A variety of processes are considered such as the unidirectional movement of a linear-motor protein along a filament, insertion of an unfolded protein into a chaperonin and release of the folded protein from it, transport of diverse substrates across the membrane by a transporter, and directed rotation of the central subunit within a rotatory motor protein complex. These topics are discussed in a unified manner within the same theoretical framework. The author argues that water plays imperative roles in the functional expression of these molecular machines. A pivotal factor is the entropic force or potential originating from the translational displacement of water molecules coexisting with the molecular machines in the entire system.

This book presents a new view of the mechanism of functional expression of ATP-driven motors (proteins or protein complexes). It is substantially different from the prevailing idea that the motor converts chemical energy to mechanical work. To facilitate understanding, the differences between the new and prevailing views are explained using many illustrations. The book is of interest to those who are not convinced of the notion of chemo-mechanical coupling. The claims presented are the following: The system, which comprises not only the motor but also water, does no mechanical work during the ATP hydrolysis cycle; a protein is moved or a protein in the complex is rotated by the entropic force generated by water. The highlight of the explanation in the book is that the mechanism of unidirectional rotation of the central shaft in F1-ATPase is discussed in detail on the basis of this new view. The hydration entropy of each subunit to which a specific chemical compound (ATP, ADP and Pi, Pi, or nothing) is bound, the hydration entropy of the 3 3 complex, and the dependence of the hydration entropy of F1-ATPase on the orientation of the subunit play essential roles.