Enterobactin and Enantioenterobactin

Advances in the Synthesis of Enterobactin, Artificial Analogues, and Enterobactin-Derived Antimicrobial Drug Conjugates and Imaging Tools for Infection Diagnosis

Iron is an essential growth factor for bacteria, but although highly abundant in nature, its bioavailability during infection in the human host or the environment is limited. Therefore, bacteria produce and secrete siderophores to ensure their supply of iron. The triscatecholate siderophore enterobactin and its glycosylated derivatives, the salmochelins, play a crucial role for iron acquisition in several bacteria. As these compounds can serve as carrier molecules for the design of antimicrobial siderophore drug conjugates as well as siderophore-derived tool compounds for the detection of infections with bacteria, their synthesis and the design of artificial analogues is of interest. In this review, we give an overview on the synthesis of enterobactin, biomimetic as well as totally artificial analogues, and related drug-conjugates covering up to 12/2021.

1 Introduction

2 Antibiotic Crisis and Sideromycins as Natural Templates for New Antimicrobial Drugs

3 Biosynthesis of Enterobactin, Salmochelins, and Microcins

4 Total Synthesis of Enterobactin and Salmochelins

5 Chemoenzymatic Semi-synthesis of Salmochelins and Microcin E492m Derivatives

6 Synthesis of Biomimetic Enterobactin Derivatives with Natural Tris-lactone Backbone

7 Synthesis of Artificial Enterobactin Derivatives without Tris-lactone Backbone

8 Conclusions

Biosynthesis and chemical diversity of β-lactone natural products

Covering: up to 2018 β-Lactones are strained rings that are useful organic synthons and pharmaceutical warheads. Over 30 core scaffolds of β-lactone natural products have been described to date, many with potent bioactivity against bacteria, fungi, or human cancer cell lines. β-Lactone natural products are chemically diverse and have high clinical potential, but production of derivatized drug leads has been largely restricted to chemical synthesis partly due to gaps in biochemical knowledge about β-lactone biosynthesis. Here we review recent discoveries in enzymatic β-lactone ring closure via ATP-dependent synthetases, intramolecular cyclization from seven-membered rings, and thioesterase-mediated cyclization during release from nonribosomal peptide synthetase assembly lines. We also comprehensively cover the diversity and taxonomy of source organisms for β-lactone natural products including their isolation from bacteria, fungi, plants, insects, and marine sponges. This work identifies computational and experimental bottlenecks and highlights future directions for genome-based discovery of biosynthetic gene clusters that may produce novel compounds with β-lactone rings.

Catechol Siderophore Transport by Vibrio cholerae

Siderophores, small iron-binding molecules secreted by many microbial species, capture environmental iron for transport back into the cell. Vibrio cholerae synthesizes and uses the catechol siderophore vibriobactin and also uses siderophores secreted by other species, including enterobactin produced by Escherichia coli. E. coli secretes both canonical cyclic enterobactin and linear enterobactin derivatives likely derived from its cleavage by the enterobactin esterase Fes. We show here that V. cholerae does not use cyclic enterobactin but instead uses its linear derivatives. V. cholerae lacked both a receptor for efficient transport of cyclic enterobactin and enterobactin esterase to promote removal of iron from the ferrisiderophore complex. To further characterize the transport of catechol siderophores, we show that the linear enterobactin derivatives were transported into V. cholerae by either of the catechol siderophore receptors IrgA and VctA, which also transported the synthetic siderophore MECAM [1,3,5-N,N',N″-tris-(2,3-dihydroxybenzoyl)-triaminomethylbenzene]. Vibriobactin is transported via the additional catechol siderophore receptor ViuA, while the Vibrio fluvialis siderophore fluvibactin was transported by all three catechol receptors. ViuB, a putative V. cholerae siderophore-interacting protein (SIP), functionally substituted for the E. coli ferric reductase YqjH, which promotes the release of iron from the siderophore in the bacterial cytoplasm. In V. cholerae, ViuB was required for the use of vibriobactin but was not required for the use of MECAM, fluvibactin, ferrichrome, or the linear derivatives of enterobactin. This suggests the presence of another protein in V. cholerae capable of promoting the release of iron from these siderophores.

