Keeping up with the F₁-ATPase

The only other proton-driven rotary device is the flagellar motor (Fig. 2b). Available evidence⁴ , although not as good at that obtained by Kinosita and colleagues, indicates that this also runs at high efficiency — but only when turning slowly. Molecular motors are not heat engines. If the energy available from the hydrolysis of ATP were uniformly distributed throughout the F₁-ATPase, it would heat up by less than 0.1 °C and then cool off with a decay time of less than 0.1 ns. The energy available from the binding of ATP or the release of inorganic phosphate (or whatever) must be stored in springs (conformational or electrostatic) which, on relaxing, drive the actin filament through 120°. The details of this mechanism should prove fascinating.

How does the F₀part of the ATPase work? The main problem is that we are not yet certain which components comprise the stator and which the rotor. Most people believe that protons move between the a and c subunits, interacting with the Arg 210 residue in a and the Asp 61 residue in c (Fig. 2a). This view has been developed quantitatively by Elston et al.⁵ , in a version of a thermal-ratchet model that was invented earlier for the flagellar motor⁶ . The proton rides with Asp 61 on the rotor. The pK_aof Asp 61 is then lowered by interaction with Arg 210, forcing the proton off whenever the rotor, moving thermally, tries to go the wrong way. One proton is carried per c subunit, 12 per revolution, four per ATP synthesized. The Cys 205 residue of the γ-subunit has been crosslinked to c subunits at positions 42, 43 or 44 without loss of ATPase activity⁷ , but it is difficult to tell whether this alters proton pumping. The interacting helical faces of a and c have also been defined by crosslinking⁸ , although it is not known whether such crosslinks block function.

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