Seongsanamides A–D: Antiallergic Bicyclic Peptides from Bacillus safensis KCTC 12796BP

Thioesterases as tools for chemoenzymatic synthesis of macrolactones

Macrocycles are a key functional group that can impart unique properties into molecules. Their synthesis has led to the development of many outstanding chemical methodologies and yet still remains challenging. Thioesterase (TE) domains are frequently responsible for macrocyclization in natural product biosynthesis and provide unique strengths for the enzymatic synthesis of macrocycles. In this feature article, we describe our work to characterize the substrate selectivity of TEs and to use these enzymes as biocatalysts. Our efforts have shown that the linear thioester activated substrates are loaded on TEs with limited substrate selectivity to generate acyl-enzyme intermediates. We show that cyclization of the acyl-enzyme intermediates can be highly selective, with competing hydrolysis of the acyl-enzyme intermediates. The mechanisms controlling TE-mediated macrocyclization versus hydrolysis are a significant unsolved problem in TE biochemistry. The potential of TEs as biocatalysts was demonstrated by using them in the chemoenzymatic total synthesis of macrocyclic depsipeptide natural products. This article highlights the strengths and potential of TEs as biocatalysts as well as their limitations, opening exciting research opportunities including TE engineering to optimize these powerful biocatalysts.

Antimetastatic activity of seongsanamide B in γ-irradiated human lung cancer

Lung cancer, which has a high incidence and mortality rates, often metastasizes and exhibits resistance to radiation therapy. Seongsanamide B has conformational features that suggest it has therapeutic potential; however, its antitumor activity has not yet been reported. We evaluated the possibility of seongsanamide B as a radiation therapy efficiency enhancer to suppress γ-irradiation-induced metastasis in non-small cell lung cancer. Seongsanamide B suppressed non-small cell lung cancer cell migration and invasion caused by γ-irradiation. Furthermore, it suppressed γ-irradiation-induced upregulation of Bcl-XL and its downstream signaling molecules, such as superoxide dismutase 2 (SOD2) and phosphorylated Src, by blocking the nuclear translocation of phosphorylated STAT3. Additionally, seongsanamide B markedly modulated the γ-irradiation-induced upregulation of E-cadherin and vimentin. Consistent with the results obtained in vitro, while seongsanamide B did not affect xenograft tumor growth, it significantly suppressed γ-irradiation-induced metastasis by inhibiting Bcl-XL/SOD2/phosphorylated-Src expression and modulating E-cadherin and vimentin expression in a mouse model. Thus, seongsanamide B may demonstrate potential applicability as a radiation therapy efficiency enhancer for lung cancer treatment.

Synthesis of the Isodityrosine Moiety of Seongsanamide A-D and Its Derivatives

The concise and highly convergent synthesis of the isodityrosine unit of seongsanamide A-D and its derivatives bearing a diaryl ether moiety is described. In this work, the synthetic strategy features palladium-catalyzed C(sp³)-H functionalization and a Cu/ligand-catalyzed coupling reaction. We report a practical protocol for the palladium-catalyzed mono-arylation of β-methyl C(sp³)-H of an alanine derivative bearing a 2-thiomethylaniline auxiliary. The reaction is compatible with a variety of functional groups, providing practical access to numerous β-aryl-α-amino acids; these acids can be converted into various tyrosine and dihydroxyphenylalanine (DOPA) derivatives. Then, a CuI/N,N-dimethylglycine-catalyzed arylation of the already synthesized DOPA derivatives with aryl iodides is described for the synthesis of isodityrosine derivatives.

In Vitro Biochemical Characterization of Excised Macrocyclizing Thioesterase Domains from Non-ribosomal Peptide Synthetases

Characterization of thioesterases (TEs) is an important step in understanding natural product biosynthesis. Studying non-ribosomal peptide synthetase (NRPS) TEs presents a unique set of challenges with specific cloning and expression issues as well as the challenging synthesis of the thioester peptides substrate required for characterization of the TE. In this method, we describe the cloning and expression of NRPS TEs, the synthesis of thioester peptides, and the in vitro biochemical characterization of the enzyme.

