TryptophanC5-,C6-andC7-Prenylating Enzymes Displaying a Preference for C-6 of the Indole Ring in the Presence of Unnatural Dimethylallyl Diphosphate Analogues

Substrate-Multiplexed Assessment of Aromatic Prenyltransferase Activity

An increasingly effective strategy to identify synthetically useful enzymes is to sample the diversity already present in Nature. Here, we construct and assay a panel of phylogenetically diverse aromatic prenyltransferases (PTs). These enzymes catalyze a variety of C-C bond forming reactions in natural product biosynthesis and are emerging as tools for synthetic chemistry and biology. Homolog screening was further empowered through substrate-multiplexed screening, which provides direct information on enzyme specificity. We perform a head-to-head assessment of the model members of the PT family and further identify homologs with divergent sequences that rival these superb enzymes. This effort revealed the first bacterial O-Tyr PT and, together, provide valuable benchmarking for future synthetic applications of PTs.

Structural insights into the diverse prenylating capabilities of DMATS prenyltransferases

Covering: 2009 up to August 2023Prenyltransferases (PTs) are involved in the primary and the secondary metabolism of plants, bacteria, and fungi, and they are key enzymes in the biosynthesis of many clinically relevant natural products (NPs). The continued biochemical and structural characterization of the soluble dimethylallyl tryptophan synthase (DMATS) PTs over the past two decades have revealed the significant promise that these enzymes hold as biocatalysts for the chemoenzymatic synthesis of novel drug leads. This is a comprehensive review of DMATSs describing the structure-function relationships that have shaped the mechanistic underpinnings of these enzymes, as well as the application of this knowledge to the engineering of DMATSs. We summarize the key findings and lessons learned from these studies over the past 14 years (2009-2023). In addition, we identify current gaps in our understanding of these fascinating enzymes.

Hydroxytryptophan biosynthesis by a family of heme-dependent enzymes in bacteria

Hydroxytryptophan serves as a chemical precursor to a variety of bioactive specialized metabolites, including the human neurotransmitter serotonin and the hormone melatonin. Although the human and animal routes to hydroxytryptophan have been known for decades, how bacteria catalyze tryptophan indole hydroxylation remains a mystery. Here we report a class of tryptophan hydroxylases that are involved in various bacterial metabolic pathways. These enzymes utilize a histidine-ligated heme cofactor and molecular oxygen or hydrogen peroxide to catalyze regioselective hydroxylation on the tryptophan indole moiety, which is mechanistically distinct from their animal counterparts from the nonheme iron enzyme family. Through genome mining, we also identify members that can hydroxylate the tryptophan indole ring at alternative positions. Our results not only reveal a conserved way to synthesize hydroxytryptophans in bacteria but also provide a valuable enzyme toolbox for biocatalysis. As proof of concept, we assemble a highly efficient pathway for melatonin in a bacterial host.

Unlocking New Prenylation Modes: Azaindoles as a New Substrate Class for Indole Prenyltransferases

Aza-substitution, the replacement of aromatic CH groups with nitrogen atoms, is an established medicinal chemistry strategy for increasing solubility, but current methods of accessing functionalized azaindoles are limited. In this work, indole-alkylating aromatic prenyltransferases (PTs) were explored as a strategy to directly functionalize azaindole-substituted analogs of natural products. For this, a series of aza-l-tryptophans (Aza-Trp) featuring N-substitution of every aromatic CH position of the indole ring and their corresponding cyclic Aza-l-Trp-l-proline dipeptides (Aza-CyWP), were synthesized as substrate mimetics for the indole-alkylating PTs FgaPT2, CdpNPT, and FtmPT1. We then demonstrated most of these substrate analogs were accepted by a PT, and the regioselectivity of each prenylation was heavily influenced by the position of the N-substitution. Remarkably, FgaPT2 was found to produce cationic N-prenylpyridinium products, representing not only a new substrate class for indole PTs but also a previously unobserved prenylation mode. The discovery that nitrogenous indole bioisosteres can be accepted by PTs thus provides access to previously unavailable chemical space in the search for bioactive indolediketopiperazine analogs.

