The worldwide emergence of antibiotic resistance poses a serious threat to human health. A molecular understanding of resistance strategies employed by bacteria is obligatory to generate less-susceptible antibiotics. Albicidin is a highly potent antibacterial compound synthesized by the plant-pathogenic bacterium Xanthomonas albilineans. The drug-binding protein AlbA confers albicidin resistance to Klebsiella oxytoca. Here we show that AlbA binds albicidin with low nanomolar affinity resulting in full inhibition of its antibacterial activity. We report on the crystal structure of the drug-binding domain of AlbA (AlbAS) in complex with albicidin. Both α-helical repeat domains of AlbAS are required to cooperatively clamp albicidin, which is unusual for drug-binding proteins of the MerR family. Structure-guided NMR binding studies employing synthetic albicidin derivatives give valuable information about ligand promiscuity of AlbAS. Our findings thus expand the general understanding of antibiotic resistance mechanisms and support current drug-design efforts directed at more effective albicidin analogs.
To investigate the pharmacophore regions of the antibiotic albicidin, derivatives with variations on the central amino acid were synthesized. Charged as well as uncharged residues were chosen to explore the influence of charge, chirality, and steric bulk. The bioactivity of the newly synthesized derivatives was determined by a microdilution technique to obtain minimum inhibitory concentrations (MIC) values. The compounds were also tested in a cell-free system to obtain information about their ability to inhibit their primary target, DNA gyrase. It was then shown that derivatives with uncharged side chains retain antibacterial activity, whereas incorporation of charged amino acid residues decreases the antibacterial activity dramatically, possibly due to restricted cell penetration of these derivatives. From the newly synthesized derivatives, the threonine derivative shows the most promising results in both tests. The information will help to develop the features of albicidin toward more drug-like structures.
Natural products represent an important source of potential novel antimicrobial drug leads. Low production by microorganisms in cell culture often delays the structural elucidation or even prevents a timely discovery. Starting from the anti‐Gram‐negative antibacterial compound albicidin produced by Xanthomonas albilineans, we describe a bioactivity‐guided approach combined with non‐targeted tandem mass spectrometry and spectral (molecular) networking for the discovery of novel antimicrobial compounds. We report eight new natural albicidin derivatives, four of which bear a β‐methoxy cyanoalanine or β‐methoxy asparagine as the central α‐amino acid. We present the total synthesis of these albicidins, which facilitated the unambiguous determination of the (2 S,3 R)‐stereoconfiguration which is complemented by the assessment of the stereochemistry on antibacterial activity.
Albicidin is a potent antibiotic and phytotoxin produced by Xanthomonas albilineans which targets the plant and bacterial DNA gyrase. We now report on a new albicidin derivative which is carbamoylated at the N-terminal coumaric acid by the action of the ATP-dependent O-carbamoyltransferase Alb15, present in the albicidin (alb) gene cluster. Carbamoyl-albicidin was characterized by tandem mass spectrometry from cultures of a Xanthomonas overproducer strain and the gene function confirmed by gene inactivation of alb15 in X. albilineans. Expression of alb15 in Escherichia coli and in vitro reconstitution of the carbamoyltransferase activity confirmed albicidin as the substrate. The chemical synthesis of carbamoyl-albicidin finally enabled us to assess its bioactivity by means of in vitro gyrase inhibition and antibacterial assays. Compared to albicidin, carbamoyl-albicidin showed a significantly higher inhibitory efficiency against bacterial gyrase (∼8 vs 49 nM), which identifies the carbamoyl group as an important structural feature of albicidin maturation.
The worrysome development and spread of multidrug-resistant bacteria demands new antibacterial agents with strong bioactivities particularly against Gram−negative bacteria. Albicidins were recently structurally characterized as highly active antibacterial natural products...
The E. coli siderophore enterobactin, the strongest FeIII chelator known to date, forms hexacoordinate complexes with SiIV, GeIV, and TiIV. Synthetic protocols have been developed to prepare non‐symmetric enterobactin analogues with varying denticities. Various benzoic acid residues were coupled to the macrocyclic lactone to afford a diverse library of ligands. These enterobactin analogues were bound to SiIV, GeIV, and TiIV, and the complexes were investigated through experimental and computational techniques. The binding behavior of the synthesized chelators enabled assessment of the contribution of each of the phenolic hydroxy groups in enterobactin to metal‐ion complexation. It was found that at least four O‐donors are needed for enterobactin derivatives to act as metal binders. Density functional theory calculations indicate that the strong binding behavior of enterobactin can be ascribed to a diminished translational entropy penalty, a common feature of the chelate effect, coupled with the structural arrangement of the three catechol moieties, which allows the triseryl base to be installed without distorting the preferred local metal‐binding geometry of the catecholate ligands.
The E. coli siderophore enterobactin, the strongest FeIII–chelator known to date, was found to form hexacoordinate complexes with SiIV, GeIV, and TiIV. Synthetic protocols were developed to prepare non‐symmetric enterobactin analogues with varying denticities and were bound to SiIV, GeIV, and TiIV. Experimental and computational studies suggest that at least four O‐donors are needed for enterobactin derivatives to act as metal binders. More information can be found in the Full Paper by M.‐H. Baik, R. D. Süssmuth et al. on page 6955.
The front cover picture shows the structure of albicidin, with modifications to the central amino acid. Albicidin, with its high activity against Gram‐negative as well as Gram‐positive bacterial strains, is a new lead structure to fulfil the strong demand for new antibiotics. The bullets in the revolver cylinder, namely Thr, Aib, Gly, Asp, and Lys represent the derivatives that were synthesized and tested. This set of amino acids allows charged as well as uncharged side chains to be introduced into the albicidin structure and, in addition, allows investigation of the influence of steric demand and chirality. The bacteria in the background are scared of the derivatives′ high potency, and somehow are not willing to accept this new nanomolar “weapon”. In our study we could show that every tested derivative is even more potent than the parent albicidin, on its molecular target: namely DNA gyrase. Nevertheless, amino acids bearing charged side chains were less active in a cell‐based assay. Our studies on variations of the central building block of albicidin will therefore be helpful in the further optimization of this lead structure toward a future antibacterial drug. More information can be found in the Communication by Roderich Süssmuth et al. on page 1499 in Issue 14, 2016 (DOI: 10.1002/cmdc.201600163).
scite is a Brooklyn-based organization that helps researchers better discover and understand research articles through Smart Citations–citations that display the context of the citation and describe whether the article provides supporting or contrasting evidence. scite is used by students and researchers from around the world and is funded in part by the National Science Foundation and the National Institute on Drug Abuse of the National Institutes of Health.