Metabolic labeling of glycoproteins with chemical tags through unnatural sialic acid biosynthesis

The importance of protein glycosylation is becoming apparent, and much effort has been directed to the investigation of glycoproteins with the aim of identifying the biological functions of the pendant glycans. Chemical synthesis provides access to oligosaccharides and their analogs for structure-function analyses in vitro. The function of cell surface glycoproteins can depend on the spatial context of their presentation. Thus, biological investigations of isolated glycoprotein molecules and those displayed on a cell surface can show different results. So, the extension of a chemical modification route to cell surface oligosaccharides is a valuable addition to the tools of cell surface biochemistry. The chapter describes the strategy for the metabolic introduction of ketone groups into cell surface sialoglycoconjugates, using the unnatural sialic acid precursor N-levulinoylmannosamine (ManLev). ManLev is converted to N-levulinoyl sialic acid (SiaLev) in cultured cells, permitting the chemical attachment of hydrazide- or aminooxy-derivatized probes to cell surface glycoproteins.