Novel Allosteric Inhibitor of Tryptophan Synthase – a New Target for Tuberculosis Therapeutics

The alarming rate of antibiotic resistance progression requires identification of new cellular targets along with novel chemistries to expand treatment options. Central metabolism remains one of the unexplored avenues with an apparently large collection of potential drug targets. Yet, conditional essentiality of many enzymes from this pool necessitates thorough understanding of metabolic fluxes and pathways regulations under certain environmental conditions, hampering a successful drug discovery campaign.

An interdisciplinary team of scientists from the Broad Institute and Harvard Medical School, led by Dr. Deborah Hung, in collaboration with researchers from the Center for Structural Genomics of Infectious Diseases (CSGID)​ at the University of Chicago and Argonne National Laboratory, led by Drs. A. Joachimiak and K. Michalska, has investigated synthesis of tryptophan in Mycobacterium tuberculosis (MTB) as a pathway with potential therapeutic importance. The study has shown that functional tryptophan synthase (TrpAB), one of the several enzymes involved in the multistep L-Trp biosynthesis, is necessary for MTB growth in vitro and in zebrafish in vivo model.

TrpAB is a heterotetrameric, PLP-dependent complex catalyzing two last reactions in L-Trp pathway: the conversion of indole-3-glycerol phosphate to indole and glyceraldehyde 3-phosphate (TrpA) followed by indole and L-Ser condensation to yield L-Trp (TrpB). During the catalytic cycle, indole is transferred from the TrpA to TrpB active site through 25 Å hydrophobic tunnel to react with the L-Ser-PLP adduct. The enzymatic activities and substrate channeling are coordinated by complex allosteric communication between the subunits.

When TrpAB is mutated or inhibited by a novel azetidine derivative, BRD4592, M. tuberculosis cannot survive. BRD4592 is a highly potent bactericidal compound specific only for certain Mycobacteria species. Detailed kinetic, biophysical and structural evidence reveals that BRD4592 is an allosteric, mixed-type inhibitor, which binds specifically to a novel site within the intersubunit channel, at the TrpA-TrpB interface. These results indicate that allosteric regulators may represent an effective tool to inactivate central metabolism enzymes, which often are allosterically regulated and might be predisposed for this type of inhibition.





Tryptophan Synthase

Investigators showed that functional tryptophan synthase (TrpAB), one of the several enzymes involved in the multistep L-Trp biosynthesis, is necessary for MTB growth in vitro.

Credit
CSGID

Investigators showed that functional tryptophan synthase (TrpAB), one of the several enzymes involved in the multistep L-Trp biosynthesis, is necessary for MTB growth in vitro.

Credit:
CSGID

Reference

Wellington S, Nag PP, Michalska K, Johnston SE, Jedrzejczak RP, Kaushik VK, Clatworthy AE, Siddiqi N, McCarren P, Bajrami B, Maltseva NI, Combs S, Fisher SL, Joachimiak A, Schreiber SL, Hung DT. A small-molecule allosteric inhibitor of Mycobacterium tuberculosis tryptophan synthase. Nat Chem Biol. 2017 Sep;13(9):943-950. doi: 10.1038/nchembio.2420. Epub 2017 Jul 3. PubMed PMID: 28671682.

Coordinates are available in the Protein Data Bank (www.rcsb.org) PDB IDs 5TCF5TCG5TCI5TCJ5TCH.

 

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