Coupling proton movements to c-ring rotation in F1Fo ATP synthase: aqueous access channels and helix rotations at the a–c interface

2002 
Abstract F 1 F o ATP synthases generate ATP by a rotary catalytic mechanism in which H + transport is coupled to rotation of a ring of c subunits within the transmembrane sector of the enzyme. Protons bind to and then are released from the aspartyl-61 residue of subunit c at the center of the membrane. Proton access channels to and from aspartyl-61 are thought to form in subunit a of the F o sector. Here, we summarize new information on the structural organization of subunit a and the mapping of aqueous accessible residues in the fourth and fifth transmembrane helices (TMHs). Cysteine substituted residues, lying on opposite faces of a TMH-4, preferentially react with either N -ethyl-maleimide (NEM) or Ag + . We propose that a TMH-4 rotates to alternately expose each helical face to aspartyl-61 of subunit c during the proton transport cycle. The concerted helical rotation of a TMH-4 and c TMH-2 are proposed to be coupled to the stepwise mechanical movement of the c -rotor.
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