Recent advances in the biosynthetic studies of bacterial organoarsenic natural products.

Covering: 1977 to presentArsenic is widely distributed throughout terrestrial and aquatic environments, mainly in highly toxic inorganic forms. To adapt to environmental inorganic arsenic, bacteria have evolved ubiquitous arsenic metabolic strategies by combining arsenite methylation and related redox reactions, which have been extensively studied. Recent reports have shown that some bacteria have specific metabolic pathways associated with structurally and biologically unique organoarsenic natural products. In this highlight, by exemplifying the cases of oxo-arsenosugars, arsinothricin, and bisenarsan, we summarize recent advances in the identification and biosynthesis of bacterial organoarsenic natural products. We also discuss the potential discoveries of novel arsenic-containing natural products of bacterial origins.

Three neo-Clerodane Diterpenoids from Tinospora cordifolia Stems and Their Arginase Inhibitory Activities.

Three neo-clerodane diterpenoids, including two new tinocordifoliols A (1) and B (2) and one known tinopanoid R (3), were isolated from the ethyl acetate-soluble fraction of the 70% ethanol extract of Tinospora cordifolia stems. The structures were elucidated by various spectroscopic methods, including one dimensional (1D) and 2D-NMR, high resolution-electrospray ionization (HR-ESI)-MS, and electronic circular dichroism (ECD) data. The T. cordifolia extract and all isolated compounds 1-3 possessed arginase I inhibitory activities. Among them, 3 exhibited moderate competitive inhibition of human arginase I (IC50 = 61.9 µM). Furthermore, docking studies revealed that the presence of a ß-substituted furan in 3 may play a key role in the arginase I inhibitory activities.

Insights into Arsenic Secondary Metabolism in Actinomycetes from the Structure and Biosynthesis of Bisenarsan.

The unique bioactivities of arsenic-containing secondary metabolites have been revealed recently, but studies on arsenic secondary metabolism in microorganisms have been extremely limited. Here, we focused on the organoarsenic metabolite with an unknown chemical structure, named bisenarsan, produced by well-studied model actinomycetes and elucidated its structure by combining feeding of the putative biosynthetic precursor (2-hydroxyethyl)arsonic acid to Streptomyces lividans 1326 and detailed NMR analyses. Bisenarsan is the first characterized actinomycete-derived arsenic secondary metabolite and may function as a prototoxin form of an antibacterial agent or be a detoxification product of inorganic arsenic species. We also verified the previously proposed genes responsible for bisenarsan biosynthesis, especially the (2-hydroxyethyl)arsonic acid moiety. Notably, we suggest that a C-As bond in bisenarsan is formed by the intramolecular rearrangement of a pentavalent arsenic species (arsenoenolpyruvate) by the cofactor-independent phosphoglycerate mutase homologue BsnN, that is entirely distinct from the conventional biological C-As bond formation through As-alkylation of trivalent arsenic species by S-adenosylmethionine-dependent enzymes. Our findings will speed up the development of arsenic natural product biosynthesis.

SolS-catalyzed sulfoxidation of labionin to solabionin drives antibacterial activity of solabiomycins.

Ribosomally synthesized and posttranslationally modified peptides (RiPPs) with polar-functionalized fatty acyl groups are newly found lipopeptide-class natural products. We recently employed a combined approach of genome mining and stable isotope labeling and discovered solabiomycins as one of the polar-functionalized fatty-acylated RiPPs (PFARs) from Streptomyces lydicus NBRC13058. The solabiomycins contained a characteristic sulfoxide group in the labionin moiety referred to as the 'solabionin' structure for the RiPP moiety. A previous gene knockout experiment indicated that solS, which encodes a putative flavin adenine dinucleotide (FAD)-nicotinamide adenine dinucleotide (phosphate) (NAD(P))-binding protein, is involved in the sulfoxidation of an alkyl sulfide in the solabionin. In this study, we isolated deoxysolabiomycins A and B from ΔsolS mutant and fully determined the chemical structures using a series of NMR experiments. We also tested the bioactivity of deoxysolabiomycins against Gram-positive bacteria, including Mycolicibacterium smegmatis, and notably found that the sulfoxide is critical for the antibacterial activity. To characterize the catalytic activity of SolS, the recombinant protein was incubated with a putative substrate, deoxysolabiomycins, and the cofactors FAD and NADPH. In vitro reactions demonstrated that SolS catalyzes the sulfoxidation, converting deoxysolabiomycins to solabiomycins.