Non-Native Site-Selective Enzyme Catalysis

The ability to site-selectively modify equivalent functional groups in a molecule has the potential to streamline syntheses and increase product yields by lowering step counts. Enzymes catalyze site-selective transformations throughout primary and secondary metabolism, but leveraging this capability for non-native substrates and reactions requires a detailed understanding of the potential and limitations of enzyme catalysis and how these bounds can be extended by protein engineering. In this review, we discuss representative examples of site-selective enzyme catalysis involving functional group manipulation and C-H bond functionalization. We include illustrative examples of native catalysis, but our focus is on cases involving non-native substrates and reactions often using engineered enzymes. We then discuss the use of these enzymes for chemoenzymatic transformations and target-oriented synthesis and conclude with a survey of tools and techniques that could expand the scope of non-native site-selective enzyme catalysis.

Biocatalytic Friedel-Crafts Reactions

Friedel-Crafts alkylation and acylation reactions are important methodologies in synthetic and industrial chemistry for the construction of aryl-alkyl and aryl-acyl linkages that are ubiquitous in bioactive molecules. Nature also exploits these reactions in many biosynthetic processes. Much work has been done to expand the synthetic application of these enzymes to unnatural substrates through directed evolution. The promise of such biocatalysts is their potential to supersede inefficient and toxic chemical approaches to these reactions, with mild operating conditions - the hallmark of enzymes. Complementary work has created many bio-hybrid Friedel-Crafts catalysts consisting of chemical catalysts anchored into biomolecular scaffolds, which display many of the same desirable characteristics. In this Review, we summarise these efforts, focussing on both mechanistic aspects and synthetic considerations, concluding with an overview of the frontiers of this field and routes towards more efficient and benign Friedel-Crafts reactions for the future of humankind.

Extension of the Terpene Chemical Space: the Very First Biosynthetic Steps

The structural diversity of terpenes is particularly notable and many studies are carried out to increase it further. In the terpene biosynthetic pathway this diversity is accessible from only two common precursors, i. e. isopentenyl diphosphate (IPP) and dimethylallyl diphosphate (DMAPP). Methods recently developed (e. g. the Terpene Mini Path) have allowed DMAPP and IPP to be obtained from a two-step enzymatic conversion of industrially available isopentenol (IOH) and dimethylallyl alcohol (DMAOH) into their corresponding diphosphates. Easily available IOH and DMAOH analogues then offer quick access to modified terpenoids thus avoiding the tedious chemical synthesis of unnatural diphosphates. The aim of this minireview is to cover the literature devoted to the use of these analogues for widening the accessible terpene chemical space.

Reprogramming Substrate and Catalytic Promiscuity of Tryptophan Prenyltransferases

Prenylation is a process widely prevalent in primary and secondary metabolism, contributing to functionality and chemical diversity in natural systems. Due to their high regio- and chemoselectivities, prenyltransferases are also valuable tools for creation of new compounds by chemoenzymatic synthesis and synthetic biology. Over the last ten years, biochemical and structural investigations shed light on the mechanism and key residues that control the catalytic process, but to date crucial information on how certain prenyltransferases control regioselectivity and chemoselectivity is still lacking. Here, we advance a general understanding of the enzyme family by contributing the first structure of a tryptophan C5-prenyltransferase 5-DMATS. Additinally, the structure of a bacterial tryptophan C6-prenyltransferase 6-DMATS was solved. Analysis and comparison of both substrate-bound complexes led to the identification of key residues for catalysis. Next, site-directed mutagenesis was successfully implemented to not only modify the prenyl donor specificity but also to redirect the prenylation, thereby switching the regioselectivity of 6-DMATS to that of 5-DMATS. The general strategy of structure-guided protein engineering should be applicable to other related prenyltransferases, thus enabling the production of novel prenylated compounds.

