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 β-d-Gal-(1 → 3)-α-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 β-d-Gal-(1 → 3)-β-d-GalNAc, β-d-Gal-(1 → 3)-β-d-GlcNAc, β-d-Gal-(1 → 6)-α-d-GalNAc, β-d-Gal-(1 → 6)-β-d-GalNAc, α-d-GalNAc, and β-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 α2,3-(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 β-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 B4, 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.