SUMMARY: High-affinity iron acquisition is mediated by siderophore-dependent pathways in the majority of pathogenic and nonpathogenic bacteria and fungi. Considerable progress has been made in characterizing and understanding mechanisms of siderophore synthesis, secretion, iron scavenging, and siderophore-delivered iron uptake and its release. The regulation of siderophore pathways reveals multilayer networks at the transcriptional and posttranscriptional levels. Due to the key role of many siderophores during virulence, coevolution led to sophisticated strategies of siderophore neutralization by mammals and (re)utilization by bacterial pathogens. Surprisingly, hosts also developed essential siderophore-based iron delivery and cell conversion pathways, which are of interest for diagnostic and therapeutic studies. In the last decades, natural and synthetic compounds have gained attention as potential therapeutics for iron-dependent treatment of infections and further diseases. Promising results for pathogen inhibition were obtained with various siderophore-antibiotic conjugates acting as “Trojan horse” toxins and siderophore pathway inhibitors. In this article, general aspects of siderophore-mediated iron acquisition, recent findings regarding iron-related pathogen-host interactions, and current strategies for iron-dependent pathogen control will be reviewed. Further concepts including the inhibition of novel siderophore pathway targets are discussed.

The synthesis of the catecholic siderophore bacillibactin is accomplished by the nonribosomal peptide synthetase (NRPS) encoded by the dhb operon. DhbE is responsible for the initial step in bacillibactin synthesis, the activation of the aryl acid 2,3-dihydroxybenzoate (DHB). The stand-alone adenylation (A) domain DhbE, the structure of which is presented here, exhibits greatest homology to other NRPS A-domains, acyl-CoA ligases and luciferases. It's structure is solved in three different states, without the ligands ATP and DHB (native state), with the product DHB-AMP (adenylate state) and with the hydrolyzed product AMP and DHB (hydrolyzed state). The 59.9-kDa protein folds into two domains, with the active site at the interface between them. In contrast to previous proposals of a major reorientation of the large and small domains on substrate binding, we observe only local structural rearrangements. The structure of the phosphate binding loop could be determined, a motif common to many adenylate-forming enzymes, as well as with bound DHB-adenylate and the hydrolyzed product DHB*AMP. Based on the structure and amino acid sequence alignments, an adapted specificity conferring code for aryl acid activating domains is proposed, allowing assignment of substrate specificity to gene products of previously unknown function.

SUMMARY: Non-ribosomally synthesized peptides have compelling biological activities ranging from antimicrobial to immunosuppressive and from cytostatic to antitumor. The broad spectrum of applications in modern medicine is reflected in the great structural diversity of these natural products. They contain unique building blocks, such as d -amino acids, fatty acids, sugar moieties, and heterocyclic elements, as well as halogenated, methylated, and formylated residues. In the past decades, significant progress has been made toward the understanding of the biosynthesis of these secondary metabolites by nonribosomal peptide synthetases (NRPSs) and their associated tailoring enzymes. Guided by this knowledge, researchers genetically redesigned the NRPS template to synthesize new peptide products. Moreover, chemoenzymatic strategies were developed to rationally engineer nonribosomal peptides products in order to increase or alter their bioactivities. Specifically, chemical synthesis combined with peptide cyclization mediated by nonribosomal thioesterase domains enabled the synthesis of glycosylated cyclopeptides, inhibitors of integrin receptors, peptide/polyketide hybrids, lipopeptide antibiotics, and streptogramin B antibiotics. In addition to the synthetic potential of these cyclization catalysts, which is the main focus of this review, different enzymes for tailoring of peptide scaffolds as well as the manipulation of carrier proteins with reporter-labeled coenzyme A analogs are discussed.

Daptomycin (Cubicin) is a branched cyclic lipopeptide antibiotic of nonribosomal origin and the prototype of the acidic lipopeptide family. It was approved in 2003 for the nontopical treatment of skin structure infections caused by gram-positive pathogens, including methicillin-resistant Staphylococcus aureus (MRSA), and in 2006 for the treatment of bacteremia. Understanding the ribosome-independent biosynthesis of daptomycin assembly will provide opportunities for the generation of daptomycin derivatives with an altered pharmaceutical spectrum to address upcoming daptomycin-resistant pathogens. Herein, the structural properties of daptomycin, its biosynthesis, recent efforts for the generation of structural diversity, and its proposed mode of action are discussed.