  Living systems at the molecular scale are composed of many constituents with
strong and heterogeneous interactions, operating far from equilibrium, and
subject to strong fluctuations. These conditions pose significant challenges to
efficient, precise, and rapid free energy transduction, yet nature has evolved
numerous molecular machines that do just this. Using a simple model of the
ingenious rotary machine FoF1-ATP synthase, we investigate the interplay
between nonequilibrium driving forces, thermal fluctuations, and interactions
between strongly coupled subsystems. This model reveals design principles for
effective free energy transduction. Most notably, while tight coupling is
intuitively appealing, we find that output power is maximized at
intermediate-strength coupling, which permits lubrication by stochastic
fluctuations with only minimal slippage.
