Enzymatic synthesis of beta-D-Gal-(1 --> 3)-[beta-D-GlcNAc-1 --> 6)]-alpha-D-GalNAc-OC6H4NO2-p as a carbohydrate unit of mucin-type 2 core.

We have established a synthetic method for obtaining beta-D-Gal-(1 --> 3)-beta-D-GlcNAc-(1 --> 6)]-alpha-D-GalNAc-OC6H4NO2-p (1), which is a carbohydrate unit of mucin-type 2 core. A beta-N-acetyl-D-hexosaminidase from Nocardia orientalis catalyzed the synthesis of the desired compound 1 with its isomers beta-D-GalNAc-(1 --> 6)-beta-D-Gal-(1 --> 3)-alpha-D-GalNAc-OC6H4NO2-p (2) beta-D-GlcNAc-(1 --> 3)-beta-D-Glc-(1 --> 3)-alpha-D-GalNAc-OC6H4NO2-p (3) through N-acetylglucosaminyl transfer from N,N'-diacetylchitobiose and beta-D-Gal-(1 --> 3)-alpha-D-GalNAc-OC6H4NO2-p. The enzyme formed the trisaccharides 1, 2, and 3 in 14% overall yield based on beta-D-Gal-(1 --> 3)-alpha-D-GalNAc-OC6H4NO2-p as an acceptor substrate, and in the ratio of 44:32:24. In this way, N-acetylglucosaminyl transfer favored O-6 of the acceptor rather than O-6', and occurred to a lesser extent at O-3'. This reaction was efficient enough to allow a one-pot preparation of the desired carbohydrate unit of mucin-type 2 core. When beta-D-Gal-(1 --> 3)-beta-D-GalNAc-OC6H4NO2-p was used as an acceptor, the enzyme also synthesized three kinds of trisaccharides in the same regioselectivity with respect to O-6 and O-6' versus O-3' of the acceptor.