IMPORTANCE

Vibrio cholerae is a major human pathogen and also serves as a model for the Vibrionaceae, which include other serious human and fish pathogens. The ability of these species to persist and acquire essential nutrients, including iron, in the environment is epidemiologically important but not well understood. In this work, we characterize the ability of V. cholerae to acquire iron by using siderophores produced by other organisms. We resolve confusion in the literature regarding its ability to use the Escherichia coli siderophore enterobactin and identify the receptor and TonB system used for the transport of several siderophores. The use of some siderophores did not require the ferric reductase ViuB, suggesting that an uncharacterized ferric reductase is present in V. cholerae.

Synthesis and structural characterization of hexacoordinate silicon, germanium, and titanium complexes of the E. coli siderophore enterobactin

The E. coli siderophore enterobactin, one of the strongest Fe(III) chelators known to date, is also capable of binding Si(IV) under physiological conditions. We report on the synthesis and structural characterization of the tris(catecholate) Si(IV) -enterobactin complex and its Ge(IV) and Ti(IV) analogues. Comparative structural analysis, supported by quantum-chemical calculations, reveals the correlation between the ionic radius and the structural changes in enterobactin upon complexation.

Siderophore-mediated cargo delivery to the cytoplasm of Escherichia coli and Pseudomonas aeruginosa: syntheses of monofunctionalized enterobactin scaffolds and evaluation of enterobactin-cargo conjugate uptake

The design and syntheses of monofunctionalized enterobactin (Ent, L- and D-isomers) scaffolds where one catecholate moiety of enterobactin houses an alkene, aldehyde, or carboxylic acid at the C5 position are described. These molecules are key precursors to a family of 10 enterobactin-cargo conjugates presented in this work, which were designed to probe the extent to which the Gram-negative ferric enterobactin uptake and processing machinery recognizes, transports, and utilizes derivatized enterobactin scaffolds. A series of growth recovery assays employing enterobactin-deficient E. coli ATCC 33475 (ent-) revealed that six conjugates based on L-Ent having relatively small cargos promoted E. coli growth under iron-limiting conditions whereas negligible-to-no growth recovery was observed for four conjugates with relatively large cargos. No growth recovery was observed for the enterobactin receptor-deficient strain of E. coli H1187 (fepA-) or the enterobactin esterase-deficient derivative of E. coli K-12 JW0576 (fes-), or when the D-isomer of enterobactin was employed. These results demonstrate that the E. coli ferric enterobactin transport machinery identifies and delivers select cargo-modified scaffolds to the E. coli cytoplasm. Pseudomonas aeruginosa PAO1 K648 (pvd-, pch-) exhibited greater promiscuity than that of E. coli for the uptake and utilization of the enterobactin-cargo conjugates, and growth promotion was observed for eight conjugates under iron-limiting conditions. Enterobactin may be utilized for delivering molecular cargos via its transport machinery to the cytoplasm of E. coli and P. aeruginosa thereby providing a means to overcome the Gram-negative outer membrane permeability barrier.

Synthesis and structure-activity relationships of N-(2-oxo-3-oxetanyl)amides as N-acylethanolamine-hydrolyzing acid amidase inhibitors

The fatty acid ethanolamides (FAEs) are a family of bioactive lipid mediators that include the endogenous agonist of peroxisome proliferator-activated receptor-alpha, palmitoylethanolamide (PEA). FAEs are hydrolyzed intracellularly by either fatty acid amide hydrolase or N-acylethanolamine-hydrolyzing acid amidase (NAAA). Selective inhibition of NAAA by (S)-N-(2-oxo-3-oxetanyl)-3-phenylpropionamide [(S)-OOPP, 7a] prevents PEA degradation in mouse leukocytes and attenuates responses to proinflammatory stimuli. Starting from the structure of 7a, a series of beta-lactones was prepared and tested on recombinant rat NAAA to explore structure-activity relationships (SARs) for this class of inhibitors and improve their in vitro potency. Following the hypothesis that these compounds inhibit NAAA by acylation of the catalytic cysteine, we identified several requirements for recognition at the active site and obtained new potent inhibitors. In particular, (S)-N-(2-oxo-3-oxetanyl)biphenyl-4-carboxamide (7h) was more potent than 7a at inhibiting recombinant rat NAAA activity (7a, IC(50) = 420 nM; 7h, IC(50) = 115 nM) in vitro and at reducing carrageenan-induced leukocyte infiltration in vivo.

