Chemoenzymatic synthesis of glycopolypeptides carrying α-Neu5Ac-(2→3)-β-D-Gal-(1→3)-α-D-GalNAc, β-D-Gal-(1→3)-α-D-GalNAc, and related compounds and analysis of their specific interactions with lectins

Glycopolypeptide (1) carrying the beta -D-Gal-(1 --> 3)-alpha -D-GalNAc unit as a kind model of asialo-type mucin was synthesized through three steps: enzymatic synthesis of p-nitrophenyl disaccharide glycoside, reduction of the p-nitrophenyl group, and coupling of the amino group with the carboxyl group of poly(L-glutamic acid)s (PGA), In a similar manner, glycopolypeptides (2-7) carrying beta -D-Gal-(1 --> 3)-beta -D-GalNAc, beta -D-Gal-(1 --> 3)-beta -D-GlcNAc, beta -D-Gal-(1 --> 6)-alpha -D-GalNAc, beta -D-Gal-(1 --> 6)-beta -D-GalNAc, alpha -D-GalNAc, and beta -D-GalNAc, respectively, were synthesized as analogous polymers of polymer 1, Glycopolypeptides 8 and 9 as a mimic of sialo-type mucin were further prepared from polymers 1 and 2 as the acceptor of CMP-Neu5Ac by alpha2,3-(O)-sialyltransferase, respectively. Interactions of these glycopolypeptides with lectins were investigated with the double-diffusion test and the hemagglutination-inhibition assay and in terms of an optical biosensor based on surface plasmon resonance. Polymers 1 and 2 reacted strongly with peanut (Arachis hypogaea) agglutinin (PNA) and Agaricus bisporus agglutinin (ABA), On the other hand, polymers 8 and 9 through sialylation from polymers 1 and 2 reacted with ABA, but did not with PNA. Other polymers 3-7 did not show any reactivity for both the lectins, These results show that PNA acts precisely in an exo manner on the beta -D-Gal-(1 --> 3)-D-GaLNAc sequence, while ABA acts in an endo manner. Polymers 6 and 7 substituted with GalNAc reacted strongly with soybean (Glycine max) agglutinin and Vicia villosa agglutinin B-4, regardless of the configuration of the glycosidic linkage, The interaction of all polymers with Bauhinia purpurea agglutinin was much stronger than that of the corresponding sugars. Polymers 8 and 9 reacted with wheat germ (Triticum vulgaris) agglutinin (WGA), to which Neu5Ac residues are needed for binding, but polymers 1 and 2 did not, These sugar-substituted glycopolypeptides interacted specifically with the corresponding lectins, Furthermore, polymers 4-7 reacted with WGA, but the corresponding sugars did not. It suggests that the N-acetyl group along the PGA backbone has a cluster effect for WGA, The artificial glycopolypeptides were shown to be useful as tools and probes of carbohydrate recognition and modeling in the analysis of glycoprotein-lectin interactions. (C) 2000 Academic Press.