Globoside as a Membrane Receptor: A Consideration of Oligosaccharide Communication with the Hydrophobic Domain

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• 作者：David H. Jones ; Clifford A. Lingwood ; Kathryn R. Barber ; and Chris W. M. Grant
• 刊名：Biochemistry
• 出版年：1997
• 出版时间：July 15, 1997
• 年：1997
• 卷：36
• 期：28
• 页码：8539 - 8547
• 全文大小：219K
• 年卷期：v.36,no.28(July 15, 1997)
• ISSN：1520-4995

文摘

Recognition of macromolecules by glycosphingolipids is closelycorrelated with the nature ofthe glycolipid carbohydrate; however, it is also thought to besecondarily modulated by the structure ofthe single fatty acid. In the present work, we sought insight intowhat physical effect a change in thisfatty acid has on the extramembranous portion of globosides atliposomal surfaces mimicking systemsfor which modulated receptor function has been recorded in the past.Protons of the exocyclichydroxymethyl group on the terminal Gal residue ofglobotriaosylceramide (Gb₃) were replaced withdeuterium. In this location, the nonperturbing probe nucleisampled cumulative conformational andorientational characteristics of the oligosaccharide chain at a sugarresidue that is critical in specific bindingof verotoxins. Deuterated Gb₃ having 18:1 fatty acidwas compared to the same species having 22:1fatty acid, at 6.3 mol % in unsonicated bilayers ofdipalmitoylphosphatidylcholine/cholesterol. Bothproduced narrow, apparently axially asymmetric ²H NMRspectra over a wide temperature range. Motionalproperties of the terminal sugar were measurably influenced by thefluidity of the host matrix; however,evidence was not found for conformational or orientational variation inthis sugar brought about by thefatty acid alteration. In related experiments, acetate protons onthe terminal N-acetyl galactosamine(GalNAc) residue of globotetraosylceramide (Gb₄) weresubstituted with deuterium, and the natural fattyacid was replaced with 18:0 or 24:0 species deuterated at C2. Onceagain, species with short vs longfatty acid were examined for evidence of headgroup differences.Spectra of Gb₄ were compared at 10mol % in unsonicated fluid bilayers of1-palmitoyl-2-oleoylphosphatidylcholine, and at 5 mol %inmembranes containing 33 mol% cholesterol. Spectral splittingsreflecting cumulative effects onconformation and order at the terminal deuterated sugar remainedunchanged between species having18:0 vs 24:0 fatty acid in POPC/cholesterol. In a purePOPC host matrix, there was clear evidence of amotional difference between the two-the longer chain Gb₄demonstrating spectral asymmetry-but thespectral width was unchanged. Transverse relaxation times,T₂, were measured. Our findings appeartohelp correlate the conclusions of a number of workers dealing with themolecular basis of crypticity. Wesuggest that changes in glycolipid receptor function based on ceramidefatty acid variation have a majororigin in the fatty acid's ability to determine the thermodynamics ofinteraction with the host matrix, asreflected in such parameters as glycolipid motional properties, localmembrane curvature, and likelyglycolipid time-dependent lateral associations. The result at lowconcentrations of glycolipid may oftenbe only a subtly altered collective surface epitope, best detected by aspecific recognition event.