Structure-guided design of novel thiazolidine inhibitors of O-acetyl serine sulfhydrylase from Mycobacterium tuberculosis.

The cysteine biosynthetic pathway is absent in humans but essential in microbial pathogens, suggesting that it provides potential targets for the development of novel antibacterial compounds. CysK1 is a pyridoxalphosphate-dependent O-acetyl sulfhydrylase, which catalyzes the formation of l-cysteine from O-acetyl serine and hydrogen sulfide. Here we report nanomolar thiazolidine inhibitors of Mycobacterium tuberculosis CysK1 developed by rational inhibitor design. The thiazolidine compounds were discovered using the crystal structure of a CysK1-peptide inhibitor complex as template. Pharmacophore modeling and subsequent in vitro screening resulted in an initial hit compound 2 (IC50 of 103.8 nM), which was subsequently optimized by a combination of protein crystallography, modeling, and synthetic chemistry. Hit expansion of 2 by chemical synthesis led to improved thiazolidine inhibitors with an IC50 value of 19 nM for the best compound, a 150-fold higher potency than the natural peptide inhibitor (IC50 2.9 μM).