The EmbB is another validated mycobacterial drug target. Ethambutol which is a
first-line TB drug inhibits EmbB target, and the mechanism of drug action is well
studied [13, 14] (Table 1; Fig. 3).
IspD a key enzyme of methylerythritol phosphate pathway and RmlC enzyme
of rhamnose pathway form a link toward biosynthesis of arabinogalactan and PG,
which is essential for the Mtb cell wall integrity and growth [16, 17]. Both these
chemotherapeutic targets are essential for Mtb growth and are absent in mammals.
In order to carry out structure-based drug discovery, both IspD and RmlC crystal
structures are available in complex with small-molecule inhibitors and its molecular
mechanism of action was clearly elucidated [18].
2.1.1 Mycolic Acid Biosynthesis Pathway Targets
InhA is an NADH-dependent key enzyme of enoyl-acyl-carrier protein reductase of
fatty acid synthase-II involved in the biosynthesis of the mycolic acid pathway
which is targeted by Isoniazid, one of the first-line anti-TB drug and a second-line
drug Ethionamide [19, 20] shown in Table 1 and Fig. 2. Hence, mycobacterial
InhA serves as a validated target for TB therapy. Isoniazid requires activation by
KatG enzyme before binding to InhA. Most of the resistance toward Isoniazid
confers to the mutations in KatG gene. So designing of inhibitors that directly target
InhA has been of interest by several research groups. Different InhA inhibitors are
already being identified using various techniques consisting of high-throughput
screening (HTS), encoded library technology, and in silico drug design techniques
[21–23]. However, most of the identified inhibitors lack good pharmacokinetic
profile. In 2016, Martínez-Hoyos et al. identified GSK693, a direct oral InhA
inhibitor with potent anti-tubercular activity against MDR and XDR clinical isolates
and also in TB murine models [24]. Mycobacterial cyclopropane synthase
(CmaA1) is another key enzyme contributing toward the persistence and virulence
of Mtb. CmaA1 is involved in the maturation of mycolic acid in a process called
cyclopropanation [25].
Wilson et al. demonstrated that Pks13 enzyme of Mtb is required for mycolic
acid biosynthesis and is an essential druggable target by discovering new classes of
thiophene-based compounds acting as cell wall synthesis inhibitors [26]. Pks13 was
known to be involved in the final step of the mycolic acid biosynthesis pathway.
MmpL3 is another key druggable transmembrane target involved in the transport of
trehalose monomycolate biosynthetic pathway, and its inhibition by SQ109 small
molecule showed good bactericidal activity. SQ109 showed synergism with the
anti-tubercular drug bedaquiline and was very effective in acute and chronic mice
models of Mtb infection [27].
Another potential Mtb druggable target involved in the cell wall assembly is
Antigen 85 (Ag85) complexes consisting of three proteins (Ag85A, B, and C).
These proteins exhibit mycolyltransferase activities through disruption of cord
factor biosynthesis by the biogenesis of trehalose dimycolate and are also useful in
controlling MDR and XDR-TB. Crystal structure of Ag85C protein was solved at
314
A. C. Pushkaran et al.
first-line TB drug inhibits EmbB target, and the mechanism of drug action is well
studied [13, 14] (Table 1; Fig. 3).
IspD a key enzyme of methylerythritol phosphate pathway and RmlC enzyme
of rhamnose pathway form a link toward biosynthesis of arabinogalactan and PG,
which is essential for the Mtb cell wall integrity and growth [16, 17]. Both these
chemotherapeutic targets are essential for Mtb growth and are absent in mammals.
In order to carry out structure-based drug discovery, both IspD and RmlC crystal
structures are available in complex with small-molecule inhibitors and its molecular
mechanism of action was clearly elucidated [18].
2.1.1 Mycolic Acid Biosynthesis Pathway Targets
InhA is an NADH-dependent key enzyme of enoyl-acyl-carrier protein reductase of
fatty acid synthase-II involved in the biosynthesis of the mycolic acid pathway
which is targeted by Isoniazid, one of the first-line anti-TB drug and a second-line
drug Ethionamide [19, 20] shown in Table 1 and Fig. 2. Hence, mycobacterial
InhA serves as a validated target for TB therapy. Isoniazid requires activation by
KatG enzyme before binding to InhA. Most of the resistance toward Isoniazid
confers to the mutations in KatG gene. So designing of inhibitors that directly target
InhA has been of interest by several research groups. Different InhA inhibitors are
already being identified using various techniques consisting of high-throughput
screening (HTS), encoded library technology, and in silico drug design techniques
[21–23]. However, most of the identified inhibitors lack good pharmacokinetic
profile. In 2016, Martínez-Hoyos et al. identified GSK693, a direct oral InhA
inhibitor with potent anti-tubercular activity against MDR and XDR clinical isolates
and also in TB murine models [24]. Mycobacterial cyclopropane synthase
(CmaA1) is another key enzyme contributing toward the persistence and virulence
of Mtb. CmaA1 is involved in the maturation of mycolic acid in a process called
cyclopropanation [25].
Wilson et al. demonstrated that Pks13 enzyme of Mtb is required for mycolic
acid biosynthesis and is an essential druggable target by discovering new classes of
thiophene-based compounds acting as cell wall synthesis inhibitors [26]. Pks13 was
known to be involved in the final step of the mycolic acid biosynthesis pathway.
MmpL3 is another key druggable transmembrane target involved in the transport of
trehalose monomycolate biosynthetic pathway, and its inhibition by SQ109 small
molecule showed good bactericidal activity. SQ109 showed synergism with the
anti-tubercular drug bedaquiline and was very effective in acute and chronic mice
models of Mtb infection [27].
Another potential Mtb druggable target involved in the cell wall assembly is
Antigen 85 (Ag85) complexes consisting of three proteins (Ag85A, B, and C).
These proteins exhibit mycolyltransferase activities through disruption of cord
factor biosynthesis by the biogenesis of trehalose dimycolate and are also useful in
controlling MDR and XDR-TB. Crystal structure of Ag85C protein was solved at
314
A. C. Pushkaran et al.