Total and Chemoenzymatic Synthesis of Seongsanamide E

The total and chemoenzymatic synthesis of the depsipeptide natural product seongsanamide E, 3, is described. The synthetic C-terminal N-acetylcysteamine thioester of linear natural product 1 was macrolactonized by the excised recombinant purified seongsanamide thioesterase (Sgd-TE) domain, generating 3. Sgd-TE also effects the ring opening of 3. Chemical synthesis provided 3 through a macrolactamization strategy. This work confirms the biosynthesis of 3 and demonstrates the power of Sgd-TE as a biocatalyst.

Recent advances in the discovery of novel marine natural products and mycosporine-like amino acid UV-absorbing compounds

Abstract

Bioactive compounds from marine environments represent a rich source of bioproducts for potential use in medicine and biotechnology. To discover and identify novel marine natural products (MNPs), evaluating diverse biological activities is critical. Increased sensitivity and specificity of omics technologies, especially next-generation high-throughput sequencing combined with liquid chromatography-mass spectrometry and nuclear magnetic resonance, are speeding up the discovery of novel bioactive compounds. Mycosporine-like amino acids (MAAs) isolated from many marine microorganisms are among highly promising MNPs characterized by ultraviolet radiation (UV) absorbing capacities and are recognized as a potential source of ecologically friendly sunscreens. MAAs absorb damaging UV radiation with maximum absorption in the range of 310–360 nm, including both UVA and UVB ranges. MAAs are also characterized by other biological activities such as anti-oxidant, anti-cancer, and anti-inflammatory activities. The application of modern omics approaches promoted some recent developments in our understanding of MAAs’ functional significance and diversity. This review will summarize the various modern tools that could be applied during the identification and characterization of MNPs, including MAAs, to further their innovative applications.

Key points

• New omics technologies are speeding up the discovery of novel bio-products

• The vast diversity of bioactive capacities of marine natural products described

• Marine microorganisms as a source of environmentally friendly sunscreens

In Vitro Metabolism Study of Seongsanamide A in Human Liver Microsomes Using Non-Targeted Metabolomics and Feature-Based Molecular Networking

Seongsanamide A is a bicyclic peptide with an isodityrosine residue discovered in Bacillus safensis KCTC 12796BP which exhibits anti-allergic activity in vitro and in vivo without significant cytotoxicity. The purpose of this study was to elucidate the in vitro metabolic pathway and potential for drug interactions of seongsanamide A in human liver microsomes using non-targeted metabolomics and feature-based molecular networking (FBMN) techniques. We identified four metabolites, and their structures were elucidated by interpretation of high-resolution tandem mass spectra. The primary metabolic pathway associated with seongsanamide A metabolism was hydroxylation and oxidative hydrolysis. A reaction phenotyping study was also performed using recombinant cytochrome P450 isoforms. CYP3A4 and CYP3A5 were identified as the major metabolic enzymes responsible for metabolite formation. Seongsanamide A did not inhibit the cytochrome P450 isoforms commonly involved in drug metabolism (IC₅₀ > 10 µM). These results will contribute to further understanding the metabolism and drug interaction potential of various bicyclic peptides.

Structural and biological aspects of natural bridged macrobicyclic peptides from marine resources

Among peptide-based drugs, naturally occurring bicyclic compounds have been established as molecules with unique therapeutic potential. The diverse pharmacological activities associated with bicyclic peptides from marine tunicates, sponges, and bacteria render them suitable to be employed as effective surrogate between complex and small therapeutic moieties. Bicyclic peptides possess greater conformational rigidity and higher metabolic stability as compared with linear and monocyclic peptides. The antibody-like affinity and specificity of bicyclic peptides enable their binding to the challenging drug targets. Bridged macrobicyclic peptides from natural marine resources represent an underexplored class of molecules that provides promising platforms for drug development owing to their biocompatibility, similarity, and chemical diversity to proteins. The present review explores major marine-derived bicyclic peptides including disulfide-bridged, histidinotyrosine-bridged, or histidinoalanine-bridged macrobicyclic peptides along with their structural characteristics, synthesis, structure-activity relationship, and bioproperties.The comparison of these macrobicyclic congeners with linear/monocyclic peptides along with their therapeutic potential are also briefly discussed.

Nyuzenamides A and B: Bicyclic Peptides with Antifungal and Cytotoxic Activity from a Marine-Derived Streptomyces sp

Two bicyclic peptides, nyuzenamides A (1) and B (2), were discovered from Streptomyces isolated from suspended matter in deep sea water collected in the Sea of Japan. Their structures were determined through nuclear magnetic resonance and mass spectrometry analyses in combination with X-ray crystallography and the chiral-phase gas chromatography-mass spectrometry method to comprise ten amino acid residues containing four unusual amino acids along with aromatic acyl units. Both compounds displayed antifungal activity against pathogenic fungi and cytotoxicity against P388 murine leukemia cells.

