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Biochemistry. 2018 Jul 24;57(29):4325-4337. doi: 10.1021/acs.biochem.8b00381. Epub 2018 Jul 11.

Key Residues Affecting Transglycosylation Activity in Family 18 Chitinases: Insights into Donor and Acceptor Subsites.

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Department of Chemistry, Biotechnology and Food Science , Norwegian University of Life Sciences (NMBU) , P.O. Box 5003, 1432 Ås , Norway.
Department of Plant Sciences , School of Life Sciences, University of Hyderabad , Gachibowli, Hyderabad , India.
Department of Medical Biochemistry, Institute for Clinical Medicine , University of Oslo , P.O. Box 4950, Nydalen, N-0424 Oslo , Norway.
Department of Microbiology, Clinic for Laboratory Medicine , Oslo University Hospital, Rikshospitalet , P.O. Box 4950, Nydalen, N-0424 Oslo , Norway.


Understanding features that determine transglycosylation (TG) activity in glycoside hydrolases is important because it would allow the construction of enzymes that can catalyze controlled synthesis of oligosaccharides. To increase TG activity in two family 18 chitinases, chitinase D from Serratia proteamaculans ( SpChiD) and chitinase A from Serratia marcescens ( SmChiA), we have mutated residues important for stabilizing the reaction intermediate and substrate binding in both donor and acceptor sites. To help mutant design, the crystal structure of the inactive SpChiD-E153Q mutant in complex with chitobiose was determined. We identified three mutations with a beneficial effect on TG activity: Y28A (affecting the -1 subsite and the intermediate), Y222A (affecting the intermediate), and Y226W (affecting the +2 subsite). Furthermore, exchange of D151, the middle residue in the catalytically important DXDXE motif, to asparagine reduced hydrolytic activity ≤99% with a concomitant increase in apparent TG activity. The combination of mutations yielded even higher degrees of TG activity. Reactions with the best mutant, SpChiD-D151N/Y226W/Y222A, led to rapid accumulation of high levels of TG products that remained stable over time. Importantly, the introduction of analogous mutations at the same positions in SmChiA (Y163A equal to Y28A and Y390F similar to Y222A) had similar effects on TG efficiency. Thus, the combination of the decreasing hydrolytic power, subsite affinity, and stability of intermediate states provides a powerful, general strategy for creating hypertransglycosylating mutants of retaining glycoside hydrolases.

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