Recently,
iron acquisition and, more specifically,
enzymes involved in
siderophore biosynthesis have become attractive targets for discovery of new
antibiotics. Accordingly, targeted inhibition of the biosynthesis of
petrobactin, a virulence-associated
siderophore encoded by the asb locus in Bacillus anthracis, may hold promise as a potential
therapy against
anthrax. This study describes the biochemical characterization of AsbC, the first reported
3,4-dihydroxybenzoic acid-
AMP ligase, and a key component in the biosynthesis of DHB-
spermidine (DHB-SP), the first isolable intermediate in
petrobactin biosynthesis. AsbC catalyzes adenylation to the corresponding
AMP ester of the unusual precursor
3,4-dihydroxybenzoate, in addition to
benzoate substrates bearing hydrogen bond-donating substituents at the para and meta positions on the phenyl ring. In a second reaction, AsbC catalyzes transfer of the activated starter unit to AsbD, an aryl
carrier protein similar to acyl and peptidyl
carrier proteins that function in
fatty acid,
polyketide, and nonribosomal peptide biosynthesis. A third
protein, AsbE, is shown to be responsible for condensation of 3,4-dihydroxybenzoyl-AsbD with
spermidine, providing the DHB-
spermidine arms that are linked to
citrate for assembly of
petrobactin. On the basis of the selective substrate profile of AsbC, a nonhydrolyzable analogue of 3,4-DHB-AMP was synthesized and shown to effectively inhibit AsbC function in vitro.