Marine natural products

This review covers the literature published between January and December in 2018 for marine natural products (MNPs), with 717 citations (706 for the period January to December 2018) referring to compounds isolated from marine microorganisms and phytoplankton, green, brown and red algae, sponges, cnidarians, bryozoans, molluscs, tunicates, echinoderms, mangroves and other intertidal plants and microorganisms. The emphasis is on new compounds (1554 in 469 papers for 2018), together with the relevant biological activities, source organisms and country of origin. Reviews, biosynthetic studies, first syntheses, and syntheses that led to the revision of structures or stereochemistries, have been included. The proportion of MNPs assigned absolute configuration over the last decade is also surveyed.

Depsipeptide synthesis using a late-stage Ag(i)-promoted macrolactonisation of peptide thioamides

Ag(i)-Triggered activation of the thioamide and attack by the C-terminal carboxylate generates an isoimide intermediate that undergoes an intramolecular acyl transfer to furnish the cyclic depsipeptide.

Suntamide A, a neuroprotective cyclic peptide from Cicadidae Periostracum

Suntamide A (1), a new cyclic peptide, was isolated from Cicadidae Periostracum. The gross structure of 1 was elucidated by detailed analysis of HRMS and 1D/2D NMR spectra, and the absolute configuration was established by C3 Marfey's method. We extended our study to examine biological activity of 1, and found that 1 protected SH-SY5Y cells against rotenone-induced neurotoxicity. This effect of 1 seemed to be attributed to antioxidant induction and protection of mitochondria from rotenone-caused injury. Along with augmentation of the antioxidant system by 1, there was an evident activation of Nrf2, a transcription factor involved in the activation of the antioxidant system. These results indicate that 1 rescued the cells from rotenone-mediated neurotoxicity by enhancing antioxidant capacity via induction of Nrf2, suggesting that the compound could be used as a therapeutic intervention in neurodegenerative diseases such as Parkinson's disease.

Marine Microorganism-Derived Macrolactins Inhibit Inflammatory Mediator Effects in LPS-Induced Macrophage and Microglial Cells by Regulating BACH1 and HO-1/Nrf2 Signals through Inhibition of TLR4 Activation

Recently, many natural products with unique structure and promising pharmacological potential have been reported from marine-derived microorganisms. The macrolactin A (MA), 15-epi-dihydromacrolactin F (DMF) and macrolactin F (MF) were obtained from the culture broth extract of a marine sediment derived microorganism Bacillus sp. HC001. In this study, MA, DMF and MF inhibited the production and expression of proinflammatory mediators of inducible nitric oxide synthase (iNOS) and cyclooxygenase–2 (COX-2) in LPS-stimulated RAW264.7 and BV2 cells. Also, MA, DMF and MF exert anti-inflammatory effects through the expression of heme oxygenase (HO) -1, a stress-inducing enzyme that converts heme to carbon monoxide (CO), iron and biliberdine. Toll-like receptor 4 (TLR4) expressed by lipopolysaccharide (LPS) was inhibited by increased expression of HO-1 transcription factor Nrf2 and down regulation of BTB Domain And CNC Homolog 1 (BACH1), inhibited phosphorylation of Mitogen-activated protein kinase kinase kinase 7 (MAP3K7, TAK1) and nuclear factor kappaB (NF-κB). These results show that MA, DMF and MF effectively inhibited TLR4 by regulating BACH1 and HO-1/Nrf2 signals in LPS-stimulated RAW264.7 and BV2 cells, which suggests the possibility of use as an anti-inflammatory agent.

Total synthesis of antiallergic bicyclic peptide seongsanamide A

The first total synthesis of antiallergic depsipeptide seongsanamide A has been achieved and also the relative and absolute stereochemistry of the natural product has been confirmed.

Survey of natural products reported by Asian research groups in 2018

The new natural products reported in 2018 in peer-reviewed articles in journals with good reputations were reviewed and analyzed. The advances made by Asian research groups in the field of natural products chemistry in 2018 were summarized. Compounds with unique structural features and/or promising bioactivities originating from Asian natural sources were discussed based on their structural classification.