Complete biosynthetic pathways of ascofuranone and ascochlorin in Acremonium egyptiacum.

Ascofuranone (AF) and ascochlorin (AC) are meroterpenoids produced by various filamentous fungi, including Acremonium egyptiacum (synonym: Acremonium sclerotigenum), and exhibit diverse physiological activities. In particular, AF is a promising drug candidate against African trypanosomiasis and a potential anticancer lead compound. These compounds are supposedly biosynthesized through farnesylation of orsellinic acid, but the details have not been established. In this study, we present all of the reactions and responsible genes for AF and AC biosyntheses in A. egyptiacum, identified by heterologous expression, in vitro reconstruction, and gene deletion experiments with the aid of a genome-wide differential expression analysis. Both pathways share the common precursor, ilicicolin A epoxide, which is processed by the membrane-bound terpene cyclase (TPC) AscF in AC biosynthesis. AF biosynthesis branches from the precursor by hydroxylation at C-16 by the P450 monooxygenase AscH, followed by cyclization by a membrane-bound TPC AscI. All genes required for AC biosynthesis (ascABCDEFG) and a transcriptional factor (ascR) form a functional gene cluster, whereas those involved in the late steps of AF biosynthesis (ascHIJ) are present in another distantly located cluster. AF is therefore a rare example of fungal secondary metabolites requiring multilocus biosynthetic clusters, which are likely to be controlled by the single regulator, AscR. Finally, we achieved the selective production of AF in A. egyptiacum by genetically blocking the AC biosynthetic pathway; further manipulation of the strain will lead to the cost-effective mass production required for the clinical use of AF.

Molecular basis for the P450-catalyzed C-N bond formation in indolactam biosynthesis.

The catalytic versatility of cytochrome P450 monooxygenases is remarkable. Here, we present mechanistic and structural characterizations of TleB from Streptomyces blastmyceticus and its homolog HinD from Streptoalloteichus hindustanus, which catalyze unusual intramolecular C-N bond formation to generate indolactam V from the dipeptide N-methylvalyl-tryptophanol. In vitro analyses demonstrated that both P450s exhibit promiscuous substrate specificity, and modification of the N13-methyl group resulted in the formation of indole-fused 6/5/6 tricyclic products. Furthermore, X-ray crystal structures in complex with substrates and structure-based mutagenesis revealed the intimate structural details of the enzyme reactions. We propose that the generation of a diradical species is critical for the indolactam formation, and that the intramolecular C(sp2)-H amination is initiated by the abstraction of the N1 indole hydrogen. After indole radical repositioning and subsequent removal of the N13 hydrogen, the coupling of the properly-folded diradical leads to the formation of the C4-N13 bond of indolactam.

Exploiting a C-N Bond Forming Cytochrome†P450 Monooxygenase for C-S Bond Formation.

C-S bond formation reactions are widely distributed in the biosynthesis of biologically active molecules, and thus have received much attention over the past decades. Herein, we report intramolecular C-S bond formation by a P450 monooxygenase, TleB, which normally catalyzes a C-N bond formation in teleocidin biosynthesis. Based on the proposed reaction mechanism of TleB, a thiol-substituted substrate analogue was synthesized and tested in the enzyme reaction, which afforded the unprecedented sulfur-containing thio-indolactam V, in addition to an unusual indole-fused 6/5/8-tricyclic product whose structure was determined by the crystalline sponge method. Interestingly, conformational analysis revealed that the SOFA conformation is stable in thio-indolactam V, in sharp contrast to the major TWIST form in indolactam V, resulting in differences in their biological activities.

