UDP-GlcNAc 2-epimerase/ManNAc kinase (GNE)

Abstract

N-Acetylneuraminic acid (Neu5Ac) or sialic acid is a pivotal structural and functional monosaccharide in any mammalian cell surface. In the membrane bound or soluble glycoconjugates, this electronegatively charged sugar is localized in the terminal position. Neu5Ac mediates recognition processes like cell-cell, cell-matrix, receptor-mediator, or cell-pathogen interactions (Varki 1997; Schauer 2004). Neu5Ac is formed in vivo by a multistep pathway (Roseman 1970) beginning with the conversion of fructose-6-P to glucosamine-6-P, which is further metabolized to UDP-N-acetylglucosamine (UDP-GlcNAc), the crucial precursor of Neu5Ac, by the hexosamine pathway. This substrate is converted to N-acetylmannosamine (ManNAc) by a specific UDP-GlcNAc 2-epimerase and then phosphorylated to ManNAc 6-P (Fig. 134.1). Both reactions are catalyzed by one enzyme, the bifunctional UDP-GlcNAc 2-epimerase/ManNAc kinase (GNE). ManNAc 6-P is then condensed with phosphoenolpyruvate forming sialic acid 9-P. This compound is further metabolized to CMP-sialic acid, which is transported to the Golgi apparatus, and they are used for the formation of sialylated glycoconjugates (Fig. 134.1).