ATP synthase astounds again.  The molecular machine that generates almost all the ATP (molecular “energy pellets”) for all life was examined by Japanese scientists for its thermodynamic efficiency.  By applying and measuring load on the top part that synthesizes ATP, they were able to determine that one cannot do better at getting work out of a motor – a motor that is also a generator.

The rotating part of the two-part machine, named F0, is embedded in the membrane, running on the proton motive force delivered by upstream machines in the electron transport chain.  These scientists concentrated on the catalytic part of the machine, named F1, with its six lobes synthesizing three ATP per revolution (for information and animation, see CMI). Writing in PNAS,1 Toyabe et al. had nothing to say about evolution but a lot to say about machine efficiency:

We found that the maximum work performed by F1-ATPase per 120° step is nearly equal to the thermodynamical maximum work that can be extracted from a single ATP hydrolysis under a broad range of conditions. Our results suggested a 100% free-energy transduction efficiency and a tight mechanochemical coupling of F1-ATPase…

The coincidence between the maximum work performed by F1-motor during the 120° rotation (Wstall) and the chemical free energy change of an ATP hydrolysis (Δμ) suggests that F1-motor serves as a highly efficient mechanochemical free-energy transducer of FoF1-ATP synthase at almost 100% thermodynamic efficiency. For such a high efficiency to be achieved, a tight mechanochemical coupling is expected, that is, one ATP is hydrolyzed during every hydrolytic-direction step, whereas one ATP is synthesized during every synthetic direction step.

The article was edited by noted Harvard expert on the bacterial flagellum, Howard Berg.  The authors used machine language repeatedly, calling ATP synthase a “nanosized biological energy transducer,” a “reversible motor/generator,” a device that runs on electricity, generates torque, and performs work just like man-made machines do – only at an efficiency humans cannot come close to matching.  Most interesting was their term “reversible motor/generator.”  ATP synthase is a motor when synthesizing ATP, because the electrically-induced rotation of Fo turns a camshaft with a power takeoff performing work against a thermodynamic gradient in F1.  It’s a generator in the other direction – when excess ATP uses its energy to turn the camshaft and make the Fo rotor turn like the turbines in a hydroelectric plant….

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