Molecular basis for the unusual ring reconstruction in fungal meroterpenoid biogenesis.

Trt14 from Aspergillus terreus is involved in unusual skeletal reconstruction during the biosynthesis of the fungal meroterpenoid terretonin. Detailed in vitro characterization revealed that this novel multifunctional enzyme catalyzes not only the D-ring expansion via intramolecular methoxy rearrangement, but also the hydrolysis of the expanded D-ring. The X-ray crystal structures of Trt14, in complex with substrate or product, and two Trt14 homologs, AusH and PrhC from Aspergillus nidulans and Penicillium brasilianum, respectively, indicated similar overall structures to those of the NTF2-like superfamily of enzymes, despite lacking sequence and functional similarities. Moreover, we gained structural insight into the mechanism of the Trt14-catalyzed ring reconstruction from the in-crystal enzyme reaction and site-directed mutagenesis to show that this reaction involves sequential ester bond cleavage and formation. Structural comparison of Trt14 and its homologs suggests that the enzymes in this new superfamily employ similar acid-base chemistry to diversify the molecular architecture of fungal meroterpenoids.

Activation of silent biosynthetic pathways and discovery of novel secondary metabolites in actinomycetes by co-culture with mycolic acid-containing bacteria.

Bacterial secondary metabolites (SM) are rich sources of drug leads, and in particular, numerous metabolites have been isolated from actinomycetes. It was revealed by recent genome sequence projects that actinomycetes harbor much more secondary metabolite-biosynthetic gene clusters (SM-BGCs) than previously expected. Nevertheless, large parts of SM-BGCs in actinomycetes are dormant and cryptic under the standard culture conditions. Therefore, a widely applicable methodology for cryptic SM-BGC activation is required to obtain novel SM. Recently, it was discovered that co-culturing with mycolic-acid-containing bacteria (MACB) widely activated cryptic SM-BGCs in actinomycetes. This "combined-culture" methodology (co-culture methodology using MACB as the partner of actinomycetes) is easily applicable for a broad range of actinomycetes, and indeed, 33 novel SM have been successfully obtained from 12 actinomycetes so far. In this review, the development, application, and mechanistic analysis of the combined-culture method were summarized.

Beijinchromes A-D, Novel Aromatic Compounds Isolated from Nocardia beijingensis NBRC 16342.

Nocardia is a potent bacterial producer of bioactive compounds. From a culture of Nocardia beijingensis NBRC 16342, we isolated four aromatic compounds, named beijinchromes A-D (1-4). We purified them by silica gel chromatography and reverse phase HPLC, and identified their structures by NMR and high resolution (HR)-MS analyses. 1, 2, and 4 are novel 1,2,3,8-tetrasubstituted naphthalenes, and 3 is a novel 3,8-disubstituted ortho-naphthoquinone. 1 and 2 exert antioxidant activities, and 3 exhibits antibiotic activity. Remarkably, the putative biosynthetic gene clusters for 1-4 are widely distributed in 37 Nocardia species, implying their potential to produce this family of compounds and important biological functions of beijinchromes.

Catenulobactins A and B, Heterocyclic Peptides from Culturing Catenuloplanes sp. with a Mycolic Acid-Containing Bacterium.

The production of two new heterocyclic peptide isomers, catenulobactins A (1) and B (2), in cultures of Catenuloplanes sp. RD067331 was significantly increased when it was cocultured with a mycolic acid-containing bacterium. The planar structures and absolute configurations of the catenulobactins were determined based on NMR/MS and chiral-phase GC-MS analyses. Catenulobactin B (2) displayed Fe(III)-chelating activity and moderate cytotoxicity against P388 murine leukemia cells.

