The Thioesterase Domain in Glycopeptide Antibiotic Biosynthesis Is Selective for Cross-Linked Aglycones

Effects of the deletion and substitution of thioesterase on bacillomycin D synthesis

OBJECTIVES

The importance of thioesterase domains on bacillomycin D synthesis and the ability of different thioesterase domains to selectively recognize and catalyze peptide chain hydrolysis and cyclization were studied by deleting and substituting thioesterase domains.

RESULTS

No bacillomycin D analogs were found in the thioesterase-deleted strain fmbJ-ΔTE, indicating that the TE domain was essential for bacillomycin D synthesis. Then the thioesterase in bacillomycin D synthetases was replaced by the thioesterase in bacillomycin F, iturin A, mycosubtilin, plipastatin and surfactin synthetases. Except for fmbJ-S-TE, all others were able to synthesize bacillomycin D homologs because a suitable recombination site was selected, which maintained the integrity of NRPSs. In particular, the yield of bacillomycin D in fmbJ-IA-TE, fmbJ-M-TE and fmbJ-P-TE was significantly increased.

CONCLUSION

This study expands our understanding of the TE domain in bacillomycin D synthetases and shows that thioesterase has excellent potential in the chemical-enzymatic synthesis of natural products or their analogs.

Newest perspectives of glycopeptide antibiotics: biosynthetic cascades, novel derivatives, and new appealing antimicrobial applications

Glycopeptide antibiotics (GPAs) are a family of non-ribosomal peptide natural products with polypeptide skeleton characteristics, which are considered the last resort for treating severe infections caused by multidrug-resistant Gram-positive pathogens. Over the past few years, an increasing prevalence of Gram-positive resistant strain "superbugs" has emerged. Therefore, more efforts are needed to study and modify the GPAs to overcome the challenge of superbugs. In this mini-review, we provide an overview of the complex biosynthetic gene clusters (BGCs), the ingenious crosslinking and tailoring modifications, the new GPA derivatives, the discoveries of new natural GPAs, and the new applications of GPAs in antivirus and anti-Gram-negative bacteria. With the development and interdisciplinary integration of synthetic biology, next-generation sequencing (NGS), and artificial intelligence (AI), more GPAs with new chemical structures and action mechanisms will constantly be emerging.

The Cytochrome P450 OxyA from the Kistamicin Biosynthesis Cyclization Cascade is Highly Sensitive to Oxidative Damage

Cytochrome P450 enzymes (P450s) are a superfamily of monooxygenases that utilize a cysteine thiolate–ligated heme moiety to perform a wide range of demanding oxidative transformations. Given the oxidative power of the active intermediate formed within P450s during their active cycle, it is remarkable that these enzymes can avoid auto-oxidation and retain the axial cysteine ligand in the deprotonated—and thus highly acidic—thiolate form. While little is known about the process of heme incorporation during P450 folding, there is an overwhelming preference for one heme orientation within the P450 active site. Indeed, very few structures to date contain an alternate heme orientation, of which two are OxyA homologs from glycopeptide antibiotic (GPA) biosynthesis. Given the apparent preference for the unusual heme orientation shown by OxyA enzymes, we investigated the OxyA homolog from kistamicin biosynthesis (OxyA_kis), which is an atypical GPA. We determined that OxyA_kis is highly sensitive to oxidative damage by peroxide, with both UV and EPR measurements showing rapid bleaching of the heme signal. We determined the structure of OxyA_kis and found a mixed population of heme orientations present in this enzyme. Our analysis further revealed the possible modification of the heme moiety, which was only present in samples where the alternate heme orientation was present in the protein. These results suggest that the typical heme orientation in cytochrome P450s can help prevent potential damage to the heme—and hence deactivation of the enzyme—during P450 catalysis. It also suggests that some P450 enzymes involved in GPA biosynthesis may be especially prone to oxidative damage due to the heme orientation found in their active sites.

Nonribosomal Peptide Synthesis Definitely Working Out of the Rules

Nonribosomal peptides are microbial secondary metabolites exhibiting a tremendous structural diversity and a broad range of biological activities useful in the medical and agro-ecological fields. They are built up by huge multimodular enzymes called nonribosomal peptide synthetases. These synthetases are organized in modules constituted of adenylation, thiolation, and condensation core domains. As such, each module governs, according to the collinearity rule, the incorporation of a monomer within the growing peptide. The release of the peptide from the assembly chain is finally performed by a terminal core thioesterase domain. Secondary domains with modifying catalytic activities such as epimerization or methylation are sometimes included in the assembly lines as supplementary domains. This assembly line structure is analyzed by bioinformatics tools to predict the sequence and structure of the final peptides according to the sequence of the corresponding synthetases. However, a constantly expanding literature unravels new examples of nonribosomal synthetases exhibiting very rare domains and noncanonical organizations of domains and modules, leading to several amazing strategies developed by microorganisms to synthesize nonribosomal peptides. In this review, through several examples, we aim at highlighting these noncanonical pathways in order for the readers to perceive their complexity.

Chain release mechanisms in polyketide and non-ribosomal peptide biosynthesis

Review covering up to mid-2021The structure of polyketide and non-ribosomal peptide natural products is strongly influenced by how they are released from their biosynthetic enzymes. As such, Nature has evolved a diverse range of release mechanisms, leading to the formation of bioactive chemical scaffolds such as lactones, lactams, diketopiperazines, and tetronates. Here, we review the enzymes and mechanisms used for chain release in polyketide and non-ribosomal peptide biosynthesis, how these mechanisms affect natural product structure, and how they could be utilised to introduce structural diversity into the products of engineered biosynthetic pathways.

