of Mtb was solved with several inhibitors by fragment-based screening and HTS
methods. These structures revealed conformational flexibility at the active sites
during catalysis in order to screen preferred chemotypes and its molecular mode of
binding. Further, in vivo mouse model of acute infection showed reduced bacterial
load using GlcB inhibitors [75, 76]. ATP synthase is another key target in
anti-mycobacterial drug discovery. This is primarily involved in cellular energy
production in all microorganisms, plants, and animals. The structural and functional
activity of the ATP synthase enzyme is same in all organisms. Bedaquiline, a
second-line TB drug, targets ATP synthase subunit e and inhibits Mtb ATP production (Table 1; Fig. 2). It was approved by FDA as MDR-TB drug [77, 78]. An
imidazopyridine amide compound (Q203) was discovered by the whole cell screen
in infected macrophages. Q203 is a potent inhibitor by disrupting the electron
transport chain of ATP synthesis and successfully entered in Phase I clinical trials
[79]. Lipoamide dehydrogenase (Lpd), another key target member of three multienzyme, complexes the energy metabolism pathway for Mtb virulence and can be
potential for anti-TB drug discovery program. Mtb of Lpd is crucial for the
metabolism of branched chain amino acids and thus essential for its pathogenicity.
The small-molecule inhibitor triazospirodimethoxybenzoyl which inhibits the Mtb
Lpd enzyme selectively without affecting the human Lpd enzyme can be successfully used for TB therapy [80, 81].
Another important druggable target in Mtb is glutamine synthetase required for
both its nitrogen metabolism and cell wall biosynthesis. Glutamine synthetase is
essential for Mtb virulence [82], and the inhibition of the enzyme resulted in reduced
growth of Mtb [83]. The tricarboxylic acid (TCA) cycle plays a key role in the
metabolism of almost all pathogens including mycobacteria. The fumarate hydratase enzyme has been identified as one of the important enzymes, which catalyzes
the reversible conversion of fumarate to (L) malate in the TCA cycle. The presence of
the human homologue of fumarate hydratase implicated the inhibitor designing
toward this target. The first selective small-molecule inhibitor for Mtb fumarate
hydratase is reported in 2016, in which the inhibitor is binding to an allosteric site
consisting of amino acid residues which are different between human and Mtb [84].
2.5 Protein Membrane Transport Targets for Mtb
Inhibition
Mtb protein export pathway system comprising SecA cytoplasmic ATPase is
required for bacterial virulence. The SecA is an important Mtb target, since no
eukaryotic homolog exists suggesting for safe development of anti-TB drug design
and discovery. Crystal structure of SecA was solved, which was further used by Li
et al. to discover new inhibitors by structure-based virtual screening (VS) [85].
Further, Chen et al. developed biochemical ATPase assay and validated these
inhibitors at the micromolar range for TB therapy [86].
320
A. C. Pushkaran et al.
methods. These structures revealed conformational flexibility at the active sites
during catalysis in order to screen preferred chemotypes and its molecular mode of
binding. Further, in vivo mouse model of acute infection showed reduced bacterial
load using GlcB inhibitors [75, 76]. ATP synthase is another key target in
anti-mycobacterial drug discovery. This is primarily involved in cellular energy
production in all microorganisms, plants, and animals. The structural and functional
activity of the ATP synthase enzyme is same in all organisms. Bedaquiline, a
second-line TB drug, targets ATP synthase subunit e and inhibits Mtb ATP production (Table 1; Fig. 2). It was approved by FDA as MDR-TB drug [77, 78]. An
imidazopyridine amide compound (Q203) was discovered by the whole cell screen
in infected macrophages. Q203 is a potent inhibitor by disrupting the electron
transport chain of ATP synthesis and successfully entered in Phase I clinical trials
[79]. Lipoamide dehydrogenase (Lpd), another key target member of three multienzyme, complexes the energy metabolism pathway for Mtb virulence and can be
potential for anti-TB drug discovery program. Mtb of Lpd is crucial for the
metabolism of branched chain amino acids and thus essential for its pathogenicity.
The small-molecule inhibitor triazospirodimethoxybenzoyl which inhibits the Mtb
Lpd enzyme selectively without affecting the human Lpd enzyme can be successfully used for TB therapy [80, 81].
Another important druggable target in Mtb is glutamine synthetase required for
both its nitrogen metabolism and cell wall biosynthesis. Glutamine synthetase is
essential for Mtb virulence [82], and the inhibition of the enzyme resulted in reduced
growth of Mtb [83]. The tricarboxylic acid (TCA) cycle plays a key role in the
metabolism of almost all pathogens including mycobacteria. The fumarate hydratase enzyme has been identified as one of the important enzymes, which catalyzes
the reversible conversion of fumarate to (L) malate in the TCA cycle. The presence of
the human homologue of fumarate hydratase implicated the inhibitor designing
toward this target. The first selective small-molecule inhibitor for Mtb fumarate
hydratase is reported in 2016, in which the inhibitor is binding to an allosteric site
consisting of amino acid residues which are different between human and Mtb [84].
2.5 Protein Membrane Transport Targets for Mtb
Inhibition
Mtb protein export pathway system comprising SecA cytoplasmic ATPase is
required for bacterial virulence. The SecA is an important Mtb target, since no
eukaryotic homolog exists suggesting for safe development of anti-TB drug design
and discovery. Crystal structure of SecA was solved, which was further used by Li
et al. to discover new inhibitors by structure-based virtual screening (VS) [85].
Further, Chen et al. developed biochemical ATPase assay and validated these
inhibitors at the micromolar range for TB therapy [86].
320
A. C. Pushkaran et al.