Molecular recognition. II. The binding of the Lewis b and Y human blood group determinants by the lectin IV of Griffoniasimplicifolia

Using a radioimmunoassay to measure the relative potencies as inhibitors of a wide number of chemically modified structures related to the Lewis b human blood group determinant, it was found that derivatives of the Lewis b (αLFuc(1→2)βDGal(1→3)[αLFuc(1→4)]βDGlcNAc) and the Y (αLFuc(1→2)PDGal(1→4)[αLFuc(1→3)]βDGlcNAc) determinants are complexed by the lectin IV of Griffoniasimplicifolia through the recognition of a topographical feature that is common to both the tetrasaccharides. This surface provides a nonpolar region formed by the two methyl groups of the fucose units and extends along the C-1—O-5—C-5 side of the αLFuc(1→2) unit and terminates at one end by a polar grouping which is formed by OH-3 and OH-4 of the βDGal unit and OH-4 of the αLFuc(1→4) unit. Association constants were determined from changes in ultraviolet absorption that occur as the result of complex formation. For the reaction of the Lewis b-OCH3 tetrasaccharide, the thermodynamic parameters were found to be ΔH = −13 kcal/mol and ΔS = −22 cal/mol/K. The inhibition data for the relevant monodeoxy derivatives indicated that OH-2 and OH-3 of both of the αLFuc units are not directly involved in the binding reaction. The basis for drawing these conclusions was strengthened by finding that the reaction of the simple Lewis b analog, methoxymethyl (1→2)βDGal(1→3)[αLFuc(1→4)]βDGlcNAcOCH3 displayed very similar thermodynamic values; namely, ΔH = −14 kcal/mol and ΔS = −26 cal/mol/K, to those mentioned above for the Leb-OCH3 tetrasaccharide. The OH-4 of the αLFuc(1→2) unit may be bound in an intramolecularly hydrogen bonded form.