C7-Prenylation of Tryptophan-Containing Cyclic Dipeptides by 7-Dimethylallyl Tryptophan Synthase Significantly Increases the Anticancer and Antimicrobial Activities

Prenylated natural products have interesting pharmacological properties and prenylation reactions play crucial roles in controlling the activities of biomolecules. They are difficult to synthesize chemically, but enzymatic synthesis production is a desirable pathway. Cyclic dipeptide prenyltransferase catalyzes the regioselective Friedel-Crafts alkylation of tryptophan-containing cyclic dipeptides. This class of enzymes, which belongs to the dimethylallyl tryptophan synthase superfamily, is known to be flexible to aromatic prenyl receptors, while mostly retaining its typical regioselectivity. In this study, seven tryptophan-containing cyclic dipeptides 1a-7a were converted to their C7-regularly prenylated derivatives 1b-7b in the presence of dimethylallyl diphosphate (DMAPP) by using the purified 7-dimethylallyl tryptophan synthase (7-DMATS) as catalyst. The HPLC analysis of the incubation mixture and the NMR analysis of the separated products showed that the stereochemical structure of the substrate had a great influence on their acceptance by 7-DMATS. Determination of the kinetic parameters proved that cyclo-l-Trp-Gly (1a) consisting of a tryptophanyl and glycine was accepted as the best substrate with a KM value of 169.7 μM and a turnover number of 0.1307 s-1. Furthermore, docking studies simulated the prenyl transfer reaction of 7-DMATS and it could be concluded that the highest affinity between 7-DMATS and 1a. Preliminary results have been clearly shown that prenylation at C7 led to a significant increase of the anticancer and antimicrobial activities of the prenylated derivatives 1b-7b in all the activity test experiment, especially the prenylated product 4b.

Chemoenzymatic synthesis of daptomycin analogs active against daptomycin-resistant strains

Abstract

Daptomycin is a last resort antibiotic for the treatment of infections caused by many Gram-positive bacterial strains, including vancomycin-resistant Enterococcus (VRE) and methicillin- and vancomycin-resistant Staphylococcus aureus (MRSA and VRSA). However, the emergence of daptomycin-resistant strains of S. aureus and Enterococcus in recent years has renewed interest in synthesizing daptomycin analogs to overcome resistance mechanisms. Within this context, three aromatic prenyltransferases have been shown to accept daptomycin as a substrate, and the resulting prenylated analog was shown to be more potent against Gram-positive strains than the parent compound. Consequently, utilizing prenyltransferases to derivatize daptomycin offered an attractive alternative to traditional synthetic approaches, especially given the molecule’s structural complexity. Herein, we report exploiting the ability of prenyltransferase CdpNPT to synthesize alkyl-diversified daptomycin analogs in combination with a library of synthetic non-native alkyl-pyrophosphates. The results revealed that CdpNPT can transfer a variety of alkyl groups onto daptomycin’s tryptophan residue using the corresponding alkyl-pyrophosphates, while subsequent scaled-up reactions suggested that the enzyme can alkylate the N1, C2, C5, and C6 positions of the indole ring. In vitro antibacterial activity assays using 16 daptomycin analogs revealed that some of the analogs displayed 2–80-fold improvements in potency against MRSA, VRE, and daptomycin-resistant strains of S. aureus and Enterococcus faecalis. Thus, along with the new potent analogs, these findings have established that the regio-chemistry of alkyl substitution on the tryptophan residue can modulate daptomycin’s potency. With additional protein engineering to improve the regio-selectivity, the described method has the potential to become a powerful tool for diversifying complex indole-containing molecules.

Key points

• CdpNPT displays impressive donor promiscuity with daptomycin as the acceptor.

• CdpNPT catalyzes N1-, C2-, C5-, and C6-alkylation on daptomycin’s tryptophan residue.

• Differential alkylation of daptomycin’s tryptophan residue modulates its activity.

Electronic supplementary material

The online version of this article (10.1007/s00253-020-10790-x) contains supplementary material, which is available to authorized users.

Acceptor substrate determines donor specificity of an aromatic prenyltransferase: expanding the biocatalytic potential of NphB

Abstract

Aromatic prenyltransferases are known for their extensive promiscuity toward aromatic acceptor substrates and their ability to form various carbon-carbon and carbon-heteroatom bonds. Of particular interest among the prenyltransferases is NphB, whose ability to geranylate cannabinoid precursors has been utilized in several in vivo and in vitro systems. It has therefore been established that prenyltransferases can be utilized as biocatalysts for the generation of useful compounds. However, recent observations of non-native alkyl-donor promiscuity among prenyltransferases indicate the role of NphB in biocatalysis could be expanded beyond geranylation reactions. Therefore, the goal of this study was to elucidate the donor promiscuity of NphB using different acceptor substrates. Herein, we report distinct donor profiles between NphB-catalyzed reactions involving the known substrate 1,6-dihydroxynaphthalene and an FDA-approved drug molecule sulfabenzamide. Furthermore, we report the first instance of regiospecific, NphB-catalyzed N-alkylation of sulfabenzamide using a library of non-native alkyl-donors, indicating the biocatalytic potential of NphB as a late-stage diversification tool.

