Biosynthesis of UDP-N-acetyl-L-fucosamine, a precursor to the biosynthesis of lipopolysaccharide in Pseudomonas aeruginosa serotype O11.

UDP-N-acetyl-L-fucosamine is a precursor to l-fucosamine in the lipopolysaccharide of Pseudomonas aeruginosa serotype O11 and the capsule of Staphylococcus aureus type 5. We have demonstrated previously the involvement of three enzymes, WbjB, WbjC, and WbjD, in the biosynthesis of UDP-2-acetamido-2,6-dideoxy-L-galactose or UDP-N-acetyl-L-fucosamine (UDP-l-FucNAc). An intermediate compound from the coupled-reaction of WbjB-WbjC with the initial substrate UDP-2-acetamido-2-deoxy-alpha-D-glucose or UDP-N-acetyl-D-glucosamine (UDP-GlcNAc) was purified, and the structure was determined by NMR spectroscopy to be UDP-2-acetamido-2,6-dideoxy-L-talose (UDP-L-PneNAc). WbjD could then convert this intermediate into a new product with the same mass, consistent with a C-2 epimerization reaction. Those results led us to propose a pathway for the biosynthesis of UDP-L-FucNAc; however, the exact enzymatic activity of each of these proteins has not been defined. Here, we describe a fast protein liquid chromatography (FPLC)-based anion-exchange procedure, which allowed the separation and purification of the products of C-2 epimerization due to WbjD. Also, the application of a cryogenically cooled probe in NMR spectrometry offers the greatest sensitivity for determining the structures of minute quantities of materials, allowing the identification of the final product of the pathway. Our results showed that WbjB is bifunctional, catalyzing firstly C-4, C-6 dehydration and secondly C-5 epimerization in the reaction with the substrate UDP-D-GlcNAc, producing two intermediates. WbjC is also bifunctional, catalyzing C-3 epimerization of the second intermediate followed by reduction at C-4. The FPLC-based procedure provided good resolution of the final product of WbjD reaction from its epimer/substrate UDP-l-PneNAc, and the use of the cryogenically cooled probe in NMR revealed unequivocally that the final product is UDP-L-FucNAc.