Molecular Mechanisms of Proton Permeation across Lipid Membranes- Effect of Cholesterol

Abstract

Despite years of study the mechanisms by which H+ permeate lipid membranes are not well understood 1, 2. H+ flux differs from that of other ions in that H+ conductance is not dependent on the actual [H+] in the solution. Combining careful permeability measurements with structural analyses of lipid bilayers using X-ray diffraction, we have developed models of water, solute and H+ permeation across membranes; the models include various headgroups, chain lengths and extent of unsaturation3,4. We compared H+ permeability with physical parameters of the lipids, such as area/lipid, hydrocarbon thickness, bending modulus and compressibility modulus. Similar to water and solutes, in membranes composed of a single phospholipid, H+ permeability varied linearly with area/lipid, and was unrelated to other physical parameters. On this basis, in single component lipid systems, the rate limiting step for H+ permeation is hypothesized to be penetration of the proton from the aqueous medium into the lipid bilayer. When cholesterol is a component of the bilayer, water permeability decreases (15.8 ±0.58 x 10−3 cm/s in absence and 6.8 ± 0.57 x 10−3 cm/s in presence of cholesterol respectively). In contrast H+ permeability increases as the proportion of cholesterol increases in the bilayer (0.056 ± 0.006 cm/s in absence and in presence of cholesterol 0.113 ± 0.005 cm/s). We conclude that mechanisms of H+ permeability differ markedly from those of water and solutes. We have developed and are testing a new model for H+ permeation, which defines how cholesterol enhances H+ flux, while impeding water and solute fluxes.