Influence of a transmembrane protein on the permeability of small molecules across lipid membranes

The influence of the nonchannel conformation of the transmembrane protein g ramicidin A on the permeability coefficients of neutral and ionized alpha-X -p-methyl-hippuric acid analogues (XMHA) (X = H, OCH3, CN, OH, COOH, and CO NH2) across egg lecithin membranes has been investigated in vesicle efflux experiments. Although 10 mol% gramicidin A increases lipid chain ordering, it enhances the transport of neutral XMHA analogues up to 8-fold, with more hydrophilic permeants exhibiting the greatest increase. Substituent contri butions to the free energies of transfer of both neutral and anionic XMHA a nalogues from water into the bilayer barrier domain were calculated. Linear free-energy relationships were established between these values and those for solute partitioning from water into decadiene, chlorobutane, butyl ethe r, and octanol to assess barrier hydrophobicity. The barrier domain is simi lar for both neutral and ionized permeants and substantially more hydrophob ic than octanol, thus establishing its location as being beyond the hydrate d headgroup region and eliminating transient water pores as the transport p athway for these permeants, as the hydrated interface or water pores would be expected to be more hydrophilic than octanol. The addition of 10 mol% gr amicidin A alters the barrier domain from a decadiene-like solvent to one p ossessing a greater hydrogen-bond accepting capacity. The permeability coef ficients for ionized XMHAs increase with Na+ or K+ concentration, exhibitin g saturability at high ion concentrations. This behavior can be quantitativ ely rationalized by Gouy-Chapman theory, though ion-pairing cannot be concl usively ruled out. The finding that transmembrane proteins alter barrier se lectivity, favoring polar permeant transport, constitutes an important step toward understanding permeability in biomembranes.