Structure and specificity of a permissive bacterial C-prenyltransferase

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.

Chemoenzymatic Modification of Daptomycin: Aromatic Group Installation on Trp1

Daptomycin is a cyclic lipodepsipeptide antibiotic used to treat infections caused by Gram-positive pathogens, including multi-drug resistant strains such as methicillin-resistant Staphylococcus aureus (MRSA) and vancomycin-resistant enterococci (VRE). The emergence of daptomycin-resistant bacterial strains has renewed interest in generating daptomycin analogs. Previous studies have shown that replacing the tryptophan of daptomycin with aromatic groups can generate analogs with enhanced potency. Additionally, we have demonstrated that aromatic prenyltransferases can attach diverse groups to the tryptophan of daptomycin. Here, we report the use of the prenyltransferase CdpNPT to derivatize the tryptophan of daptomycin with a library of benzylic and heterocyclic pyrophosphates. An analytical-scale study revealed that CdpNPT can transfer various aromatic groups onto daptomycin. Subsequent scaled-up and purified reactions indicated that the enzyme can attach aromatic groups to N1, C2, C5 and C6 positions of Trp1 of daptomycin. In vitro antibacterial activity assays using six of these purified compounds identified aromatic substituted daptomycin analogs show potency against both daptomycin-susceptible and resistant strains of Gram-positive bacteria. These findings suggest that installing aromatic groups on the Trp1 of daptomycin can lead to the generation of potent daptomycin analogs.

The utility of Streptococcus mutans undecaprenol kinase for the chemoenzymatic synthesis of diverse non-natural isoprenoids

Isoprene chemoenzymatic cascades (ICCs) overcome the complexity of natural pathways by leveraging a streamlined two-enzyme cascade, facilitating efficient synthesis of C5-isoprene diphosphate precursors from readily available alcohol derivatives. Despite the documented promiscuity of enzymes in ICCs, exploration of their potential for accessing novel compounds remains limited, and existing methods require additional enzymes for generating longer-chain diphosphates. In this study, we present the utility of Streptococcus mutans undecaprenol kinase (SmUdpK) for the chemoenzymatic synthesis of diverse non-natural isoprenoids. Using a library of 50 synthetic alcohols, we demonstrate that SmUdpK's promiscuity extends to allylic chains as small as four carbons and benzylic alcohols with various substituents. Subsequently, SmUdpK is utilized in an ICC with isopentenyl phosphate kinase and aromatic prenyltransferase to generate multiple non-natural isoprenoids. This work provides evidence that, with proper optimization, SmUdpK can act as the first enzyme in these ICCs, enhancing access to both valuable and novel compounds.

De Novo Discovery of Pseudo-Natural Prenylated Macrocyclic Peptide Ligands

Prenylation of peptides is widely observed in the secondary metabolites of diverse organisms, granting peptides unique chemical properties distinct from proteinogenic amino acids. Discovery of prenylated peptide agents has largely relied on isolation or genome mining of naturally occurring molecules. To devise a platform technology for de novo discovery of artificial prenylated peptides targeting a protein of choice, here we have integrated the thioether-macrocyclic peptide (teMP) library construction/selection technology, so-called RaPID (Random nonstandard Peptides Integrated Discovery) system, with a Trp-C3-prenyltransferase KgpF involved in the biosynthesis of a prenylated natural product. This unique enzyme exhibited remarkably broad substrate tolerance, capable of modifying various Trp-containing teMPs to install a prenylated residue with tricyclic constrained structure. We constructed a vast library of prenylated teMPs and subjected it to in vitro selection against a phosphoglycerate mutase. This selection platform has led to the identification of a pseudo-natural prenylated teMP inhibiting the target enzyme with an IC50 of 30 nM. Importantly, the prenylation was essential for the inhibitory activity, enhanced serum stability, and cellular uptake of the peptide, highlighting the benefits of peptide prenylation. This work showcases the de novo discovery platform for pseudo-natural prenylated peptides, which is readily applicable to other drug targets.

