Structure of Bacterial Polysaccharide Synthesizing Enzyme May Lead to New Drugs

Both humans and a range of other animal species are susceptible to melioidosis, and the production of a group III polysaccharide capsule in B. pseudomallei is essential for virulence. B. pseudomallei capsular polysaccharide (CPS) I comprises unbranched manno-heptopyranose residues and is encoded by a 34.5 kilobase locus on chromosome 1. Despite the importance of this locus, the role of all of the genes within this region is unclear.

The principal B. pseudomallei CPS consists of a linear repeat of -3)-2-O-acetyl-6-deoxy-β-d-manno-heptopyranose-(1-. This CPS is critical to the virulence of this emerging pathogen and represents a key target for the development of novel therapeutics, Wcbl is one of several genes in the CPS biosynthetic cluster whose deletion leads to significant attenuation of the pathogen; unlike most others, it has no homologues of known function and no detectable sequence similarity to any protein with an extant structure.

Investigators at the University of Exeter (United Kingdom) recently presented the X-ray crystallographic structure of the WcbL protein from B. pseudomallei. They reported in the December 17, 2015, issue of the journal Chemistry and Biology that WcbL operated enzymatically through a sequential ordered Bi-Bi mechanism, loading the heptose first and then ATP. Dimeric WcbL bound ATP anti-cooperatively in the absence of heptose, and cooperatively in its presence. Modeling of WcbL suggested that heptose binding caused an elegant switch in the hydrogen-bonding network, facilitating the binding of a second ATP molecule.

In addition, the investigators screened a library of drug-like fragments, identifying hits that potently inhibited WcbL. These results provided a novel mechanism for control of substrate binding and emphasized WcbL as an attractive antimicrobial target for Gram-negative bacteria.

"We identified the most important parts of this protein that is involved in making up the outer structure of some pathogenic bacteria and found that the protein regulated itself in quite an unusual way," said first author Dr. Mirella Vivoli, associate research fellow at the University of Exeter. "We were then able to test a number of compounds and find one that blocked this action and its ability to make the sugar."

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