Phospholipid-Dependent Regulation of Cytochrome c3-Mediated Electron Transport across Membranes

Abstract

Cytochrome c 3 (cyt c 3) can mediate electron transport across phosphatidylcholine (PC)/cardiolipin (CL) and PC/phosphatidylglycerol (PG) membranes. A two-molecule process is involved in the electron transport across PC/CL membranes in the liquid-crystalline state. In contrast, a single-molecule process dominates the electron transport across PC/CL membranes in the gel state and PC/PG membranes in the liquid-crystalline and gel states. Namely, the electron transport mechanism differs with the phospholipid composition and membrane fluidity. The rate-limiting step of the two-molecule process was lateral diffusion of cyt c 3 in membranes. The rate constants for the three single-molecule process cases were similar to each other. To elucidate these reaction processes, interactions between cyt c 3 and phosphate groups and between cyt c 3 and the glycerol backbones of phospholipid bilayers were investigated by means of 31P and 2H solid-state NMR, respectively, for CL and PC/CL membranes. The results showed that the polar headgroups of both phosphatidylcholine and CL are involved in the binding of cyt c 3. Also, cyt c 3 penetrates into membranes, which would induce distortion of the lipid bilayer. The molecular mechanisms underlying the single- and two-molecule processes are discussed in terms of membrane structure.