Mechanics of coupling proton movements to c‐ring rotation in ATP synthase

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Abstract
F1F0 ATP synthases generate ATP by a rotary catalytic mechanism in which H+ transport is coupled to rotation of an oligomeric ring of c subunits extending through the membrane. Protons bind to and then are released from the aspartyl‐61 residue of subunit c at the center of the membrane. Subunit a of the F0 sector is thought to provide proton access channels to and from aspartyl‐61. Here, we summarize new information on the structural organization of Escherichia coli subunit a and the mapping of aqueous‐accessible residues in the second, fourth and fifth transmembrane helices (TMHs). Aqueous‐accessible regions of these helices extend to both the cytoplasmic and periplasmic surface. We propose that aTMH4 rotates to alternately expose the periplasmic or cytoplasmic half‐channels to aspartyl‐61 of subunit c during the proton transport cycle. The concerted rotation of interacting helices in subunit a and subunit c is proposed to be the mechanical force driving rotation of the c‐rotor, using a mechanism akin to meshed gears.