Anionic Lipid-Dependent Gliding of Cytochrome C across Bioenergetic Membranes

Abstract

Bioenergetic membranes found in mitochondria, thylakoids, and chromatophores are primary sites of ATP production in living cells. These membranes contain an electron transport chain (ETC) in which electrons are shuttled between a series of redox proteins during the generation of ATP via oxidative phosphorylation. Classical examples of electron shuttles include members of the cytochrome c family. Phospholipid composition plays a role in determining the local electrostatic environment of the surface of the owing to the spatial distribution of their charged head groups. Cardiolipin (CDL) is a characteristic phospholipid in bioenergetic membranes, and is a significant contributor of negative surface charge. Interactions between cytochromes and phospholipid head groups in the membrane can in principle affect the rate of travel between ETC components, influencing the rate of ATP turnover. We used Highly Mobile Membrane Mimetic (HMMM) simulations of cytochrome c2 (cyt.c2) to study protein-lipid interactions in the context of diffusion across a model bioenergetic membrane (PC:PE:PG:CDL = 10:30:46:14). Using HMMM, we extend the time-scales of molecular dynamics to aggressively sample protein and lipid diffusion and interactions at an atomic resolution. We observed a bias for cyt.c2 to bind anionic lipids, with a clear preference for CDL. During diffusion, cyt.c2 maintains a relatively fixed tilt with respect to the membrane normal with wider fluctuations in the angle with respect to axes in the plane of the membrane, caused by its prominent dipole moment. The cyt.c2 diffusion coefficient calculated from simulation trajectories is 0.7 Å2 /ns. Our simulation supports a mode of diffusion in which cyt.c2 skips across the membrane, rather than a purely two dimensional, on the surface of the membrane, or a three dimensional mode of diffusion.