The emergence of viral strains that are resistant to the standard regimen for treatment of HIV, known as highly active anti-retroviral therapy (HAART), has led to interest in therapeutic approaches that target alternative stages of HIV infectivity. The interaction between the HIV envelope glycoprotein and the carbohydrate beta -galactosyl ceramide (GalCer) has been implicated as an early step in infectivity. The long term goal of this proposal is the elucidation of the molecular basis of this recognition process. This information is relevant to the development of potent inhibitors of gp120-GalCer binding, and ultimately, to novel HIV entry inhibitors. Our working hypothesis is that GalCer binds gp120 through key contact points in the sugar and the more distal region of the lipid segment, and that the central polar head segment of the ceramide acts primarily as a scaffold that controls the relative spatial positions of the sugar and lipid regions. This model is based on existing structure-activity data, and our observation that a relatively simple O-glycoside, binds gp120 considerably more strongly than its C-glycoside analog (ie. The glycosidic oxygen is replaced by CH2). To interrogate these conformational requirements, we have designed O-and C- galacto-lipid probes in which the lipid samples different regions of conformational space, relative to the galactose segment. The O-glycoside structures will be prepared by standard glycosidation procedures, and the C- glycosides via a novel C-glycosidation methodology that was developed in our laboratory. The conformational and gp120 binding properties of these molecules will be determined in collaboration with Dr. Jesus Jimenez Barbero (CSlC, Madrid) and Professor Jacquelyn Gervay-Hague (UC Davis). On the assumption that ligands that are more preorganized for binding will be more active, than ones that are not, we expect to obtain a clearer picture of the bioactive conformation. On a broader note, this study promises fundamental contributions in new areas of carbohydrate synthesis and conformation. This information will lay the groundwork for the extension of these structure activity probes to other problems in protein-carbohydrate recognition.
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