The loose coupling mechanism in molecular machines of living cells.

For a bacterial flagellar motor driven by a proton flux, a loose coupling mechanism has been proposed in which the movement of the proton is indirectly and loosely coupled with the rotation of the motor. This mechanism assures the efficient and smooth conversion of both electrical and chemical potential energies of the proton of the same order as the energy of the thermal fluctuation. Loose coupling has been also assumed for the proton ATPase. A proton flux produces a rotational movement of protein molecules and this movement promotes the synthesis of ATP. In the proposed mechanism, the number of protons necessary for the synthesis of one ATP molecule is not an integer but varies depending on the environmental condition. In the case of muscle, the coupling between the hydrolysis of ATP and the shortening was found to be extremely loose. It is likely that molecular machines in living cells often adopt a loose coupling mechanism in which the chemical reaction and the physical cycle have not always a definite one-to-one correspondence.