Maramycin, a Cytotoxic Isoquinolinequinone Terpenoid Produced through Heterologous Expression of a Bifunctional Indole Prenyltransferase/Tryptophan Indole-Lyase in S. albidoflavus

Isoquinolinequinones represent an important family of natural alkaloids with profound biological activities. Heterologous expression of a rare bifunctional indole prenyltransferase/tryptophan indole-lyase enzyme from Streptomyces mirabilis P8-A2 in S. albidoflavus J1074 led to the activation of a putative isoquinolinequinone biosynthetic gene cluster and production of a novel isoquinolinequinone alkaloid, named maramycin (1). The structure of maramycin was determined by analysis of spectroscopic (1D/2D NMR) and MS spectrometric data. The prevalence of this bifunctional biosynthetic enzyme was explored and found to be a recent evolutionary event with only a few representatives in nature. Maramycin exhibited moderate cytotoxicity against human prostate cancer cell lines, LNCaP and C4-2B. The discovery of maramycin (1) enriched the chemical diversity of natural isoquinolinequinones and also provided new insights into crosstalk between the host biosynthetic genes and the heterologous biosynthetic genes in generating new chemical scaffolds.

Selective Late-Stage Functionalization of Tryptophan-Containing Peptides To Facilitate Bioorthogonal Tetrazine Ligation

Site-selective modification of complex peptides and the functionalization of their C-H bonds hold great promise for expanding their use in therapeutics and biomedical research. Herein, we leverage the power of late-stage chemoenzymatic catalysis using an indole prenyltransferase (IPT) enzyme and alkyl diphosphates to specifically modify the indole ring of tryptophan in clinically relevant peptides. Furthermore, the installed handle enables bioorthogonal click chemistry through an inverse electron-demand Diels-Alder (IEDDA) reaction with a biotin-conjugated tetrazine probe.

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.

Enzymatic β-elimination in natural product O- and C-glycoside deglycosylation

Biological degradation of natural product glycosides involves, alongside hydrolysis, β-elimination for glycosidic bond cleavage. Here, we discover an O-glycoside β-eliminase (OGE) from Agrobacterium tumefaciens that converts the C3-oxidized O-β-D-glucoside of phloretin (a plant-derived flavonoid) into the aglycone and the 2-hydroxy-3-keto-glycal elimination product. While unrelated in sequence, OGE is structurally homologous to, and shows effectively the same Mn2+ active site as, the C-glycoside deglycosylating enzyme (CGE) from a human intestinal bacterium implicated in β-elimination of 3-keto C-β-D-glucosides. We show that CGE catalyzes β-elimination of 3-keto O- and C-β-D-glucosides while OGE is specific for the O-glycoside substrate. Substrate comparisons and mutagenesis for CGE uncover positioning of aglycone for protonic assistance by the enzyme as critically important for C-glycoside cleavage. Collectively, our study suggests convergent evolution of active site for β-elimination of 3-keto O-β-D-glucosides. C-Glycoside cleavage is a specialized feature of this active site which is elicited by substrate through finely tuned enzyme-aglycone interactions.

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.

Promiscuous Enzymes for Residue-Specific Peptide and Protein Late-Stage Functionalization

The late-stage functionalization of peptides and proteins holds significant promise for drug discovery and facilitates bioorthogonal chemistry. This selective functionalization leads to innovative advances in in vitro and in vivo biological research. However, it is a challenging endeavor to selectively target a certain amino acid or position in the presence of other residues containing reactive groups. Biocatalysis has emerged as a powerful tool for selective, efficient, and economical modifications of molecules. Enzymes that have the ability to modify multiple complex substrates or selectively install nonnative handles have wide applications. Herein, we highlight enzymes with broad substrate tolerance that have been demonstrated to modify a specific amino acid residue in simple or complex peptides and/or proteins at late-stage. The different substrates accepted by these enzymes are mentioned together with the reported downstream bioorthogonal reactions that have benefited from the enzymatic selective modifications.

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.

