The Structure of the UDP-Glc/GlcNAc 4-Epimerase from the Human Pathogen Campylobacter jejuni

Abstract

Worldwide, the food-born pathogen Campylobacter jejuni is the leading bacterial source of human gastroenteritis. C. jejuni produces a large number of atypical cell-surface carbohydrates that facilitate infections. A major component of these oligo- and polysaccharides is the sugar N-acetylgalactosamine (GalNAc). The sole source of this critical sugar is derived by the epimerization of UDP-N-acetylglucosamine (GlcNAc) catalyzed by the enzyme UDP-GlcNAc 4-epimerase (GNE). This enzyme is unique among known bacterial epimerases in that it also catalyzes the equivalent reaction with the non-N-acetylated sugars. Understanding how CjGNE catalyzes these various interconversions is critical to designing novel inhibitors of this enzyme. Here, to further the mechanistic understanding we present a 2.0Å crystal structure of CjGNE with its NAD+ co-factor bound. Based on novel features found in the structure we perform a variety of biochemical studies to probe the mechanism and compare these results to the only other structurally characterized bi-functional epimerase, the human homolog GalE. We further show that ebselen, previously identified for inhibition of HsGalE, is active against CjGNE, suggesting a route for antibiotic development.