Protein-Carbohydrate Interactions: Deoxylactosides as probes for the bindingspecificity of betha-galactoside binding lectin

Abstract: a) The use of a solid phase reaction (silver silicate) for avoiding ortho ester formation in some difficult glycosyl donor/acceptor systems. Thus seven monodeoxygenated analogues of 2-(Trimethylsilyl)ethyl betha-lactoside were synthesiszed. Silver silcate was successfully used in betha 1-4 coupling of 2- and 3-deoxy glucosides to to acetobromogalactose. By similar means, the monodeoxylactosyl bromides and acetates were coupled to azidosphingosine. Yhe yields of betha-lactosides were higher than 70% in all cases except the 2-deoxylactoside, that was transformed to the alpha-chloride and then betha-sphingosinylated over silver silicate in 45% yield. b) The use of deoxylactosides as probes for investigating binding selectivities in a low affinity lectin-sugar system (complexation of lactose by Erythrina corallodendron lectin, the Lac-EcorL interaction). The Lac-EcorL interaction was studied by titration microcalorimetry, and a competitive inhibition assay where EcorL binding to lymphocytes was measured with a fluorescence activated cell sorter (FACS). The 3'- , 4'- and 6'-deoxylactoside was inactive, whereas the other monodeoxylactosides retained their activity. These tesults correlate well with published X-ray chrystallographic data. Our measurements showed that LacNAc binds about 15 times stronger than lactose. c) Computer simulations of the EcorL-Lac and EcorL-NAcLac complexes showed only minor differences concerning their hydrogen bonding properties. Interestingly, Lac O-3', O-4' seems to interact with both side chain oxygens in asp89 whereas only one of the oxygens was involved in binding to LacNAc.