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UDP-N-acetylglucosamine 4,6-dehydratase/5-epimerase The FnlA enzyme is the first step in the biosynthesis of UDP-FucNAc from UDP-GlcNAc in E. coli (along with FnlB and FnlC). The proteins identified by this model include FnlA homologs in the O-antigen clusters of O4, O25, O26, O29 (Shigella D11), O118, O145 and O172 serotype strains, all of which produce O-antigens containing FucNAc (or the further modified FucNAm). A homolog from Pseudomonas aerugiosa serotype O11, WbjB, also involved in the biosynthesis of UDP-FucNAc has been characterized and is now believed to carry out both the initial 4,6-dehydratase reaction and the subsequent epimerization of the resulting methyl group at C-5. A phylogenetic tree of related sequences shows a distinct clade of enzymes involved in the biosynthesis of UDP-QuiNAc (Qui=qinovosamine). This clade appears to be descendant from the common ancestor of the Pseudomonas and E. coli fucose-biosynthesis enzymes. It has been hypothesized that the first step in the biosynthesis of these two compounds may be the same, and thus that these enzymes all have the same function. At present, lacking sufficient confirmation of this, the current model trusted cutoff only covers the tree segment surrounding the E. coli genes. The clades containing the Pseudomonas and QuiNAc biosynthesis enzymes score above the noise cutoff. Immediately below the noise cutoff are enzymes involved in the biosynthesis of UDP-RhaNAc (Rha=rhamnose), which again may or may not produce the same product. |
Jiyao W et al.(2023). "The conserved domain database in 2023.",