Abstract
Nonulosonic acids are found in the surface polysaccharides of many bacterial species and are often implicated in pathogenesis. Here, the structure of a novel 5,7-diacetamido-3,5,7,9-tetradeoxynon-2-ulosonic acid recovered from the capsular polysaccharide of a multiply antibiotic resistant Acinetobacter baumannii isolate was determined. The isolate carries a sugar synthesis module that differs by only a single gene from the module for the synthesis of 5,7-diacetamido-3,5,7,9-tetradeoxy-L-glycero-L-altro-non-2-ulosonic acid or 5,7-di-N-acetylacinetaminic acid, recently discovered in the capsule of another A. baumannii isolate. The new monosaccharide is the C8-epimer of acinetaminic acid (8eAci; 5,7-diacetamido-3,5,7,9-tetradeoxy-D-glycero-L-altro-non-2-ulosonic acid) and the C7-epimer of legionaminic acid. This monosaccharide had not previously been detected in a biological sample but had been synthesized chemically.
Highlights
The nonulosonate superfamily of monosaccharides includes various acetyl and acyl derivatives of nine-carbon acidic sugars such as neuraminic acid and sialic acid, which have important implications for human health[1]
The biosynthesis pathways for the 5,7-acetylated forms of Pseudaminic acid (Pse) and legionaminic acid (Leg) are inferred for A. baumannii based on experimental data from other species, and involve quite different steps[8]; Pse synthesis is mediated by Psa enzymes[9], whereas Leg synthesis involves different Lga enzymes[10]
This module was in a novel capsule biosynthesis gene cluster at the K locus, designated KL73 (GenBank accession number MF362178)
Summary
The nonulosonate superfamily of monosaccharides includes various acetyl and acyl derivatives of nine-carbon acidic sugars such as neuraminic acid and sialic acid, which have important implications for human health[1]. In 2015, the 7,8-epimer of Leg (L-glycero-L-altro isomer; Fig. 1B) was found in the capsular polysaccharide (CPS) of a multiply antibiotic resistant Acinetobacter baumannii isolate, and named acinetaminic acid (Aci)[7] Each of these five naturally occurring 5,7-diamino-3,5,7,9-tetradeoxynon-2-ulosonic acids may have various acetyl or other acyl decorations on nitrogens 5 and 7 that can give rise to further structural diversity[2], and the 5,7-di-N-acetylated derivative of acinetaminic acid, Aci5Ac7Ac, was produced by the A. baumannii isolate investigated[7]. The K12 capsule was found to include Aci and the additional genes required for its synthesis were named aciABCD7 Though these genes were found in more than one KL gene cluster, to date they have not been observed outside of the species, suggesting that the sugar is so far unique to A. baumannii. The sugar content of the CPS produced by this strain was examined to determine the structure of the 5,7-diamino-3,5,7,9-tetradeoxynon-2-ulosonic acid present
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