Integrated target-based and phenotypic screening approaches for the identification of anti-tubercular agents that bind to the mycobacterial adenylating enzyme MbtA
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Abstract
Iron is essential for the pathogenicity and virulence of Mycobacterium tuberculosis, which synthesises salicyl-capped siderophores (mycobactins) to acquire this element from the host. MbtA is the adenylating enzyme that catalyses the initial reaction of mycobactins’ biosynthesis and is solely expressed by mycobacteria. A 3,200-member library comprised of lead-like, structurally-diverse compounds was screened against M. tuberculosis for whole-cell inhibitory activity. A set of 846 compounds that inhibited the tubercle bacilli growth were then tested for their ability to bind to MbtA using a fluorescence-based thermal shift assay and NMR-based Water-LOGSY and Saturation Transfer Difference (STD) experiments. We identified an attractive hit-molecule, 5-hydroxy-indol-3-ethylamino-(2-nitro-4-trifluoromethyl)benzene (5), that bound with high affinity to MbtA and produced a MIC90 of 13 µM. The ligand was docked into the MbtA crystal structure and displayed an excellent fit within the MbtA active pocket, adopting a different binding mode to the established MbtA inhibitor Sal-AMS.