The present invention is broadly concerned with new in vitro glycosylation methods that provide rational approaches for producing glycosylated proteins, and the use of glycosylated proteins. In more detail, the present invention comprises methods of glycosylating a starting protein having an amino sidechain with a nucleophilic moiety, comprising the step of reacting the protein with a carbohydrate having an oxazoline moiety on the reducing end thereof, to covalently bond the amino sidechain of the starting protein with the oxazoline moiety, wherein the glycosylated protein substantially retains the structure and function of the starting protein. Target proteins include oxidase, oxidoreductase and dehydrogenase enzymes. The glycosylated proteins advantageously have molecular weights of at least about 7500 Daltons. In a further embodiment, the present invention concerns the use of glycosylated proteins, fabricated by the methods disclosed herein, in the assembly of amperometric biosensors.