A two-step enzymatic glycosylation of polypeptides with complex N-glycans

A chemoenyzmatic method for direct glycosylation of polypeptides is described. The method consists of two site-specific enzymatic glycosylation steps: introduction of a glucose moiety at the consensus N-glycosylation sequence (NXS/T) in a polypeptide by an N-glycosyltransferase (NGT) and attachment of a complex N-glycan to the glucose primer by an endoglycosidase (ENGase)-catalyzed transglycosylation. Our experiments demonstrated that a relatively small excess of the UDP-Glc (the donor substrate) was sufficient for an effective glucosylation of polypeptides by the NGT, and different high-mannose and complex type N-glycans could be readily transferred to the glucose moiety by ENGases to provide full-size glycopeptides. The usefulness of the chemoenzymatic method was exemplified by an efficient synthesis of a complex glycoform of polypeptide C34, a potent HIV inhibitor derived from HIV-1 gp41. A comparative study indicated that the Glc-peptide was equally efficient as the natural GlcNAc-peptide to serve as an acceptor in the transglycosylation with sugar oxazoline as the donor substrate. Interestingly, the Glc-Asn linked glycopeptide was completely resistant to PNGase F digestion, in contrast to the GlcNAc-Asn linked natural glycopeptide that is an excellent substrate for hydrolysis. In addition, the Glc-Asn linked glycopeptide showed at least 10-fold lower hydrolytic activity toward Endo-M than the natural GlcNAc-Asn linked glycopeptide. The chemoenzymatic glycosylation method described here provides an efficient way to introducing complex N-glycans into polypeptides, for gain of novel properties that could be valuable for drug discovery.