Molecular basis for bacterial N-glycosylation by a soluble HMW1C-like N-glycosyltransferase

Rational design of N-glycosyltransferases from Aggregatibacter aphrophilus to synthesize Gal-modified glycoconjugates targeting hepatocellular carcinoma cells

Hepatocellular carcinoma (HCC) significantly impacts human health due to its tendency to develop multi-drug resistance. Regarding the treatment of HCC, galactosylated conjugates can target drug delivery. Bacterial N-glycosyltransferases (NGTs) catalyze glucosylation or galactosylation of the Asn residue motif Asn0-X+1-Ser/Thr+2 (X ≠ P). However, the limited utilization of the sugar donor UDP-Gal by NGTs severely restricts the potential applications in the development of glycol-drugs and vaccines. In this study, we rationally designed an NGT derived from Aggregatibacter aphrophilus and generated a mutant called AaFQ, which has a catalytic activity 18.8 times greater than that of wild-type towards UDP-Gal. AaFQ could rapidly synthesize Gal-modified peptides, which have been demonstrated to possess high targeting activity for HCC cells HepG2. Moreover, AaFQ exhibited a marked affinity for sugar donors with an equatorial bond-linked hydroxyl group at the C-2 position in monosaccharides. The mutant led to a significantly enhanced binding of K440 to the hydroxyl group at the C-2 position, which might be a major factor contributing to the significantly increased ability to utilize UDP-Gal. This work provides a crucial enzymatic synthetic tool for the targeted therapy of HCC and also offers a new perspective for the engineering modification and application of NGTs.

A kinetic model reveals the critical gating motifs for donor-substrate loading into Actinobacillus pleuropneumoniae N-glycosyltransferase

Soluble N-glycosyltransferase from Actinobacillus pleuropneumoniae (ApNGT) catalyzes the glycosylation of asparagine residues, and represents one of the most encouraging biocatalysts for N-glycoprotein production. Since the sugar tolerance of ApNGT is restricted to limited monosaccharides (e.g., Glc, GlcN, Gal, Xyl, and Man), tremendous efforts are devoted to expanding the substrate scope of ApNGT via enzyme engineering. However, rational design of novel NGT variants suffers from an elusive understanding of the substrate-binding process from a dynamic point of view. Here, by employing extensive all-atom molecular dynamics (MD) simulations integrated with a kinetic model, we reveal, at the atomic level, the complete donor-substrate binding process from the bulk solvent to the ApNGT active-site, and the key intermediate states of UDP-Glc during its loading dynamics. We are able to determine the critical transition event that limits the overall binding rate, which guides us to pinpoint the key ApNGT residues dictating the donor-substrate entry. The functional roles of several identified gating residues were evaluated through site-directed mutagenesis and enzymatic assays. Two single-point mutations, N471A and S496A, could profoundly enhance the catalytic activity of ApNGT. Our work provides deep mechanistic insights into the structural dynamics of the donor-substrate loading process for ApNGT, which sets a rational basis for design of novel NGT variants with desired substrate specificity.