Biosynthesis of UDP-GlcNAc(3NAc)A by WbpB, WbpE, and WbpD: Enzymes in the Wbp Pathway Responsible for O-Antigen Assembly in Pseudomonas aeruginosa PAO1

Abstract

The B-band O-antigen of the lipopolysaccharide found in the opportunistic pathogen Pseudomonas aeruginosa PAO1 (serotype O5) comprises a repeating trisaccharide unit that is critical for virulence and protection from host defense systems. One of the carbohydrates in this repeating unit, the rare diacetylated aminuronic acid derivative 2,3-diacetamido-2,3-dideoxy-β-d-mannuronic acid (ManNAc(3NAc)A), is thought to be produced by five enzymes (WbpA, WbpB, WbpE, WbpD, and WbpI) in a stepwise manner starting from UDP-GlcNAc. Although the genes responsible for the biosynthesis of this sugar are known, only two of the five encoded proteins (WbpA and WbpI) have been thoroughly investigated. In this report, we describe the cloning, overexpression, purification, and biochemical characterization of the three central enzymes in this pathway, WbpB, WbpE, and WbpD. Using a combination of capillary electrophoresis, RP-HPLC, and NMR spectroscopy, we show that WbpB and WbpE are a dehydrogenase/aminotransferase pair that converts UDP-GlcNAcA to UDP-GlcNAc(3NH₂)A in a coupled reaction via a unique NAD⁺ recycling pathway. In addition, we confirm that WbpD catalyzes the acetylation of UDP-GlcNAc(3NH₂)A to give UDP-GlcNAc(3NAc)A. Notably, WbpA, WbpB, WbpE, WbpD, and WbpI can be combined in vitro to generate UDP-ManNAc(3NAc)A in a single reaction vessel, thereby providing supplies of this complex glycosyl donor for future studies of lipopolysaccharide assembly. This work completes the biochemical characterization of the enzymes in this pathway and provides novel targets for potential therapeutics to combat infections with drug resistant P. aeruginosa strains.