Elucidation of the biochemical significance of GMb1 ganglioside in model membranes of perdeuterated 1,2-dimyristoyl-L-[alpha]-phosphatidylcholine : a Fourier Transform Infrared spectroscopic investigation.

Abstract

The monosialoganglioside GM1 is an anionic glycosphingolipid found ubiquitously in the plasma membrane of mammalian cells. Although its exact normal biological function(s) and physical properties are unknown, it is speculated that GM1 may play a role in calcium transport and synaptic transmission in neural tissues. Fourier Transform Infrared (FTIR) spectroscopy was employed to test the model of bovine brain GM1 ganglioside incorporated into perdeuterated dimyristoylphosphatidylcholine (DMPCd54) multilamellar dispersions. The interaction of Ca 2+ ions with the mixed DMPCd54/GM1 bilayers and its effect on the hydrogen bonding at the interfacial region was also investigated. No thermal phase transition was displayed by the GM1 micellar suspensions over the temperature range from 5 to 60°C, even at concentrations of 110 mg/ml. On the other hand, in the mixed DMPCd54 /GM1 multilamellar bilayers, both the acyl chains of GM1 and the perdeuterated hydrocarbon chains of DMPCd54 exhibited a single, cooperative gel to liquid crystalline phase transition, occurring at a higher temperature than in pure DMPCd54 dispersions. This suggests that the ceramide moiety of the glycolipid inserts into the hydrophobic core of the phospholipid bilayer and that the mixed liposomes consisted of a homogeneous mixture of GM1 and DMPCd54 molecules. GM1 had an ordering effect on the mixed bilayers that was further enhanced by the divalent cation C2+. Spectral changes of the amide I and ester carbonyl bands at the interfacial zone indicated a reduced hydrogen bonding of these groups in the mixed liposomal bilayers. Our results also suggested that the ceramide moiety of GM1 inserts into the lipid bilayer of shorter chain symmetric phospholipids and terminates at the bilayer center (i.e., no interdigitation), forcing the carbohydrate headgroup to protrude farther out of the bilayer surface to allow a better binding to Ca2+ ions than DMPCd54. The divalent cation increased the probability of hydrogen bonding at the interfacial region. It is strongly believed that GM1 is involved in the translocation of extracellular signals across the plasma membrane and that the glycolipid affects interfacial hydrogen bonding and local hydrophobicity of the cell surface, which might modulate various surface events such as cell/cell and cell/surface protein interactions.