Contribution of Membrane Elastic Energy to Rhodopsin Function

We considered the issue of whether shifts in the metarhodopsin I (MI)-metarhodopsin II (MII) equilibrium from lipid composition are fully explicable by differences in bilayer curvature elastic stress. A series of six lipids with known spontaneous radii of monolayer curvature and bending elastic moduli were added at increasing concentrations to the matrix lipid 1-palmitoyl-2-oleoyl-sn-glycero-3-phosphocholine (POPC) and the MI-MII equilibrium measured by flash photolysis followed by recording UV-vis spectra. The average area-per-lipid molecule and the membrane hydrophobic thickness were derived from measurements of the ²H NMR order parameter profile of the palmitic acid chain in POPC. For the series of ethanolamines with different levels of headgroup methylation, shifts in the MI-MII equilibrium correlated with changes in membrane elastic properties as expressed by the product of spontaneous radius of monolayer curvature, bending elastic modulus, and lateral area per molecule. However, for the entire series of lipids, elastic energy explained the shifts only partially. Additional contributions correlated with the capability of the ethanolamine headgroups to engage in hydrogen bonding with the protein, independent of the state of ethanolamine methylation, with introduction of polyunsaturated sn-2 hydrocarbon chains, and with replacement of the palmitic acid sn-1 chains by oleic acid. The experiments point to the importance of interactions of rhodopsin with particular lipid species in the first layer of lipids surrounding the protein as well as to membrane elastic stress in the lipid-protein domain.