Biological, chemical, and biochemical strategies for modifying glycopeptide antibiotics

Since the discovery of vancomycin in the 1950s, the glycopeptide antibiotics (GPAs) have been of great interest to the scientific community. These nonribosomally biosynthesized peptides are highly cross-linked, often glycosylated, and inhibit bacterial cell wall assembly by interfering with peptidoglycan synthesis. Interest in glycopeptide antibiotics covers many scientific disciplines, due to their challenging total syntheses, complex biosynthesis pathways, mechanism of action, and high potency. After intense efforts, early enthusiasm has given way to a recognition of the challenges in chemically synthesizing GPAs and of the effort needed to study and modify GPA-producing strains to prepare new GPAs to address the increasing threat of microbial antibiotic resistance. Although the preparation of GPAs, either by modifying the pendant groups such as saccharides or by functionalizing the N- or C-terminal moieties, is readily achievable, the peptide core of these molecules-the GPA aglycone-remains highly challenging to modify. This review aims to present a summary of the results of GPA modification obtained with the three major approaches developed to date: in vivo strain manipulation, total chemical synthesis, and chemoenzymatic synthesis methods.

Genomic analysis of siderophore β-hydroxylases reveals divergent stereocontrol and expands the condensation domain family

Genome mining of biosynthetic pathways streamlines discovery of secondary metabolites but can leave ambiguities in the predicted structures, which must be rectified experimentally. Through coupling the reactivity predicted by biosynthetic gene clusters with verified structures, the origin of the β-hydroxyaspartic acid diastereomers in siderophores is reported herein. Two functional subtypes of nonheme Fe(II)/α-ketoglutarate-dependent aspartyl β-hydroxylases are identified in siderophore biosynthetic gene clusters, which differ in genomic organization-existing either as fused domains (IβH_Asp) at the carboxyl terminus of a nonribosomal peptide synthetase (NRPS) or as stand-alone enzymes (TβH_Asp)-and each directs opposite stereoselectivity of Asp β-hydroxylation. The predictive power of this subtype delineation is confirmed by the stereochemical characterization of β-OHAsp residues in pyoverdine GB-1, delftibactin, histicorrugatin, and cupriachelin. The l-threo (2S, 3S) β-OHAsp residues of alterobactin arise from hydroxylation by the β-hydroxylase domain integrated into NRPS AltH, while l-erythro (2S, 3R) β-OHAsp in delftibactin arises from the stand-alone β-hydroxylase DelD. Cupriachelin contains both l-threo and l-erythro β-OHAsp, consistent with the presence of both types of β-hydroxylases in the biosynthetic gene cluster. A third subtype of nonheme Fe(II)/α-ketoglutarate-dependent enzymes (IβH_His) hydroxylates histidyl residues with l-threo stereospecificity. A previously undescribed, noncanonical member of the NRPS condensation domain superfamily is identified, named the interface domain, which is proposed to position the β-hydroxylase and the NRPS-bound amino acid prior to hydroxylation. Through mapping characterized β-OHAsp diastereomers to the phylogenetic tree of siderophore β-hydroxylases, methods to predict β-OHAsp stereochemistry in silico are realized.

The biosynthetic implications of late-stage condensation domain selectivity during glycopeptide antibiotic biosynthesis

Non-ribosomal peptide synthesis is a highly important biosynthetic pathway for the formation of many secondary metabolites of medical relevance. Due to the challenges associated with the chemical synthesis of many of the products of these assembly lines, understanding the activity and selectivity of non-ribosomal peptide synthetase (NRPS) machineries is an essential step towards the redesign of such machineries to produce new bioactive peptides. Whilst the selectivity of the adenylation domains responsible for amino acid activation during NRPS synthesis has been widely studied, the selectivity of the essential peptide bond forming domains - known as condensation domains - is not well understood. Here, we present the results of a combination of in vitro and in vivo investigations into the final condensation domain from the NRPS machinery that produces the glycopeptide antibiotics (GPAs). Our results show that this condensation domain is tolerant for a range of peptide substrates and even those with unnatural stereochemistry of the peptide C-terminus, which is in contrast to the widely ascribed role of these domains as a stereochemical gatekeeper during NRPS synthesis. Furthermore, we show that this condensation domain has a significant preference for linear peptide substrates over crosslinked peptides, which indicates that the GPA crosslinking cascade targets the heptapeptide bound to the final module of the NRPS machinery and reinforces the role of the unique GPA X-domain in this process. Finally, we demonstrate that the peptide bond forming activity of this condensation domain is coupled to the rate of amino acid activation performed by the subsequent adenylation domain. This is a significant result with implications for NRPS redesign, as it indicates that the rate of amino acid activation of modified adenylation domains must be maintained to prevent unwanted peptide hydrolysis from the NRPS due to a loss of the productive coupling of amino acid selection and peptide bond formation. Taken together, our results indicate that assessing condensation domain activity is a vital step in not only understanding the biosynthetic logic and timing of NRPS-mediated peptide assembly, but also the rules which redesign efforts must obey in order to successfully produce functional, modified NRPS assembly lines.