Abstract:
Menaquinone (MQ) is a lipid-soluble electron carrier central to mitochondrial energy metabolism in Mycobacterium tuberculosis (Mtb) and other Gram-positive pathogenic bacteria. Menaquinone biosynthesis presents a potential target for therapeutic intervention, as it is not produced in humans. In this work, libraries of novel inhibitor candidates targeted at the essential enzyme MenD were designed, synthesized, and evaluated to explore the potential utility of this enzyme as a therapeutic target. A multidisciplinary approach was taken, as part of a wider collaborative effort, that involved synthesis of novel small molecule classes, in silico inhibitor docking and molecular design, protein crystallography, enzyme kinetics and microbiology.
Initially, a series of inhibitor candidates derived from the natural substrate isochorismate were synthesized. Unexpectedly, these were shown by X-ray crystallography to bind to a previously unreported allosteric regulation site on MenD, which was identified by our collaborators during the period of this work. Based on these findings, subsequent inhibitor
series were designed with the aid of in silico docking and enumeration tools, with the aim of targeting the allosteric site, that were then shown to weakly inhibit Mtb. Iterative cycles of enzyme co-crystallisation, in silico docking and inspiration from natural products ultimately led to the design and synthesis of a new class of heterocyclic inhibitor candidates, based on the 3-carboxy-4-quinolone N-oxide framework.
Process optimisation of the synthetic route for 4-quinolone N-oxide preparation enabled the assembly of a compound library, to address potential issues of solubility, hydrogen bonding interactions in the allosteric site, and cross-membrane transport. Collaborative work across the multidisciplinary team is on-going, in order to establish a complete picture of how the compounds prepared in this work interact with the target micro-organism, from detailed molecular interactions at the allosteric site, to in vitro enzyme inhibition, and growth inhibition in whole-cell culture against Mtb and other clinically relevant MQ-dependent pathogenic bacteria.