Hinduchelins A-D, Noncytotoxic Catechol Derivatives from Streptoalloteichus hindustanus.

Four new catechol derivatives, hinduchelins A-D (1-4), composed of 2,3- dihydroxybenzoic acid, threonine, and decarboxylated phenylalanine, were isolated from Streptoalloteichus hindustanus. Their structures and absolute configurations were elucidated by interpretation of NMR and HRMS data and quantum chemical ECD calculations. The iron-binding properties of the compounds were evaluated by a pyoverdine production assay in Pseudomonas aeruginosa, and compound 4 showed moderate ability to induce pyoverdine production at 50 µM. None of the compounds were cytotoxic toward HL-20, A549, SMMC-7721, MCF-7, and SW-480 tumor cell lines.

Mirilactams C-E, Novel Polycyclic Macrolactams Isolated from Combined-Culture of Actinosynnema mirum NBRC 14064 and Mycolic Acid-Containing Bacterium.

Mycolic acid-containing bacteria (MACB) are known to activate cryptic natural product biosynthesis in co-cultures with actinobacteria. We cultured Actinosynnema mirum NBRC 14064, a producer of the mono-cyclic polyene macrolactam mirilactam A (6), with the MACB Tsukamurella pulmonis TP-B0596. As a result, three novel compounds (mirilactams C-E, 1-3) were produced in the co-culture conditions. Compounds 1-3 were likely derived from 6 by epoxidation and subsequent spontaneous cyclization. The chemical structures and stereochemistries of 1-3 were determined by spectroscopic analyses (NMR and MS), conformational searches in the optimized potentials for liquid simulations-3 (OPLS3) force field, and calculations of electronic circular dichroism (ECD).

Genome-Based Discovery of an Unprecedented Cyclization Mode in Fungal Sesterterpenoid Biosynthesis.

Sesterterpenoids are a group of terpenoid natural products that are primarily biosynthesized via cyclization of the C25 linear substrate geranylfarnesyl pyrophosphate (GFPP). Although the long carbon chain of GFPP in theory allows for many different cyclization patterns, sesterterpenoids are relatively rare species among terpenoids, suggesting that many intriguing sesterterpenoid scaffolds have been overlooked. Meanwhile, the recent identification of the first sesterterpene synthase has allowed the discovery of new sesterterpenoids by the genome mining approach. In this study, we characterized the unusual fungal sesterterpene synthase EvQS and successfully obtained the sesterterpene quiannulatene (1) with a novel and unique highly congested carbon skeleton, which is further oxidized to quiannulatic acid (2) by the cytochrome P450 Qnn-P450. A mechanistic study of its cyclization from GFPP indicated that the biosynthesis employs an unprecedented cyclization mode, which involves three rounds of hydride shifts and two successive C-C bond migrations to construct the 5-6-5-5-5 fused ring system of 1.

Structural Diversification of Lyngbyatoxin†A by Host-Dependent Heterologous Expression of the tleABC Biosynthetic Gene Cluster.

Natural products have enormous structural diversity, yet little is known about how such diversity is achieved in nature. Here we report the structural diversification of a cyanotoxin-lyngbyatoxin A-and its biosynthetic intermediates by heterologous expression of the Streptomyces-derived tleABC biosynthetic gene cluster in three different Streptomyces hosts: S. lividans, S. albus, and S. avermitilis. Notably, the isolated lyngbyatoxin derivatives, including four new natural products, were biosynthesized by crosstalk between the heterologous tleABC gene cluster and the endogenous host enzymes. The simple strategy described here has expanded the structural diversity of lyngbyatoxin A and its biosynthetic intermediates, and provides opportunities for investigation of the currently underestimated hidden biosynthetic crosstalk.

Dietziamides, novel tetramic acid dimers from Dietzia timorensis MZ-3 with antioxidative activity.

