Rational design of a highly active N-glycosyltransferase mutant using fragment replacement approach

The modularity of carbohydrate-active enzymes facilitates that enzymes with different functions have similar fragments. However, because of the complex structure of the enzyme active sites and the epistatic effects of various mutations on enzyme activity, it is difficult to design enzymes with multiple mutation sites using conventional methods. In this study, we designed multi-point mutants by fragment replacement in the donor-acceptor binding pocket of Actinobacillus pleuropneumoniae N-glycosyltransferase (ApNGT) to obtain novel properties. Candidate fragments were selected from a customized glycosyltransferase database. The stability and substrate-binding energy of the three fragment replacement mutants were calculated in comparison with wild-type ApNGT, and mutants with top-ranking stability and middle-ranking substrate-binding energy were chosen for priority experimental verification. We found that a mutant called F13, which increased the glycosylation efficiency of the natural substrate by 1.44 times, the relative conversion of UDP-galactose by 14.2 times, and the relative conversion of UDP-xylose from almost 0 to 78.6%. Most importantly, F13 mutant acquired an entirely new property, the ability to utilize UDP-glucuronic acid. On one hand, this work shows that replacing similar fragments in the donor-acceptor binding pocket of the enzyme might provide new ideas for designing mutants with new properties; on the other hand, F13 mutant is expected to play an important role in targeted drug delivery.