Identification of (2S,3S)-β-Methyltryptophan as the Real Biosynthetic Intermediate of Antitumor Agent Streptonigrin

L-Proline catalyzed multi-component synthesis of N-pyridyl-tetrahydroisoquinolines and their α-C(sp3)-H oxygenation

Herein, we report an effective multi-component synthesis that starts with readily available starting materials and accesses poly-substituted pyridine derivatives by using L-proline as a benign catalyst. This process uses cyclic amines, aldehydes, and malononitrile in a condensation reaction to produce a variety of pyridine derivatives under mild conditions. Furthermore, depending on the catalysts used, the selective synthesis of an amide and/or an aldehyde functionality is achieved through α-C(sp3)-H oxygenation of the tertiary amine moiety in the resultant pyridine derivatives. The pyridine ring's nitrogen atom plays a crucial role in accelerating C-H oxygenation at the α-position of the tertiary amine, highlighting the synthetic versatility and usefulness of this method.

A Bifunctional Methyltransferase in Biosynthesis of Antitumor Antibiotic Streptonigrin

Streptonigrin (STN, 1) is a highly functionalized aminoquinone alkaloid antibiotic with broad and potent antitumor activity. STN structurally contains four methyl groups belonging to two types: C-methyl group and O-methyl groups. Here, we report the biochemical characterization of the O-methyltransferase StnQ2 that can catalyze both the methylation of a hydroxyl group and a carboxyl group in the biosynthesis of streptonigrin. This work not only provides a new insight into methyltransferases, but also advances the elucidation of the complete biosynthetic pathway of streptonigrin.

Flavoprotein StnP2 Catalyzes the β-Carboline Formation during the Streptonigrin Biosynthesis

β-Carboline (βC) alkaloids constitute a large family of indole alkaloids that exhibit diverse pharmacological properties, such as antitumor, antiviral, antiparasitic, and antimicrobial activities. Here, we report that a flavoprotein StnP2 catalyzes the dehydrogenation at C1-N2 of a tetrahydro-β-carboline (THβC) generating a 3,4-dihydro-β-carboline (DHβC), and the DHβC subsequently undergoes a spontaneous dehydrogenation to βC formation involved in the biosynthesis of the antitumor agent streptonigrin. Biochemical characterization showed that StnP2 catalyzed the highly regio- and stereo-selective dehydrogenation, and StnP2 exhibits promiscuity toward different THβCs. This study provides an alternative kind of enzyme catalyzing the biosynthesis of βC alkaloids and enhances the importance of flavoproteins.

Biocatalytic One-Carbon Transfer – A Review

This review provides an overview of different C1 building blocks as substrates of enzymes, or part of their cofactors, and the resulting­ functionalized products. There is an emphasis on the broad range of possibilities of biocatalytic one-carbon extensions with C1 sources of different oxidation states. The identification of uncommon biosynthetic strategies, many of which might serve as templates for synthetic or biotechnological applications, towards one-carbon extensions is supported by recent genomic and metabolomic progress and hence we refer principally to literature spanning from 2014 to 2020.

1 Introduction

2 Methane, Methanol, and Methylamine

3 Glycine

4 Nitromethane

5 SAM and SAM Ylide

6 Other C1 Building Blocks

7 Formaldehyde and Glyoxylate as Formaldehyde Equivalents

8 Cyanide

9 Formic Acid

10 Formyl-CoA and Oxalyl-CoA

11 Carbon Monoxide

12 Carbon Dioxide

13 Conclusions

A novel streptonigrin type alkaloid from the Streptomyces flocculus CGMCC 4.1223 mutant ΔstnA/Q2

Streptonigrin (STN) is a highly functionalized aminoquinone alkaloid with broad and potent antitumor activities. Previously, the biosynthetic gene cluster of STN was identified in Streptomyces flocculus CGMCC 4.1223, revealing an α/β-hydrolase (StnA) and a methyltransferase (StnQ2). In this work, a double mutant ΔstnA/Q2 was constructed by genetic manipulation and produced a novel derivative of STN, named as streptonigramide. Structure of streptonigramide was established by spectroscopic analyses. Its biosynthetic pathway has been proposed as well.

Identification of the Antifungal Metabolite Chaetoglobosin P From Discosia rubi Using a Cryptococcus neoformans Inhibition Assay: Insights Into Mode of Action and Biosynthesis

Cryptococcus neoformans is an important human pathogen with limited options for treatments. We have interrogated extracts from fungal fermentations to find Cryptococcus-inhibiting natural products using assays for growth inhibition, differential thermosensitivity, and synergy with existing antifungal drugs. Extracts from fermentations of strains of Discosia rubi from eastern Texas showed anticryptococcal bioactivity with preferential activity in agar zone of inhibition assays against C. neoformans at 37°C versus 25°C. Assay-guided fractionation led to the purification and identification of chaetoglobosin P as the active component of these extracts. Genome sequencing of these strains revealed a biosynthetic gene cluster consistent with chaetoglobosin biosynthesis and β-methylation of the tryptophan residue. Proximity of genes of the actin-binding protein twinfilin-1 to the chaetoglobosin P and K gene clusters suggested a possible self-resistance mechanism involving twinfilin-1 which is consistent with the predicted mechanism of action involving interference with the polymerization of the capping process of filamentous actin. A C. neoformans mutant lacking twinfilin-1 was hypersensitive to chaetoglobosin P. Chaetoglobosins also potentiated the effects of amphotericin B and caspofungin on C. neoformans.