Structure-guided Mutagenesis Reveals the Catalytic Residue that Controls the Regiospecificity of C6-Indole Prenyltransferases

Indole is a significant structural moiety and functionalization of the C-H bond in indole-containing molecules expands their chemical space, and modifies their properties and/or activities. Indole prenyltransferases (IPTs) catalyze the direct regiospecific installation of prenyl, C5 carbon units, on indole-derived compounds. IPTs have shown relaxed substrate flexibility enabling them to be used as tools for indole functionalization. However, the mechanism by which certain IPTs target a specific carbon position is not fully understood. Herein, we use structure-guided site-directed mutagenesis, in vitro enzymatic reactions, kinetics and structural-elucidation of analogs to verify the key catalytic residues that control the regiospecificity of all characterized regiospecific C6 IPTs. Our results also demonstrate that substitution of PriB_His312 to Tyr leads to the synthesis of analogs prenylated at different positions than C6. This work contributes to understanding of how certain IPTs can access a challenging position in indole-derived compounds.

Discovery, biochemical characterization, and bioengineering of cyanobactin prenyltransferases

Prenylation is a post-translational modification (PTM) widely found in primary and secondary metabolism. This modification can enhance the lipophilicity of molecules, enabling them to interact with lipid membranes more effectively. The prenylation of peptides is often carried out by cyanobactin prenyltransferases (PTases) from cyanobacteria. These enzymes are of interest due to their ability to add prenyl groups to unmodified peptides, thus making them more effective therapeutics through the subsequent acquisition of increased membrane permeability and bioavailability. Herein we review the current knowledge of cyanobactin PTases, focusing on their discovery, biochemistry, and bioengineering, and highlight the potential application of them as peptide alkylation biocatalysts to generate peptide therapeutics.

Prenylation: A Critical Step for Biomanufacturing of Prenylated Aromatic Natural Products

Prenylated aromatic natural products (PANPs) have received much attention due to their biomedical benefits for human health. The prenylation of aromatic natural products (ANPs), which is mainly catalyzed by aromatic prenyltransferases (aPTs), contributes significantly to their structural and functional diversity by providing higher lipophilicity and enhanced bioactivity. aPTs are widely distributed in bacteria, fungi, animals, and plants and play a key role in the regiospecific prenylation of ANPs. Recent studies have greatly advanced our understanding of the characteristics and application of aPTs. In this review, we comment on research progress regarding sources, evolutionary relationships, structural features, reaction mechanism, engineering modification, and application of aPTs. Particular emphasis is also placed on recent advances, challenges, and prospects about applications of aPTs in microbial cell factories for producing PANPs. Generally, this review could provide guidance for using aPTs as robust biocatalytic tools to produce various PANPs with high efficiency.

Catalytic potential of a fungal indole prenyltransferase toward β-carbolines, harmine and harman, and their prenylation effects on antibacterial activity

The prenylation of compounds has attracted much attention, since it often adds bioactivity to non-prenylated compounds. We employed an enzyme assay with CdpNPT, an indole prenyltransferase from Aspergillus fumigatus with two naturally occurring β-carbolines, harmine (3) and harman (4) as prenyl acceptors, in the presence of dimethylallyl diphosphate (DMAPP) as the prenyl donor. The enzyme accepted these two prenyl acceptor substrates to produce 6-(3',3'-dimethylallyl)harmine (5) from 3 and 9-(3',3'-dimethylallyl)harman (6) and 6-(3',3'-dimethylallyl)harman (7) from 4. The X-ray crystal structure analysis of the CdpNPT (38-440) truncated mutant complexed with 4, and docking simulation studies of DMAPP to the crystal structure of the CdpNPT (38-440) mutant, suggested that CdpNPT could employ the two-step prenylation mechanism to produce 7, while the enzyme produced 6 with either one- or two-step prenylation mechanisms. Furthermore, the antibacterial assays revealed that the 3',3'-dimethylallylation of 3 and 4, as well as harmol (1), at C-6 enhanced the activities against Staphylococcus aureus and Bacillus subtilis.

