Molecular Dynamics Simulations of the Rotary Motor F0 under External Electric Fields across the Membrane

The membrane-bound component F-0, which is a major component of the F0F1-ATP synthase, works as a rotary motor and plays a central role in driving the F-1 component to transform chemiosmotic energy into ATP synthesis. We conducted molecular dynamics simulations of b(2)-free F-0 in a 1-palmitoyl-2-oleoyl-phosphatidylcholine lipid bilayer for tens of nanoseconds with two different protonation states of the cAsp-61 residue at the interface of the a-c complex in the absence of electric fields and under electric fields of +/-0.03 V/nm across the membrane. To our surprise, we observed that the upper half of the N-terminal helix of the cl subunit rotated about its axis clockwise by 30 degrees. An energetic analysis revealed that the electrostatic repulsion between this N-terminal helix and subunit c(12) was a major contributor to the observed rotation. A correlation map analysis indicated that the correlated motions of residues in the interface of the a-c complex were significantly reduced by external electric fields. The deuterium order parameter (S-CD) profile calculated by averaging all the lipids in the F-0-bound bilayer was not very different from that of the pure bilayer system, in agreement with recent H-2 solid-state NMR experiments. However, by delineating the lipid properties according to their vicinity to F-0, we found that the SCD profiles of different lipid shells were prominently different. Lipids close to F-0 formed a more ordered structure. Similarly, the lateral diffusion of lipids on the membrane surface also followed a shell-dependent behavior. The lipids in the proximity of F-0 exhibited very significantly reduced diffusional motion. The numerical value Of SCD was anticorrelated with that of the diffusion coefficient, i.e., the more ordered lipid structures led to slower lipid diffusion. Our findings will help elucidate the dynamics of F-0 depending on the protonation state and electric field, and may also shed some light on the interactions between the motor F-0 and its surrounding lipids under physiological conditions, which could help to rationalize its extraordinary energy conversion efficiency.