Orientation of the saccharide chains of glycolipids at the membrane surface: conformational analysis of the glucose-ceramide and the glucose-glyceride linkages using molecular mechanics (MM3).

Preferred conformations of the saccharide-ceramide linkage of glucosylceramides with different ceramide structures (normal and hydroxy fatty acids) were investigated by molecular mechanics (MM3) calculations and compared with conformational features obtained for glucosylglycerolipids (diacyl and dialkyl analogues). Relaxed energy map calculations with MM3 were performed for the three bonds (C1'-O1-C1-C2, torsion angles phi, psi, and theta 1) of the glucose-ceramide/diglyceride linkage at different values of the dielectric constant. For the phi torsion of the glycosidic C1'-O1 bond the calculations show a strict preference for the +sc range whereas the psi/theta 1 energy surface is dependent on the structure of the lipid moiety as well as on the dielectric constant (epsilon). Calculations performed on glucosylceramide with normal and hydroxy fatty acids at epsilon = 4 (bilayer subsurface conditions) show three dominating conformers (psi/theta 1 = ap/-sc, -sc/ap, and ap/ap). The ap/-sc conformer, which represents the global energy minimum, is stabilized by polar interactions involving the amide group. The +sc rotamer of theta 1 is unfavored in sphingolipids due to a Hassel-Ottar effect involving the sphingosine O3 and O1 oxygen atoms. Comparative calculations on glycosylglycerolipid analogues (ester and ether derivatives) show a distinct preference for the ap rotamer of theta 1. An evaluation of the steric hindrance imposed by the surrounding membrane surface shows that in a bilayer arrangement the range of possible conformations for the saccharide-lipid linkage is considerably reduced. The significance of preferred conformations of the saccharide-ceramide linkage for the presentation and recognition of the saccharide chains of glycosphingolipids at the membrane surface is discussed.