Total (bio)synthesis: strategies of nature and of chemists

The biosynthetic pathways to a number of natural products have been reconstituted in vitro using purified enzymes. Many of these molecules have also been synthesized by organic chemists. Here we compare the strategies used by nature and by chemists to reveal the underlying logic and success of each total synthetic approach for some exemplary molecules with diverse biosynthetic origins.

Oxinobactin, a siderophore analogue to enterobactin involving 8-hydroxyquinoline subunits: synthesis and iron binding ability

Oxinobactin, a siderophore analogue to enterobactin but possessing 8-hydroxyquinoline instead of catechol complexing subunits, has been synthesized starting from L-serine and 8-hydroxyquinoline. Comparative iron binding studies showed that oxinobactin is as effective as enterobactin for the complexation of Fe(III) at physiological pH but with improved complexing ability at acidic pH.

High yield selective acylation of polyamines: proton as protecting group

An advance in the selective acylation of polyamines having identical or similar amine functions is reported. While nucleophilicity differences between the various amine functions are slight, the corresponding conjugate acids exhibit pKa values over a significant range. We have used proton as polyamine protecting group: the monoamine resulting from single deprotonation of a polyammonium compound has allowed for high yields of selective acylation.

Dynamic covalent chemistry

Dynamic covalent chemistry relates to chemical reactions carried out reversibly under conditions of equilibrium control. The reversible nature of the reactions introduces the prospects of "error checking" and "proof-reading" into synthetic processes where dynamic covalent chemistry operates. Since the formation of products occurs under thermodynamic control, product distributions depend only on the relative stabilities of the final products. In kinetically controlled reactions, however, it is the free energy differences between the transition states leading to the products that determines their relative proportions. Supramolecular chemistry has had a huge impact on synthesis at two levels: one is noncovalent synthesis, or strict self-assembly, and the other is supramolecular assistance to molecular synthesis, also referred to as self-assembly followed by covalent modification. Noncovalent synthesis has given us access to finite supermolecules and infinite supramolecular arrays. Supramolecular assistance to covalent synthesis has been exploited in the construction of more-complex systems, such as interlocked molecular compounds (for example, catenanes and rotaxanes) as well as container molecules (molecular capsules). The appealing prospect of also synthesizing these types of compounds with complex molecular architectures using reversible covalent bond forming chemistry has led to the development of dynamic covalent chemistry. Historically, dynamic covalent chemistry has played a central role in the development of conformational analysis by opening up the possibility to be able to equilibrate configurational isomers, sometimes with base (for example, esters) and sometimes with acid (for example, acetals). These stereochemical "balancing acts" revealed another major advantage that dynamic covalent chemistry offers the chemist, which is not so easily accessible in the kinetically controlled regime: the ability to re-adjust the product distribution of a reaction, even once the initial products have been formed, by changing the reaction's environment (for example, concentration, temperature, presence or absence of a template). This highly transparent, yet tremendously subtle, characteristic of dynamic covalent chemistry has led to key discoveries in polymer chemistry. In this review, some recent examples where dynamic covalent chemistry has been demonstrated are shown to emphasise the basic concepts of this area of science.

Synthesis of L-glucose from D-gulono-1,4-lactone

An efficient method for the synthesis of L-glucose from D-gulono-1,4-lactone via 1,2,3,4,5-penta-O-benzyl/acetyl/benzoyl-D-gulitol is described in 34-53% overall yield.