Key Points

• NphB can utilize the antibacterial drug sulfabenzamide as an acceptor.

• The donor profile of NphB changes dramatically with the choice of acceptor.

• NphB performs a previously unknown regiospecific N-alkylation on sulfabenzamide.

• Prenyltransferases like NphB can be utilized as drug-alkylating biocatalysts.

Electronic supplementary material

The online version of this article (10.1007/s00253-020-10529-8) contains supplementary material, which is available to authorized users.

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.

Bioactive secondary metabolites from the marine-associated fungus Aspergillus terreus

Three new compounds, including a prenylated tryptophan derivative, luteoride E (1), a butenolide derivative, versicolactone G (2), and a linear aliphatic alcohol, (3E,7E)-4,8-dimethyl-undecane-3,7-diene-1,11-diol (3), together with nine known compounds (4-12), were isolated and identified from a coral-associated fungus Aspergillus terreus. Their structures were elucidated by HRESIMS, one- and two-dimensional NMR analysis, and the absolute configuration of 2 was determined by comparison of its electronic circular dichroism (ECD) spectrum with the literature. Structurally, compound 1 featured an unusual (E)-oxime group, which occurred rarely in natural products. Compounds 1-3 were evaluated for the α-glucosidase inhibitory activity, and compound 2 showed potent inhibitory potency with IC₅₀ value of 104.8 ± 9.5 μM, which was lower than the positive control acarbose (IC₅₀ = 154.7 ± 8.1 µM). Additionally, all the isolated compounds were evaluated for the anti-inflammatory activity against NO production, and compounds 1-3, 5-7, and 10 showed significant inhibitory potency with IC₅₀ values ranging from 5.48 to 29.34 μM.

C3-Selective alkenylation of N-acylindoles with unactivated internal alkynes by cooperative nickel/aluminium catalysis

Highly regio- and stereoselective alkenylation of N-acylindoles with unactivated internal alkynes has been accomplished by cooperative nickel/aluminium catalysis to afford C3-alkenylated indoles.

Characterisation of 6-DMATSMo from Micromonospora olivasterospora leading to identification of the divergence in enantioselectivity, regioselectivity and multiple prenylation of tryptophan prenyltransferases

Identification of a new tryptophan prenyltransferase 6-DMATS_Mo and different behaviours of DMATS enzymes for regiospecific mono- and diprenylations of l- and d-tryptophan as well as methylated derivatives.

Impacts and perspectives of prenyltransferases of the DMATS superfamily for use in biotechnology

Prenylated compounds are ubiquitously found in nature and demonstrate interesting biological and pharmacological activities. Prenyltransferases catalyze the attachment of prenyl moieties from different prenyl donors to various acceptors and contribute significantly to the structural and biological diversity of natural products. In the last decade, significant progress has been achieved for the prenyltransferases of the dimethylallyltryptophan synthase (DMATS) superfamily. More than 40 members of these soluble enzymes are identified in microorganisms and characterized biochemically. These enzymes were also successfully used for production of a large number of prenylated derivatives. N1-, C4-, C5-, C6-, and C7-prenylated tryptophan and N1-, C2-, C3-, C4-, and C7-prenylated tryptophan-containing peptides were obtained by using DMATS enzymes as biocatalysts. Tyrosine and xanthone prenyltransferases were used for production of prenylated derivatives of their analogs. More interestingly, the members of the DMATS superfamily demonstrated intriguing substrate and catalytic promiscuity and also used structurally quite different compounds as prenyl acceptors. Prenylated hydroxynaphthalenes, flavonoids, indolocarbazoles, and acylphloroglucinols, which are typical bacterial or plant metabolites, were produced by using several fungal DMATS enzymes. Furthermore, the potential usage of these enzymes was further expanded by using natural or unnatural DMAPP analogs as well as by coexpression with other genes like NRPS and by development of whole cell biocatalyst.