Steric effects in the interaction between transmembrane proteins and polyunsaturated phospholipids

Abstract

The methylene-interrupted, all-cis configuration is overwhelmingly favoured in polyunsaturated fatty acids (PUFA) of vertebrate membranes, particularly the highly unsaturated membranes of electrically active neural tissue. The hypothesis that hexaene and pentaene homoallylic groups are sterically allowed to follow the groove of a transmembrane α-helix from bovine rhodopsin was investigated using energy minimization (EM) and molecular dynamics (MD) simulations with the GROMOS96 force field. Docosahexaenoic acid (22:6n-3) and docosapentaenoic acid (22:5n-6) were arranged along arbitrary midand C-terminal paths of a model of α-helix 2 of bovine rhodopsin, with the doubly allylic CH 2 hydrogens directed inward and the ethylenic CH hydrogens directed out of the helix. By performing EM and MD simulations for 22:6n-3 and 22:5n-6, it was shown that the rotationally constrained homoallylic regions are more stably contained within the groove than are the saturated regions. The analyses of the initial conformations of 22:6n-3 in phosphatidylcholine-22:6n-3;34:5n-3, and of 22:5n-6 in phosphatidylcholine-22:5n-6;34:5n-3, and of conformations of 22:6n-3 in phosphatidylcholine-22:6n-3;34:5n-3 obtained after EM, showed that the homoallylic regions fit loosely within the groove while the saturated regions are much closer to the outer groove boundary. EM calculations of the phosphatidylcholine22:6n-3;34:5n-3 showed that the 22:6n-3 chain can follow the groove nearest a membrane– water interface, while the homoallylic region of the very long chain 34:5n-3 can follow the groove nearer the membrane centre, tethered by its extended saturated region. These results illustrate that the homoallylic polyunsaturated fatty acid motif is not sterically prevented from occupying the groove of a transmembrane α-helix. Speculations are provided about the biophysical stability and properties conferred by this configuration.