Modulation of the transglycosylation activity of plant family GH18 chitinase by removing or introducing a tryptophan side chain

•Transglycosylation of family GH18 chitinase was eliminated by removing a Trp side chain from site +3.•Transglycosylation of family GH18 chitinase was enhanced by introducing a Trp side chain into site +1.•The introduced tryptophan side chain was found to be stacked with the sugar residue at subsite +1. Transglycosylation (TG) activity of a family GH18 chitinase from the cycad, Cycas revoluta, (CrChiA) was modulated by removing or introducing a tryptophan side chain. The removal from subsite +3 through mutation of Trp168 to alanine suppressed TG activity, while introduction into subsite +1 through mutation of Gly77 to tryptophan (CrChiA-G77W) enhanced TG activity. The crystal structures of an inactive double mutant of CrChiA (CrChiA-G77W/E119Q) with one or two N-acetylglucosamine residues occupying subsites +1 or +1/+2, respectively, revealed that the Trp77 side chain was oriented toward +1 GlcNAc to be stacked with it face-to-face, but rotated away from subsite +1 in the absence of GlcNAc at the subsite. Aromatic residues in the aglycon-binding site are key determinants of TG activity of GH18 chitinases.