Enzymatic formation of a prenyl β-carboline by a fungal indole prenyltransferase

CdpNPT from Aspergillus fumigatus is a fungal indole prenyltransferase (IPT) with remarkable substrate promiscuity to generate prenylated compounds. Our first investigation of the catalytic potential of CdpNPT against a β-carboline, harmol (1), revealed that the enzyme also accepts 1 as the prenyl acceptor with dimethylallyl diphosphate (DMAPP) as the prenyl donor and selectively prenylates the C-6 position of 1 by the "regular-type" dimethylallylation to produce 6-(3-dimethylallyl)harmol (2). Furthermore, our X-ray crystal structure analysis of the C-His₆-tagged CdpNPT (38-440) truncated mutant complexed with 1 and docking studies of DMAPP to the crystal structure of the CdpNPT (38-440) mutant suggested that CdpNPT could employ the two-step prenylation system to produce 2.

Late-Stage Chemoenzymatic Installation of Hydroxy-Bearing Allyl Moiety on the Indole Ring of Tryptophan-Containing Peptides

The late‐stage functionalization of indole‐ and tryptophan‐containing compounds with reactive moieties facilitates downstream diversification and leads to changes in their biological properties. Here, the synthesis of two hydroxy‐bearing allyl pyrophosphates is described. A chemoenzymatic method is demonstrated which uses a promiscuous indole prenyltransferase enzyme to install a dual reactive hydroxy‐bearing allyl moiety directly on the indole ring of tryptophan‐containing peptides. This is the first report of late‐stage indole modifications with this reactive group.

Structure and Function of a Dual Reductase-Dehydratase Enzyme System Involved in p-Terphenyl Biosynthesis

We report the identification of the ter gene cluster responsible for the formation of the p-terphenyl derivatives terfestatins B and C and echoside B from the Appalachian Streptomyces strain RM-5-8. We characterize the function of TerB/C, catalysts that work together as a dual enzyme system in the biosynthesis of natural terphenyls. TerB acts as a reductase and TerC as a dehydratase to enable the conversion of polyporic acid to a terphenyl triol intermediate. X-ray crystallography of the apo and substrate-bound forms for both enzymes provides additional mechanistic insights. Validation of the TerC structural model via mutagenesis highlights a critical role of arginine 143 and aspartate 173 in catalysis. Cumulatively, this work highlights a set of enzymes acting in harmony to control and direct reactive intermediates and advances fundamental understanding of the previously unresolved early steps in terphenyl biosynthesis.

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.

Bacterial terpenome

Covering: up to mid-2020 Terpenoids, also called isoprenoids, are the largest and most structurally diverse family of natural products. Found in all domains of life, there are over 80 000 known compounds. The majority of characterized terpenoids, which include some of the most well known, pharmaceutically relevant, and commercially valuable natural products, are produced by plants and fungi. Comparatively, terpenoids of bacterial origin are rare. This is counter-intuitive to the fact that recent microbial genomics revealed that almost all bacteria have the biosynthetic potential to create the C5 building blocks necessary for terpenoid biosynthesis. In this review, we catalogue terpenoids produced by bacteria. We collected 1062 natural products, consisting of both primary and secondary metabolites, and classified them into two major families and 55 distinct subfamilies. To highlight the structural and chemical space of bacterial terpenoids, we discuss their structures, biosynthesis, and biological activities. Although the bacterial terpenome is relatively small, it presents a fascinating dichotomy for future research. Similarities between bacterial and non-bacterial terpenoids and their biosynthetic pathways provides alternative model systems for detailed characterization while the abundance of novel skeletons, biosynthetic pathways, and bioactivies presents new opportunities for drug discovery, genome mining, and enzymology.

Regiospecific Synthesis of Calcium-Independent Daptomycin Antibiotics using a Chemoenzymatic Method

Daptomycin (DAP) is a calcium (Ca²⁺ )-dependent FDA-approved antibiotic drug for the treatment of Gram-positive infections. It possesses a complex pharmacophore hampering derivatization and/or synthesis of analogues. To mimic the Ca²⁺ -binding effect, we used a chemoenzymatic approach to modify the tryptophan (Trp) residue of DAP and synthesize kinetically characterized and structurally elucidated regiospecific Trp-modified DAP analogues. We demonstrated that the modified DAPs are several times more active than the parent molecule against antibiotic-susceptible and antibiotic-resistant Gram-positive bacteria. Strikingly, and in contrast to the parent molecule, the DAP derivatives do not rely on calcium or any additional elements for activity.

