In order to improve the catalytic efficiency of recombinant Photobacterium sp. JT-ISH-224 α2-6-sialyltransferase Psp2,6ST(15-501)-His6 in sialylating α-GalNAc-containing acceptors for the synthesis of tumor-associated carbohydrate antigens sialyl Tn (STn), protein crystal structure-based mutagenesis studies were carried out. Among several mutants obtained by altering the residues close to the acceptor substrate binding pocket, mutant A366G was shown to improve the sialyltransferase activity of Psp2,6ST(15-501)-His6 toward α-GalNAc-containing acceptors by 21-115% without significantly affecting its sialylation activity to β-galactosides. Furthermore, the expression level was improved from 18-40 mg L(-1) for the wild-type enzyme to 72-110 mg L(-1) for the A366G mutant. In situ generation of CMP-sialic acid in a one-pot two-enzyme system was shown effective in overcoming the high donor hydrolysis of the enzyme. Mutant A366G performed better than the wild-type Psp2,6ST(15-501)-His6 for synthesizing Neu5Acα2-6GalNAcαOSer/Thr STn antigens.

Two novel synthetic α2-6-linked disialyl hexasaccharides, disialyllacto-N-neotetraose (DSLNnT) and α2-6-linked disialyllacto-N-tetraose (DS'LNT), were readily obtained by highly efficient one-pot multienzyme (OPME) reactions. The sequential OPME systems described herein allowed the use of an inexpensive disaccharide and simple monosaccharides to synthesize the desired complex oligosaccharides with high efficiency and selectivity. DSLNnT and DS'LNT were shown to protect neonatal rats from necrotizing enterocolitis (NEC) and are good therapeutic candidates for preclinical experiments and clinical application in treating NEC in preterm infants.

Fucosylated human milk oligosaccharides (HMOs) have important biological functions. Enzymatic synthesis of such compounds requires robust fucosyltransferases. A C-terminal 66-amino acid truncated version of Helicobacter pylori α1-3-fucosyltransferase (Hp3FT) is a good candidate. Hp3FT was biochemically characterized to identify optimal conditions for enzymatic synthesis of fucosides. While N-acetyllactosamine (LacNAc) and lactose were both suitable acceptors, the former is preferred. At a low guanosine 5'-diphospho-β-L-fucose (GDP-Fuc) to acceptor ratio, Hp3FT selectively fucosylated LacNAc. Based on these enzymatic characteristics, diverse fucosylated HMOs, including 3-fucosyllactose (3-FL), lacto-N-fucopentaose (LNFP) III, lacto-N-neofucopentaose (LNnFP) V, lacto-N-neodifucohexaose (LNnDFH) II, difuco- and trifuco-para-lacto-N-neohexaose (DF-paraLNnH and TF-para-LNnH), were synthesized enzymatically by varying the ratio of the donor and acceptor as well as controlling the order of multiple glycosyltransferase-catalyzed reactions.

Quantification, characterization and biofunctional studies of N-glycans on proteins remain challenging tasks due to complexity, diversity and low abundance of these glycans. The availability of structurally defined N-glycans (especially isomers) libraries is essential to help on solving these tasks. We reported herein an efficient chemoenzymatic strategy, namely Core Synthesis/Enzymatic Extension (CSEE), for rapid production of diverse N-glycans. Starting with 5 chemically prepared building blocks, 8 N-glycan core structures containing one or two terminal N-acetyl-D-glucosamine (GlcNAc) residue(s) were chemically synthesized via consistent use of oligosaccharyl thioethers as glycosylation donors in the convergent fragment coupling strategy. Each of these core structures was then extended to 5 to 15 N-glycan sequences by enzymatic reactions catalyzed by 4 robust glycosyltransferases. Success in synthesizing N-glycans with Neu5Gc and core-fucosylation further expanded the ability of enzymatic extension. High performance liquid chromatography with an amide column enabled rapid and efficient purification (>98% purity) of N-glycans in milligram scales. A total of 73 N-glycans (63 isomers) were successfully prepared and characterized by MS² and NMR. The CSEE strategy provides a practical approach for "mass production" of structurally defined N-glycans, which are important standards and probes for Glycoscience.