Abstract
With new strains of Acinetobacter baumannii undergoing genomic analysis, it has been possible to define regions of genomic plasticity (RGPs), encoding specific adaptive elements. For a selected RGP from a community-derived isolate of A. baumannii, we outline sequences compatible with biosynthetic machinery of surface polysaccharides, specifically enzymes utilized in the dehydration and conversion of UDP-N-acetyl-D-glucosamine (UDP-D-GlcNAc). We have determined the crystal structure of one of these, the epimerase Ab-WbjB. This dehydratase belongs to the ‘extended’ short-chain dehydrogenase/reductase (SDR) family, related in fold to previously characterised enzymes CapE and FlaA1. Our 2.65Å resolution structure of Ab-WbjB shows a hexamer, organised into a trimer of chain pairs, with coenzyme NADP+ occupying each chain. Specific active-site interactions between each coenzyme and a lysine quaternary group of a neighbouring chain interconnect adjacent dimers, so stabilising the hexameric form. We show UDP-GlcNAc to be a specific substrate for Ab-WbjB, with binding evident by ITC (Ka = 0.23 μmol-1). The sequence of Ab-WbjB shows variation from the consensus active-site motifs of many SDR enzymes, demonstrating a likely catalytic role for a specific threonine sidechain (as an alternative to tyrosine) in the canonical active site chemistry of these epimerases.
Highlights
Acinetobacter baumannii is a Gram-negative opportunistic pathogen responsible for a range of infections [1, 2]
Whilst the same island recurs in its entirety in the clinicallysourced A. baumannii strain MDR-TJ [42], it is absent from other A. baumannii genomes analysed to date
Based on the high sequence identity to their P. aeruginosa and S. aureus homologs, we propose these enzymes to be responsible for the conversion of UDP-D-GlcNAc to UDP-L-FucNAc [43] via the five-step reaction cascade characterized for these relatives (Fig 1B)
Summary
Acinetobacter baumannii is a Gram-negative opportunistic pathogen responsible for a range of infections [1, 2]. It poses a serious clinical challenge due to its resistance to several classes of antimicrobial drugs [3, 4] and is today grouped as one of the highly multidrug resistant “ESKAPE” pathogens (Enterococcus faecium, Staphylococcus aureus, Klebsiella pneumoniae, A. baumannii, Pseudomonas aeruginosa and Enterobacter species) [5]. A. baumannii is responsible for both community and hospital-acquired (nosocomial) infections that are difficult to control and treat [7]. Reports of community-acquired A. baumannii infections have intensified over.
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