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.

Indole C6 Functionalization of Tryprostatin B Using Prenyltransferase CdpNPT

Tryprostatin A and B are prenylated, tryptophan-containing, diketopiperazine natural products, displaying cytotoxic activity through different mechanisms of action. The presence of the 6-methoxy substituent on the indole moiety of tryprostatin A was shown to be essential for the dual inhibition of topoisomerase II and tubulin polymerization. However, the inability to perform late-stage modification of the indole ring has limited the structure–activity relationship studies of this class of natural products. Herein, we describe an efficient chemoenzymatic approach for the late-stage modification of tryprostatin B using a cyclic dipeptide N-prenyltransferase (CdpNPT) from Aspergillus fumigatus, which generates novel analogs functionalized with allylic, benzylic, heterocyclic, and diene moieties. Notably, this biocatalytic functionalizational study revealed high selectivity for the indole C6 position. Seven of the 11 structurally characterized analogs were exclusively C6-alkylated, and the remaining four contained predominant C6-regioisomers. Of the 24 accepted substrates, 10 provided >50% conversion and eight provided 20–50% conversion, with the remaining six giving <20% conversion under standard conditions. This study demonstrates that prenyltransferase-based late-stage diversification enables direct access to previously inaccessible natural product analogs.

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.

Broadening the scope of biocatalytic C-C bond formation

The impeccable control over chemo-, site-, and stereoselectivity possible in enzymatic reactions has led to a surge in the development of new biocatalytic methods. Despite carbon-carbon (C-C) bonds providing the central framework for organic molecules, development of biocatalytic methods for their formation has been largely confined to the use of a select few lyases over the last several decades, limiting the types of C-C bond-forming transformations possible through biocatalytic methods. This Review provides an update on the suite of enzymes available for highly selective biocatalytic C-C bond formation. Examples will be discussed in reference to the (1) native activity of enzymes, (2) alteration of activity through protein or substrate engineering for broader applicability, and (3) utility of the biocatalyst for abiotic synthesis.

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.

Enzymatic studies on aromatic prenyltransferases

Aromatic prenyltransferases (PTases), including ABBA-type and dimethylallyl tryptophan synthase (DMATS)-type enzymes from bacteria and fungi, play important role for diversification of the natural products and improvement of the biological activities. For a decade, the characterization of enzymes and enzymatic synthesis of prenylated compounds by using ABBA-type and DMATS-type PTases have been demonstrated. Here, I introduce several examples of the studies on chemoenzymatic synthesis of unnatural prenylated compounds and the enzyme engineering of ABBA-type and DMATS-type PTases.

Structure Determination, Functional Characterization, and Biosynthetic Implications of Nybomycin Metabolites from a Mining Reclamation Site-Associated Streptomyces

We report the isolation and characterization of three new nybomycins (nybomycins B-D, 1-3) and six known compounds (nybomycin, 4; deoxynyboquinone, 5; α-rubromycin, 6; β-rubromycin, 7; γ-rubromycin, 8; and [2α(1E,3E),4β]-2-(1,3-pentadienyl)-4-piperidinol, 9) from the Rock Creek (McCreary County, KY) underground coal mine acid reclamation site isolate Streptomyces sp. AD-3-6. Nybomycin D (3) and deoxynyboquinone (5) displayed moderate (3) to potent (5) cancer cell line cytotoxicity and displayed weak to moderate anti-Gram-(+) bacterial activity, whereas rubromycins 6-8 displayed little to no cancer cell line cytotoxicity but moderate to potent anti-Gram-(+) bacterial and antifungal activity. Assessment of the impact of 3 or 5 cancer cell line treatment on 4E-BP1 phosphorylation, a predictive marker of ROS-mediated control of cap-dependent translation, also revealed deoxynyboquinone (5)-mediated downstream inhibition of 4E-BP1p. Evaluation of 1-9 in a recently established axolotl embryo tail regeneration assay also highlighted the prototypical telomerase inhibitor γ-rubromycin (8) as a new inhibitor of tail regeneration. Cumulatively, this work highlights an alternative nybomycin production strain, a small set of new nybomycin metabolites, and previously unknown functions of rubromycins (antifungal activity and inhibition of tail regeneration) and also provides a basis for revision of the previously proposed nybomycin biosynthetic pathway.