Biosynthetic Pathways to Nonproteinogenic α-Amino Acids

Natural nonproteinogenic amino acids vastly outnumber the well-known 22 proteinogenic amino acids. Such amino acids are generated in specialized metabolic pathways. In these pathways, diverse biosynthetic transformations, ranging from isomerizations to the stereospecific functionalization of C-H bonds, are employed to generate structural diversity. The resulting nonproteinogenic amino acids can be integrated into more complex natural products. Here we review recently discovered biosynthetic routes to freestanding nonproteinogenic α-amino acids, with an emphasis on work reported between 2013 and mid-2019.

Structural basis of the mechanism of β-methyl epimerization by enzyme MarH

Structures of free MarH and MarH in complex with l-Trp, the analogue of substrate, were determined and the mechanism of MarH-catalyzed stereospecific β-methyl epimerization was proposed.

A3 foresight network on natural products

Discovery and development of natural products (NPs) have played important roles in the fields of human medicine and other biotechnology fields for the past several decades. Recent genome-mining approaches for the isolation of novel and cryptic NP biosynthetic gene clusters (BGCs) have led to the growing interest in NP research communities including Asian NP researchers from China, Japan, and Korea. Recently, a three-nation government-sponsored program named 'A3 Foresight Network on Chemical and Synthetic Biology of NPs' has been launched with a goal of establishing an Asian hub for NP research-&-personnel exchange program. This brief commentary describes introduction, main researchers, and future perspective of A3 NP network program.

StnK2 catalysing a Pictet–Spengler reaction involved in the biosynthesis of the antitumor reagent streptonigrin

StnK2 is a new Pictet–Spenglerase specifically catalyzing a (1R,3S,4S)-tetrahydro-β-carboline scaffold as the biosynthetic intermediate of the antitumor reagent streptonigrin.

Xantholipin B produced by the stnR inactivation mutant Streptomyces flocculus CGMCC 4.1223 WJN-1

Xantholipin is a polycyclic xanthone antibiotic that exhibits potent cytotoxic and antibacterial activity. In this study, a new xanthone-type antibiotic, xantholipin B (1), was isolated for the first time along with its known derivative, xantholipin (2), from strain WJN-1, an aminotransferase inactivation mutant of the streptonigrin-producer Streptomyces flocculus CGMCC 4.1223. The structure of 1 was established based on spectroscopic analysis and supports the previously proposed biosynthetic pathway as a key intermediate of 2. Moreover, 1 showed 3- to 10-fold greater cytotoxicity than 2 against a select panel of human cancer cell lines. In addition, 1 demonstrated powerful antimicrobial activity against both Gram-positive bacteria and fungi. Importantly, both 1 and 2 inhibited the methicillin-resistant strain Staphylococcus aureus Mu50, with the MIC value of 0.025 μg ml-1. The new structural features of 1 enrich the structural diversity of xantholipin family compounds and shed new light on the structure-activity relationship of 1 as a promising antitumor drug candidate.

Tryptophan Synthase Uses an Atypical Mechanism To Achieve Substrate Specificity

Tryptophan synthase (TrpS) catalyzes the final steps in the biosynthesis of l-tryptophan from l-serine (Ser) and indole-3-glycerol phosphate (IGP). We report that native TrpS can also catalyze a productive reaction with l-threonine (Thr), leading to (2S,3S)-β-methyltryptophan. Surprisingly, β-substitution occurs in vitro with a 3.4-fold higher catalytic efficiency for Ser over Thr using saturating indole, despite a >82000-fold preference for Ser in direct competition using IGP. Structural data identify a novel product binding site, and kinetic experiments clarify the atypical mechanism of specificity: Thr binds efficiently but decreases the affinity for indole and disrupts the allosteric signaling that regulates the catalytic cycle.

Synthesis of β-Branched Tryptophan Analogues Using an Engineered Subunit of Tryptophan Synthase

We report that l-threonine may substitute for l-serine in the β-substitution reaction of an engineered subunit of tryptophan synthase from Pyrococcus furiosus, yielding (2S,3S)-β-methyltryptophan (β-MeTrp) in a single step. The trace activity of the wild-type β-subunit on this substrate was enhanced more than 1000-fold by directed evolution. Structural and spectroscopic data indicate that this increase is correlated with stabilization of the electrophilic aminoacrylate intermediate. The engineered biocatalyst also reacts with a variety of indole analogues and thiophenol for diastereoselective C-C, C-N, and C-S bond-forming reactions. This new activity circumvents the 3-enzyme pathway that produces β-MeTrp in nature and offers a simple and expandable route to preparing derivatives of this valuable building block.