Methyl scanning: total synthesis of demethylasterriquinone B1 and derivatives for identification of sites of interaction with and isolation of its receptor(s)

Peptide-to-Small Molecule: Discovery of Non-Covalent, Active-Site Inhibitors of β-Herpesvirus Proteases

Viral proteases, the key enzymes that regulate viral replication and assembly, are promising targets for antiviral drug discovery. Herpesvirus proteases are enzymes with no crystallographically confirmed noncovalent active-site binders, owing to their shallow and polar substrate-binding pockets. Here, we applied our previously reported "Peptide-to-Small Molecule" strategy to generate novel inhibitors of β-herpesvirus proteases. Rapid selection with a display technology was used to identify macrocyclic peptide 1 bound to the active site of human cytomegalovirus protease (HCMVPro) with high affinity, and pharmacophore queries were defined based on the results of subsequent intermolecular interaction analyses. Membrane-permeable small molecule 19, designed de novo according to this hypothesis, exhibited enzyme inhibitory activity (IC50 = 10-6 to 10-7 M) against β-herpesvirus proteases, and the design concept was proved by X-ray cocrystal analysis.

Hypoiodite-catalysed oxidative homocoupling of arenols and tandem oxidation/cross-coupling of hydroquinones with arenes

We report the hypoiodite-catalyzed oxidative C-C homocoupling of arenols to biarenols or biquinones using aqueous hydrogen peroxide as an oxidant. In addition, by combining hypoiodite catalysis and lipophilic Lewis acid-assisted Brønsted acid catalysis under aqueous conditions, we achieved a tandem oxidation/cross-coupling reaction of hydroquinones with electron-rich arenes. These results highlight the substantial scope of hypoiodite/acid co-catalysis for use in oxidative coupling reactions.

One-Pot Mechanochemical Synthesis of Mono- and Bis-Indolylquinones via Solvent-Free Multiple Bond-Forming Processes

Bis-indolylquinones are fungal natural products endowed with interesting pharmacological properties. Most of the previously described methodologies in solution for the construction of the bis-indolylquinone framework show disadvantages associated with long reaction times and difficult, waste-generating purifications. A one-pot mechanochemical methodology for the synthesis of indolylquinones was developed, starting from indoles and dihaloquinones in the presence of FeCl₃ or p-TsOH as catalysts and Fetizon's reagent as an oxidant. In contrast to solution chemistry, mechanochemical activation allowed the double addition of indole to a quinone substrate in one pot, leading to symmetrical or non-symmetrical bis-indolylquinones via a domino processes comprising up to six steps. In terms of sustainability, the method has several advantages over the solution protocol, including much shorter reaction times, no external heating, one-pot operation, and the absence of chromatography, leading to a drastically better performance in green metrics and demonstrating the application of several principles of green chemistry, in particular principles 2, 3, and 5.

Identification of the first highly selective inhibitor of human lactate dehydrogenase B

Lactate dehydrogenase (LDH) catalyses the conversion of pyruvate to lactate and NADH to NAD⁺; it has two isoforms, LDHA and LDHB. LDHA is a promising target for cancer therapy, whereas LDHB is necessary for basal autophagy and cancer cell proliferation in oxidative and glycolytic cancer cells. To the best of our knowledge, selective inhibitors for LDHB have not yet been reported. Here, we developed a high-throughput mass spectrometry screening system using an LDHB enzyme assay by detecting NADH and NAD⁺. As a result, we identified a small-molecule LDHB selective inhibitor AXKO-0046, an indole derivative. This compound exhibited uncompetitive LDHB inhibition (EC₅₀ = 42 nM). X-ray crystallography revealed that AXKO-0046 bound to the potential allosteric site away from the LDHB catalytic active site, suggesting that targeting the tetramerisation interface of the two dimers is critical for the enzymatic activity. AXKO-0046 and its derivatives can be used to validate LDHB-associated pathways in cancer metabolism.

Methyl Scanning and Revised Binding Mode of 2-Pralidoxime, an Antidote for Nerve Agent Poisoning

Organophosphorus nerve agents (OPNAs) inhibit acetylcholinesterase (AChE) and, despite the Chemical Weapons Convention arms control treaty, continue to represent a threat to both military personnel and civilians. 2-Pralidoxime (2-PAM) is currently the only therapeutic countermeasure approved by the United States Food and Drug Administration for treating OPNA poisoning. However, 2-PAM is not centrally active due to its hydrophilicity and resulting poor blood-brain barrier permeability; hence, these deficiencies warrant the development of more hydrophobic analogs. Specifically, gaps exist in previously published structure activity relationship (SAR) studies for 2-PAM, thereby making it difficult to rationally design novel analogs that are concomitantly more permeable and more efficacious. In this study, we methodically performed a methyl scan on the core pyridinium of 2-PAM to identify ring positions that could tolerate both additional steric bulk and hydrophobicity. Subsequently, SAR-guided molecular docking was used to rationalize hydropathically feasible binding modes for 2-PAM and the reported derivatives. Overall, the data presented herein provide new insights that may facilitate the rational design of more efficacious 2-PAM analogs.