Probing the Substrate Promiscuity of Isopentenyl Phosphate Kinase as a Platform for Hemiterpene Analogue Production

Isoprenoids are a large class of natural products with wide-ranging applications. Synthetic biology approaches to the manufacture of isoprenoids and their new-to-nature derivatives are limited due to the provision in nature of just two hemiterpene building blocks for isoprenoid biosynthesis. To address this limitation, artificial chemo-enzymatic pathways such as the alcohol-dependent hemiterpene (ADH) pathway serve to leverage consecutive kinases to convert exogenous alcohols into pyrophosphates that could be coupled to downstream isoprenoid biosynthesis. To be successful, each kinase in this pathway should be permissive of a broad range of substrates. For the first time, we have probed the promiscuity of the second enzyme in the ADH pathway-isopentenyl phosphate kinase from Thermoplasma acidophilum-towards a broad range of acceptor monophosphates. Subsequently, we evaluate the suitability of this enzyme to provide unnatural pyrophosphates and provide a critical first step in characterizing the rate-limiting steps in the artificial ADH pathway.

Structural Basis of Tryptophan Reverse N-Prenylation Catalyzed by CymD

Indole prenyltransferases catalyze the prenylation of l-tryptophan (l-Trp) and other indoles to produce a diverse set of natural products in bacteria, fungi, and plants, many of which possess useful biological properties. Among this family of enzymes, CymD from Salinispora arenicola catalyzes the reverse N1 prenylation of l-Trp, an unusual reaction given the poor nucleophilicity of the indole nitrogen. CymD utilizes dimethylallyl diphosphate (DMAPP) as the prenyl donor, catalyzing the dissociation of the diphosphate leaving group followed by nucleophilic attack of the indole nitrogen at the tertiary carbon of the dimethylallyl cation. To better understand the structural basis of selective indole N-alkylation reactions in biology, we have determined the X-ray crystal structures of CymD, the CymD-l-Trp complex, and the CymD-l-Trp-DMSPP complex (DMSPP is dimethylallyl S-thiolodiphosphate, an unreactive analogue of DMAPP). The orientation of l-Trp with respect to DMSPP reveals how the active site contour of CymD serves as a template to direct the reverse prenylation of the indole nitrogen. Comparison to PriB, a C6 bacterial indole prenyltransferase, offers further insight regarding the structural basis of regioselective indole prenylation. Isothermal titration calorimetry measurements indicate a synergistic relationship between l-Trp and DMSPP binding. Finally, activity assays demonstrate the selectivity of CymD for l-Trp and indole as prenyl acceptors. Collectively, these data establish a foundation for understanding and engineering the regioselectivity of indole prenylation by members of the prenyltransferase protein family.

Baraphenazines A-G, Divergent Fused Phenazine-Based Metabolites from a Himalayan Streptomyces

The structures and bioactivities of three unprecedented fused 5-hydroxyquinoxaline/alpha-keto acid amino acid metabolites (baraphenazines A-C, 1-3), two unique diastaphenazine-type metabolites (baraphenazines D and E, 4 and 5) and two new phenazinolin-type (baraphenazines F and G, 6 and 7) metabolites from the Himalayan isolate Streptomyces sp. PU-10A are reported. This study highlights the first reported bacterial strain capable of producing diastaphenazine-type, phenazinolin-type, and izumiphenazine A-type metabolites and presents a unique opportunity for the future biosynthetic interrogation of late-stage phenazine-based metabolite maturation.

