Pseudaminic acid as a target for antimicrobial therapy

Antimicrobial resistance represents an ever-increasing threat to world health. A drive for novel, narrower-range antimicrobials is necessary and new bacterial structural targets are being considered. Bacterial flagella are extracellular protein structures commonly found in liquid-borne bacteria. They are used for motility, aid colonisation, and play an essential role in bacterial pathogenesis. It has been found that some bacteria glycosylate their flagella as an important mechanism in flagella assembly. Pseudaminic acid (Pse) is a sialic-acid family nonulosonic sugar, unique to microbes, which decorates the flagella of pathogenic bacteria such as Campylobacter jejuni, Helicobacter pylori and Aeromonas caviae (the focus of this work). The 6-step bacterial biosynthetic pathway for Pse has previously been elucidated with the pseudaminic acid synthase enzyme (PseI) playing a key role. It is hypothesised that inhibiting PseI will disrupt bacterial flagella glycosylation and affect bacterial motility and colonisation without affecting the host and therefore provide a potential new method of antimicrobial therapy.
In this project, two potential PseI inhibitors have been designed based on the literature precedent for the inhibition of analogous ulosonic acid synthase enzymes. After the assessment of parallel chemical models, an efficient method of accessing one of the proposed inhibitors was determined and demonstrated on a model phenylpropanal-derived compound. Furthermore, the synthetic route uses a key step aldol reaction, the product of which is a common intermediate that can be used to furnish both inhibitors via a proposed divergent synthesis. Expression and purification of the enzymes used in the A. caviae pseudaminic acid biosynthesis was carried out in order to test inhibition. LCMS reactions confirmed the activity of the expressed enzymes and provided a simple inhibition assay. This was used to test a set of compounds derived from a different potential PseI inhibitor from the literature. Work is ongoing towards the first acquisition of a crystal structure of the PseI enzyme to better characterise its active site.