Chemoenzymatic Synthesis of Chito-oligosaccharides with Alternating N-d-Acetylglucosamine and d-Glucosamine.

Chito-oligosaccharides (CHOS) are homo- or hetero-oligomers of N-acetylglucosamine (GlcNAc, A) and d-glucosamine (GlcN, D). Production of well-defined CHOS-mixtures, or even pure CHOS, with specific lengths and sugar compositions, is of great interest since these oligosaccharides have interesting bioactivities. While direct chemical synthesis of CHOS is not straightforward, chemo-enzymatic approaches have shown some promise. We have used engineered glycoside hydrolases to catalyze oligomerization of activated DA building blocks through transglycosylation reactions. The building blocks were generated from readily available (GlcNAc)2-para-nitrophenol through deacetylation of the nonreducing end sugar with a recombinantly expressed deacetylase from Aspergillus niger (AnCDA9). This approach, using a previously described hyper-transglycosylating variant of ChiA from Serratia marcescens (SmChiA) and a newly generated transglycosylating variant of Chitinase D from Serratia proteamaculans (SpChiD), led to production of CHOS containing up to ten alternating D and A units [(DA)2, (DA)3, (DA)4, and (DA)5]. The most abundant compounds were purified and characterized. Finally, we demonstrate that (DA)3 generated in this study may serve as a specific inhibitor of the human chitotriosidase. Inhibition of this enzyme has been suggested as a therapeutic strategy against systemic sclerosis.