Dietziamides A and B, two novel tetramic acid dimers, were isolated from the rare actinomycetes Dietzia timorensis MZ-3 in the course of our HPLC-diode array screening of our collection of terrestrial actinomycetes. The spectroscopic analysis revealed the chemical structures of the first secondary metabolites characterized in the genus Dietzia. Dietziamides A and B showed moderate DPPH (1,1-diphenyl-2-picrylhydrazyl) radical scavenging activities.

Niizalactams A-C, Multicyclic Macrolactams Isolated from Combined Culture of Streptomyces with Mycolic Acid-Containing Bacterium.

A terrestrial bacterium, Streptomyces sp. NZ-6, produced niizalactams A-C (1-3), unprecedented di- and tricyclic macrolactams, by coculturing with the mycolic acid-containing bacterium Tsukamurella pulmonis TP-B0596. Their complete structures, including absolute configurations, were elucidated on the basis of spectroscopic data and chemical derivatization. Their unique skeletons are proposed to be biosynthesized from a common 26-membered macrolactam intermediate by SN2 cyclization or an intramolecular Diels-Alder reaction.

A methyltransferase initiates terpene cyclization in teleocidin B biosynthesis.

Teleocidin B is an indole terpenoid isolated from Streptomyces. Due to its unique chemical structure and ability to activate protein kinase C, it has attracted interest in the areas of organic chemistry and cell biology. Here, we report the identification of genes encoding enzymes for teleocidin B biosynthesis, including nonribosomal peptide synthetase (tleA), P-450 monooxygenase (tleB), prenyltransferase (tleC), and methyltransferase (tleD). The tleD gene, which is located outside of the tleABC cluster on the chromosome, was identified by transcriptional analysis and heterologous expression. Remarkably, TleD not only installs a methyl group on the geranyl moiety of the precursor but also facilitates the nucleophilic attack from the electron-rich indole to the resultant cation, to form the indole-fused six-membered ring. This is the first demonstration of a cation, generated from methylation, triggering successive terpenoid ring closure.

Five pimarane diterpenoids from Kaempferia champasakensis and their cytotoxic activities.

A phytochemical investigation of Kaempferia champasakensis rhizomes led to the isolation of five new pimarane diterpenes, kaempferiols E-I (1-5). The structures of 1-5 were elucidated by extensive spectroscopic techniques, including HR-ESI-MS, UV, IR, and 1D and 2D NMR. The absolute configurations of 1-3 were determined by the modified Mosher method, and those of 4 and 5 were established by ECD calculations. Further cytotoxic assay for all isolated compounds against three human cancer cell lines, lung cancer (A549), cervical cancer (HeLa), and breast cancer (MCF-7) indicated that 5 showed moderate cytotoxic activities against the three tested cell lines, with IC50 values of 44.78, 25.97, and 41.39 Mµ for A549, HeLa, and MCF-7 cell lines, respectively.

A new 3,4-seco-isopimarane and three new isopimarane diterpenoids from Kaempferia champasakensis collected from Vietnam and their cytotoxic activities.

A phytochemical investigation of Kaempferia champasakensis rhizomes led to the isolation of a new 3,4-seco-isopimarane diterpene, kaempferiol A (1), and three new isopimarane diterpenes, kaempferiols B-D (2-4), together with six known isopimarane diterpenes (5-10). The structures of 1-4 were elucidated by extensive spectroscopic analyses, including HR-ESI-MS, UV, IR, and 1D and 2D NMR. The absolute configurations of 1, 3, and 4 were determined by ECD calculations, while that of 2 was established using the modified Mosher method. All isolated compounds were tested for cytotoxicity against three human cancer cell lines, lung cancer (A549), cervical cancer (HeLa), and breast cancer (MCF-7). Among them, 6 and 7 showed moderate cytotoxic activities against the three tested cell lines, with IC50 values ranging from 38.04 to 27.77 µM, respectively.