Biochemical engineering of the N-acyl side chain of sialic acid: biological implications

N-Acetylneuraminic acid is the most prominent sialic acid in eukaryotes. Th e structural diversity of sialic acid is exploited by viruses, bacteria, an d toxins and by the sialoglycoproteins and sialoglycolipids involved in cel l-cell recognition in their highly specific recognition and binding to cell ular receptors. The physiological precursor of all sialic acids is N-acetyl D-mannosamine (ManNAc). By recent findings it could be shown that syntheti c N-acyl-modified D-mannosamines can be taken up by cells and efficiently m etabolized to the respective N-acyl-modified neuraminic acids in vitro and in vivo. Successfully employed D-mannosamines with modified N-acyl side cha ins include N-propanoyl- (ManNProp), N-butanoyl- (ManNBut)-, N-pentanoyl- ( ManNPent), N-hexanoyl- (ManNHex), N-crotonoyl- (ManNCrot), N-levulinoyl- (M anNLev), N-glycolyl-(ManNGc), and N-azidoacetyl D-mannosamine (ManNAc-azido ). All of these compounds are metabolized by the promiscuous sialic acid bi osynthetic pathway and are incorporated into cell surface sialoglycoconjuga tes replacing in a cell type-specific manner 10-85% of normal sialic acids. Application of these compounds to different biological systems has reveale d important and unexpected functions of the N-acyl side chain of sialic aci ds, including its crucial role for the interaction of different viruses wit h their sialylated host cell receptors. Also, treatment with ManNProp, whic h contains only one additional methylene group compared to the physiologica l precursor ManNAc, induced proliferation of astrocytes, microglia, and per ipheral T-lymphocytes. Unique, chemically reactive ketone and azido groups can be introduced biosynthetically into cell surface sialoglycans using N-a cyl-modified sialic acid precursors, a process offering a variety of applic ations viral gene delivery. This group of novel sialic acid precursors enab led studies on sialic acid modifications on the surface of living cells and has improved our understanding of carbohydrate receptors in their native e nvironment. The biochemical engineering of the side chain of sialic acid of fers new tools to study its biological relevance and to exploit it as a tag for therapeutic and diagnostic applications.