FgaPT2, a biocatalytic tool for alkyl-diversification of indole natural products

Demonstration of FgaPT2 catalyzed alkyl-diversification of indole containing natural products.

Aromatic prenyltransferases from natural product biosynthetic pathways display relaxed specificity for their aromatic substrates. While a growing body of evidence suggests aromatic prenyltransferases to be more tolerant towards their alkyl-donor substrates, most studies aimed at probing their donor-substrate specificity are limited to only a small set of alkyl pyrophosphate donors, restricting their broader utility as biocatalysts for synthetic applications. Here, we assess the donor substrate specificity of an l-tryptophan C4-prenyltransferase, also known as C4-dimethylallyltryptophan synthase, FgaPT2 from Aspergillus fumigatus, using an array of 34 synthetic unnatural alkyl-pyrophosphate analogues, and demonstrate FgaPT2 can catalyze the transfer of 25 of the 34 non-native alkyl groups from their corresponding synthetic alkyl-pyrophosphate analogues at N1, C3, C4 and C5 position of tryptophan in a normal and reverse manner. The kinetic studies and regio-chemical analysis of the alkyl-l-tryptophan products suggest that the alkyl-donor transfer by FgaPT2 is a function of the stability of the carbocation and the steric factors in the active site of the enzyme. Further, to demonstrate the biocatalytic utility of FgaPT2, this study also highlights the FgaPT2-catalyzed synthesis of a small set of alkyl-diversified indolocarbazole analogues. These results reveal FgaPT2 to be more tolerant to diverse non-native alkyl-donor substrates beyond their known acceptor substrate promiscuity and set the stage for its development as a novel biocatalytic tool for the differential alkylation of natural products for drug discovery and other synthetic applications.

Biocatalysis for terpene-based polymers

Accelerated generation of bio-based materials is vital to replace current synthetic polymers obtained from petroleum with more sustainable options. However, many building blocks available from renewable resources mainly contain unreactive carbon-carbon bonds, which obstructs their efficient polymerization. Herein, we highlight the potential of applying biocatalysis to afford tailored functionalization of the inert carbocyclic core of multicyclic terpenes toward advanced materials. As a showcase, we unlock the inherent monomer reactivity of norcamphor, a bicyclic ketone used as a monoterpene model system in this study, to afford polyesters with unprecedented backbones. The efficiencies of the chemical and enzymatic Baeyer-Villiger transformation in generating key lactone intermediates are compared. The concepts discussed herein are widely applicable for the valorization of terpenes and other cyclic building blocks using chemoenzymatic strategies.

An Artificial Pathway for Isoprenoid Biosynthesis Decoupled from Native Hemiterpene Metabolism

Isoprenoids are constructed in nature using hemiterpene building blocks that are biosynthesized from lengthy enzymatic pathways with little opportunity to deploy precursor-directed biosynthesis. Here, an artificial alcohol-dependent hemiterpene biosynthetic pathway was designed and coupled to several isoprenoid biosynthetic systems, affording lycopene and a prenylated tryptophan in robust yields. This approach affords a potential route to diverse non-natural hemiterpenes and by extension isoprenoids modified with non-natural chemical functionality. Accordingly, the prototype chemo-enzymatic pathway is a critical first step toward the construction of engineered microbial strains for bioconversion of simple scalable building blocks into complex isoprenoid scaffolds.

Puromycins B-E, Naturally Occurring Amino-Nucleosides Produced by the Himalayan Isolate Streptomyces sp. PU-14G

The isolation and structure elucidation of four new naturally occurring amino-nucleoside [puromycins B-E (1-4)] metabolites from a Himalayan isolate ( Streptomyces sp. PU-14-G, isolated from the Bara Gali region of northern Pakistan) is reported. Consistent with prior reports, comparative antimicrobial assays revealed the need for the free 2″-amine for anti-Gram-positive bacteria and antimycobacterial activity. Similarly, comparative cancer cell line cytotoxicity assays highlighted the importance of the puromycin-free 2″-amine and the impact of 3'-nucleoside substitution. These studies extend the repertoire of known naturally occurring puromycins and their corresponding SAR. Notably, 1 represents the first reported naturally occurring bacterial puromycin-related metabolite with a 3'- N-amino acid substitution that differs from the 3'- N-tyrosinyl of classical puromycin-type natural products. This discovery suggests the biosynthesis of 1 in Streptomyces sp. PU-14G may invoke a uniquely permissive amino-nucleoside synthetase and/or multiple synthetases and sets the stage for further studies to elucidate, and potentially exploit, new biocatalysts for puromycin chemoenzymatic diversification.

