Probing glycosyltransferase activities with the Staudinger ligation.

The development of rapid screening methods for probing glycosyltransferase activities is essential for advancing the field of glycobiology. While assays for specific glycosyltransferases exist, there is no generalizable method that can be applied across the enzyme superfamily. Herein we describe a novel glycosyltransferase assay that exploits their unnatural substrate tolerance and the unique chemical reactivity of the azide. We applied this "azido-ELISA" to the family of polypeptide alpha-N-acetylgalactosaminyltransferases (ppGalNAcTs), all of which were able to transfer N-azidoacetylgalactosamine (GalNAz) from the unnatural nucleotide sugar donor UDP-GalNAz. The azide was detected and quantified by Staudinger ligation with a phosphine probe in a microtiter plate format. This approach should be applicable to any glycosyltransferase or group-transfer enzyme that tolerates unnatural azido substrates.