Glycoside hydrolases require carboxyl groups as catalysts for their activity. A retaining xylanase from Streptomyces olivaceoviridis E-86 belonging to glycoside hydrolase family 10 possesses Glu128 and Glu236 that respectively function as acid/base and nucleophile. We previously developed a unique mutant of the retaining xylanase, N127S/E128H, whose deglycosylation is triggered by azide. A crystallographic study reported that the transient formation of a Ser–His catalytic dyad in the reaction cycle possibly reduced the azidolysis reaction. In the present study, we engineered a catalytic dyad with enhanced stability by site-directed mutagenesis and crystallographic study of N127S/E128H. Comparison of the Michaelis complexes of N127S/E128H with pNP-X2 and with xylopentaose showed that Ser127 could form an alternative hydrogen bond with Thr82, which disrupts the formation of the Ser–His catalytic dyad. The introduction of T82A mutation in N127S/E128H produces an enhanced first-order rate constant (6 times that of N127S/E128H). We confirmed the presence of a stable Ser–His hydrogen bond in the Michaelis complex of the triple mutant, which forms the productive tautomer of His128 that acts as an acid catalyst. Because the glycosyl azide is applicable in the bioconjugation of glycans by using click chemistry, the enzyme-assisted production of the glycosyl azide may contribute to the field of glycobiology.
In cereals, granule-bound starch synthase I (GBSSI)-deficient mutants accumulate glutinous (amylose-free) starch in their storage tissues. The amylose-free starch produced by waxy (wx) mutants of hexaploid bread wheat (Triticum aestivum L.) is used in cakes and breads. However, wx mutants of diploid wheat (T. monococcum L.) have so far no commercial applications. In this study, we identified a mutation in exon 6 of GBSSI in a diploid wheat wx mutant that resulted in the replacement of Trp355 with a stop codon. Molecular markers were developed for the rapid screening of the mutation, which should allow the selection of heterozygous and homozygous plants during backcrossing. This will facilitate the improvement of the agricultural traits of the wx mutant and the generation of new amylose-free wx lines.