Many pathogenic bacteria biosynthesize
and excrete small molecule
metallophores, known as siderophores, that are used to extract ferric
iron from host sources to satisfy nutritional need. Native siderophores
are often structurally complex multidentate chelators that selectively
form high-affinity octahedral ferric iron complexes with defined chirality
recognizable by cognate protein receptors displayed on the bacterial
cell surface. Simplified achiral analogues can serve as synthetically
tractable siderophore mimics with potential utility as chemical probes
and therapeutic agents to better understand and treat bacterial infections,
respectively. Here, we demonstrate that synthetic spermidine-derived
mixed ligand bis-catecholate monohydroxamate siderophores (compounds 1–3) are versatile structural and biomimetic
analogues of two native siderophores, acinetobactin and fimsbactin,
produced by Acinetobacter baumannii, a multidrug-resistant Gram-negative human pathogen. The metal-free
and ferric iron complexes of the synthetic siderophores are growth-promoting
agents of A. baumannii, while the Ga(III)-complexes
are potent growth inhibitors of A. baumannii with
MIC values <1 μM. The synthetic siderophores compete with
native siderophores for uptake in A. baumannii and
maintain comparable apparent binding affinities for ferric iron (K
Fe) and the siderophore-binding protein BauB
(K
d). Our findings provide new insight
to guide the structural fine-tuning of these compounds as siderophore-based
therapeutics targeting pathogenic strains of A. baumannii.
Desferrioxamine siderophores are assembled by the nonribosomal-peptide-synthetase-independent siderophore (NIS) synthetase enzyme DesD via ATP-dependent iterative condensation of three N 1 -hydroxy-N 1 -succinyl-cadaverine (HSC) units. Current knowledge of NIS enzymology and the desferrioxamine biosynthetic pathway does not account for the existence of most known members of this natural product family, which differ in substitution patterns of the N-and C-termini. The directionality of desferrioxamine biosynthetic assembly, N-to-C versus C-to-N, is a longstanding knowledge gap that is limiting further progress in understanding the origins of natural products in this structural family. Here, we establish the directionality of desferrioxamine biosynthesis using a chemoenzymatic approach with stable isotope incorporation and dimeric substrates. We propose a mechanism where DesD catalyzes the N-to-C condensation of HSC units to establish a unifying biosynthetic paradigm for desferrioxamine natural products in Streptomyces.
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