Differential Peptidoglycan Recognition Assay Using Varied Surface Presentations

Bacterial peptidoglycan (PG) is recognized by the human innate immune system to generate an appropriate response. To gain an appreciation of how this essential polymer is sensed, a surface plasmon resonance (SPR) assay using varied PG surface presentation was developed. PG derivatives were synthesized and immobilized on the surface at different positions on the molecule to assess effects of ligand orientation on the binding affinities of NOD-like receptors (NLRs). NLRP1 and NOD2 are cytosolic innate immune proteins known to generate an immune response to PG. Both possess conserved leucine rich repeat domains (LRR) as proposed sites of molecular recognition, though limited biochemical evidence exists regarding the mechanisms of PG recognition. Here direct biochemical evidence for the association of PG fragments to NOD2 and NLRP1 with nanomolar affinity is shown. The orientations in which the fragments were presented on the SPR surface influenced the strength of PG recognition by both NLRs. This assay displays fundamental differences in binding preferences for PG by innate immune receptors and reveals unique recognition mechanisms between the LRRs. Each receptor uses specific ligand structural features to achieve optimal binding, which will be critical information to manipulate these responses and combat diseases.

Positional Analogue Scanning: An Effective Strategy for Multiparameter Optimization in Drug Design

Minimizing the number and duration of design cycles needed to optimize hit or lead compounds into high-quality chemical probes or drug candidates is an ongoing challenge in biomedical research. Small structure modifications to hit or lead compounds can have meaningful impacts on pharmacological profiles due to significant effects on molecular and physicochemical properties and intra- and intermolecular interactions. Rapid pharmacological profiling of an efficiently prepared series of positional analogues stemming from the systematic exchange of methine groups with heteroatoms or other substituents in aromatic or heteroaromatic ring-containing hit or lead compounds is one approach toward minimizing design cycles (e.g., exchange of aromatic or heteroaromatic CH groups with N atoms or CF, CMe, or COH groups). In this Perspective, positional analogue scanning is shown to be an effective strategy for multiparameter optimization in drug design, whereby substantial improvements in a variety of pharmacological parameters can be achieved.

B(C6F5)3-Catalyzed C-C Coupling of 1,4-Naphthoquinones with the C-3 Position of Indole Derivatives in Water

An atom-economical and environmentally benign approach for the synthesis of indole-substituted 1,4-naphthoquinones from indoles and 1,4-naphthoquinones using readily available Lewis acidic B(C₆F₅)₃ in water and with the recycling of water and part of the catalyst is reported. The reaction proceeded through the B(C₆F₅)₃-catalyzed C(sp²)-H and C(sp²)-H bond coupling of 1,4-naphthoquinones with the C-3 position of indole derivatives in water. This methodology provides a facile protocol for the synthesis of some new indole-substituted 1,4-naphthoquinones in satisfactory yields and with a broad substrate scope. When compared to known methods for the synthesis of indole-substituted 1,4-naphthoquinones, this protocol is practical and efficient and does not require a transition-metal catalyst or toxic organic solvents. In addition, we utilized a simple filtration process for complete recycling of the solvent and the part of the catalyst in each reaction cycle.

A free radical alkylation of quinones with olefins

We demonstrated herein an Fe(iii)-mediated radical alkylation of quinones. A wide range of bioactive molecules with quinone motifs can be rapidly synthesized by using readily available and inexpensive NaBH4/NaBD4 with alkenes at room temperature under open flask conditions.

Copper-catalyzed tandem reaction of 2-alkynylanilines with benzoquinones: efficient access to 3-indolylquinones

A simple, mild, catalytic and efficient method for the straightforward synthesis of an interesting class of 2-aryl/alkyl-substituted-3-indolyl quinones in good to high yields is reported for the first time. This atom-efficient method proceeds via copper-catalyzed one-pot sequential intramolecular hydroamination (C-N bond formation) of 2-alkynylanilines followed by oxidative C-C coupling with benzoquinones.

Pharmacological characterization and binding modes of novel racemic and optically active phenylalanine-based antagonists of AMPA receptors

In order to map out molecular determinants for the competitive blockade of AMPA receptor subtypes, a series of racemic aryl-substituted phenylalanines was synthesized and pharmacologically characterized in vitro at native rat ionotropic glutamate receptors. Most of the compounds showed micromolar affinity and preference for AMPA receptors. Individual stereoisomers of selected compounds were further evaluated at recombinant homomeric rat GluA2 and GluA3 receptors. The most potent compound, (-)-2-amino-3-(6-chloro-2',5'-dihydroxy-5-nitro-[1,1'-biphenyl]-3-yl)propanoic acid, the expected R-isomer showing K_i of 1.71 μM at the GluA2 subtype, was found to competitively antagonize GluA2(Q)_i receptors in TEVC electrophysiological experiments (K_b = 2.13 μM). Molecular docking experiments allowed us to compare two alternative antagonist binding modes for the synthesized phenylalanines at the GluA2 binding core, showing the direction for further structural modifications.

