Regulation of the F0F1-ATP synthase: The conformation of subunit ε might be determined by directionality of subunit γ rotation

F0F1-ATP synthase couples ATP synthesis/hydrolysis with transmembrane proton transport. The catalytic mechanism involves rotation of the γεc∼10-subunits complex relative to the rest of the enzyme.In the absence of protonmotive force the enzyme is inactivated by the tight binding of MgADP. Subunit ε also modulates the activity: its conformation can change from a contracted to extended form with C-terminus stretched towards F1. The latter form ihnibits ATP hydrolysis (but not synthesis).We propose that the directionality of the coiled-coil subunit γ rotation determines whether subunit ε is in contracted or extended form. Block of rotation by MgADP presumably induces the extended conformation of subunit ε. This conformation might serve as a safety lock, stabilizing the ADP-inhibited state upon de-energization and preventing spontaneous re-activation and wasteful ATP hydrolysis. The hypothesis merges the known regulatory effects of ADP, protonmotive force and conformational changes of subunit ε into a consistent picture.