Eurotiumins A⁻E, Five New Alkaloids from the Marine-Derived Fungus Eurotium sp. SCSIO F452

Three new prenylated indole 2,5-diketopiperazine alkaloids (1–3) with nine known ones (5–13), one new indole alkaloid (4), and one new bis-benzyl pyrimidine derivative (14) were isolated and characterized from the marine-derived fungus Eurotium sp. SCSIO F452. 1 and 2, occurring as a pair of diastereomers, both presented a hexahydropyrrolo[2,3-b]indole skeleton. Their chemical structures, including absolute configurations, were elucidated by 1D and 2D NMR, HRESIMS, quantum chemical calculations of electronic circular dichroism, and single crystal X-ray diffraction experiments. Most isolated compounds were screened for antioxidative potency. Compounds 3, 5, 6, 7, 9, 10, and 12 showed significant radical scavenging activities against DPPH with IC₅₀ values of 13, 19, 4, 3, 24, 13, and 18 µM, respectively. Five new compounds were evaluated for cytotoxic activities.

Self-association of a highly charged arginine-rich cell-penetrating peptide

Small-angle X-ray scattering (SAXS) measurements reveal a striking difference in intermolecular interactions between two short highly charged peptides-deca-arginine (R10) and deca-lysine (K10). Comparison of SAXS curves at high and low salt concentration shows that R10 self-associates, while interactions between K10 chains are purely repulsive. The self-association of R10 is stronger at lower ionic strengths, indicating that the attraction between R10 molecules has an important electrostatic component. SAXS data are complemented by NMR measurements and potentials of mean force between the peptides, calculated by means of umbrella-sampling molecular dynamics (MD) simulations. All-atom MD simulations elucidate the origin of the R10-R10 attraction by providing structural information on the dimeric state. The last two C-terminal residues of R10 constitute an adhesive patch formed by stacking of the side chains of two arginine residues and by salt bridges formed between the like-charge ion pair and the C-terminal carboxyl groups. A statistical analysis of the Protein Data Bank reveals that this mode of interaction is a common feature in proteins.

Bioinspired Indole Prenylation Reactions in Water

Isoprene units derived from dimethylallyl diphosphate (DMAPP) are an important motif in many natural products including terpenoids, carotenoids, steroids, and natural rubber. Understanding the chemical characteristics of DMAPP is an important topic in natural products chemistry, organic chemistry, and biochemistry. We have developed a direct bioinspired indole prenylation reaction using DMAPP or its equivalents as the electrophile in homogeneous aqueous acidic media in the absence of enzyme to provide prenylated indole products. After establishing the bioinspired indole prenylation reaction, this was then used to achieve the synthesis of a series of natural products, namely, N-prenylcyclo-l-tryptophyl-l-proline, tryprostatins, rhinocladins, and terezine D.

Bi- and Tetracyclic Spirotetronates from the Coal Mine Fire Isolate Streptomyces sp. LC-6-2

The structures of 12 new "enantiomeric"-like abyssomicin metabolites (abyssomicins M-X) from Streptomyces sp. LC-6-2 are reported. Of this set, the abyssomicin W (11) contains an unprecedented 8/6/6/6 tetracyclic core, while the bicyclic abyssomicin X (12) represents the first reported naturally occurring linear spirotetronate. Metabolite structures were determined based on spectroscopic data and X-ray crystallography, and Streptomyces sp. LC-6-2 genome sequencing also revealed the corresponding putative biosynthetic gene cluster.