Siderophore-Mediated Iron Acquisition: Target for the Development of Selective Antibiotics Towards Mycobacterium tuberculosis - Iron Acquisition by the Genus Mycobacterium - page 77

Fig. 5.14 Adenylate-forming activity of A-domains in NRPS ( left ) and examples of selective
inhibitors ( right ) [ 73 , 74 ]
A-domains: GrsA, a phenylalanine-activating (PheA) domain from Bacillus bre-
vis , and a leucine-activating (LeuA) domain from Bacillus subtilis . The report of
the crystal structure of GrsA in the presence of adenosine monophosphate and
phenylalanine provided an insight in the mode of action of the A-domain. From
the inhibition assays, the presence of 61 inhibited the activity of PheA with L-Phe
(K i = 61 nM) and D-Phe (K i = 63 nM), while 62 inhibited LeuA with L-Leu
(K i = 8.4 nM). Since no cross-inhibition was observed between domains, this
work demonstrated the ability to develop selective NRPS A-domain inhibitors
where minor structural changes could afford specificity.
Inspired by the previous work, May et al. [ 75 ] demonstrated inhibition of DltA,
a carrier protein ligase that selectively activates D-Ala during the D-Ala adenyla-
tion of lipoteichoic and teichoic acid in B. subtilis and other Gram-positive bacte-
ria. Compound 63 (Fig. 5.15 ) was found to be a potent inhibitor of DltA isolated
from B. subtilis (K i = 232 nM). Because structurally related ascamycin ( 64 ) is
found in fermentation of Stroptomyces , it was expected that 63 would display the
ability to diffuse through cell membranes. Growth inhibition using B. subtilis and
a DltA-deletion mutant showed that the phenotype observed in the presence of 63
was consistent with the DltA-deletion mutant. Additional evidence of selectivity
was obtained when compound 63 did not inhibit A-domains PheA and DhbE at a
concentration of 2 mM.
Ferreras et al. [ 76 ] studied the inhibition of adenylate-forming enzymes MbtA
from M. tuberculosis and YbtE from Yersinia pestis . Because other mechanisti-
cally-related enzymes had been shown to bind their adenylate intermediates 2-3
times more strongly than the corresponding substrates, it was proposed that com-
pound 65 (Fig. 5.16 ) could inhibit both MbtA and YbtE. The report of a crystal
structure of DhbE with 66 led to the conclusion that 66 would bind in a similar
fashion. Recognition appeared to be based primarily on H-bonding and not elec-
trostatic interactions and therefore 65 was expected to be an adequate surrogate.
Under iron-limited conditions, compound 65 was demonstrated to inhibit the pro-
duction of mycobactin in M. tuberculosis (IC 50 = 2.2 ± 0.3 μ M) and yersiniabac-
tin in Y. pestis (IC 50 = 51.2 ± 4.7 μ M). No inhibition of Y. pestis was observed in
iron-sufficient medium, while M. tuberculosis displayed a 18-fold reduced potency