Heterogeneously Catalyzed Domino Synthesis of 3-Indolylquinones Involving Direct Oxidative C-C Coupling of Hydroquinones and Indoles

A domino synthesis of 3-indolylquinones was achieved successfully via direct oxidative C-C coupling of hydroquinones with indoles over Ag2O and Fe3O4/povidone-phosphotungstic acid (PVP-PWA) catalysts using H2O2 in tetrahydrofuran at room temperature. Ag2O catalyzed the in situ oxidation of hydroquinone and 3-indolylhydroquinone intermediates, whereas ferrite solid acid, Fe3O4/PVP-PWA, with a 1:4:1 ratio of Fe3O4, PVP, and PWA, catalyzed the activation of quinones. The efficiency of this catalytic domino approach was established by a broad scope of substrates involving a variety of hydroquinones and quinones to give high yields (81-97%) of 3-indolylquinones. Fe3O4/PVP-PWA was separated magnetically, whereas simple filtration could separate Ag2O, both of which could be recycled several times without losing their activities.

Design, synthesis and structure-activity relationships of novel phenylalanine-based amino acids as kainate receptors ligands

A new series of carboxyaryl-substituted phenylalanines was designed, synthesized and pharmacologically characterized in vitro at native rat ionotropic glutamate receptors as well as at cloned homomeric kainate receptors GluK1-GluK3. Among them, six compounds bound to GluK1 receptor subtypes with reasonable affinity (Ki values in the range of 4.9-7.5μM). A structure-activity relationship (SAR) for the obtained series, focused mainly on the pharmacological effect of structural modifications in the 4- and 5-position of the phenylalanine ring, was established. To illustrate the results, molecular docking of the synthesized series to the X-ray structure of GluK1 ligand binding core was performed. The influence of individual substituents at the phenylalanine ring for both the affinity and selectivity at AMPA, GluK1 and GluK3 receptors was analyzed, giving directions for future studies.

Mutant cycle analysis with modified saxitoxins reveals specific interactions critical to attaining high-affinity inhibition of hNaV1.7

Improper function of voltage-gated sodium channels (NaVs), obligatory membrane proteins for bioelectrical signaling, has been linked to a number of human pathologies. Small-molecule agents that target NaVs hold considerable promise for treatment of chronic disease. Absent a comprehensive understanding of channel structure, the challenge of designing selective agents to modulate the activity of NaV subtypes is formidable. We have endeavored to gain insight into the 3D architecture of the outer vestibule of NaV through a systematic structure-activity relationship (SAR) study involving the bis-guanidinium toxin saxitoxin (STX), modified saxitoxins, and protein mutagenesis. Mutant cycle analysis has led to the identification of an acetylated variant of STX with unprecedented, low-nanomolar affinity for human NaV1.7 (hNaV1.7), a channel subtype that has been implicated in pain perception. A revised toxin-receptor binding model is presented, which is consistent with the large body of SAR data that we have obtained. This new model is expected to facilitate subsequent efforts to design isoform-selective NaV inhibitors.

Applications of Bartoli indole synthesis

In 1989, the reaction of vinyl magnesium halides with ortho-substituted nitroarenes leading to indoles was discovered. This reaction is now frequently reported as the "Bartoli reaction" or the "Bartoli indole synthesis" (BIS). It has rapidly become the shortest and most flexible route to 7-substituted indoles, because the classical indole syntheses generally fail in their preparation. The flexibility of the Bartoli reaction is great as it can be extended to heteroaromatic nitro derivatives and can be run on solid support. This review will focus on the use of the Bartoli indole synthesis as the key step in preparations of complex indoles, which appeared in the literature in the last few years.

Plant chemical genomics: gravity sensing and response

The gene families that encode the vesicle trafficking machinery in plants are highly expanded compared to those from protists and animals. As such, classical genetic screens for mutants with lesions in these genes are fraught with issues of redundancy and lethality. A chemical genomics approach can, in theory, circumvent these issues because inhibitory or stimulatory molecules may be applied at any point in development at sublethal concentrations. This chapter describes the protocols for a chemical genomics screen designed to identify components of the plant cell vesicle trafficking machinery. A two-tiered screen was designed where the primary screen assayed for chemicals that modified the gravitropic response, a process that in plant cells is intimately tied to vesicle trafficking; the secondary screen employed fluorescent marker lines that were treated with gravitropic inhibitors or inducers to assay for changes in endomembrane system morphology. We thus identified four compounds by which we can further explore the relationship between gravitropic signal transduction and vesicle trafficking.

AstPT catalyses both reverse N1- and regular C2 prenylation of a methylated bisindolyl benzoquinone

Prenylated bisindolyl benzoquinones exhibit interesting biological activities, such as antidiabetic or anti-HIV activities. A number of these compounds, including asterriquinones, have been isolated from Aspergillus terreus. In this study, we identified two putative genes by genome mining, ATEG_09980 and ATEG_00702, which share high sequence similarity with the known bisindolyl benzoquinone prenyltransferase TdiB from Aspergillus nidulans. The coding sequences were cloned and overexpressed in E. coli. The overproduced recombinant proteins were purified to near homogeneity and used for enzyme assays with asterriquinone D in the presence of dimethylallyl diphosphate. HPLC analysis showed that product formation was only detected in enzyme assays with EAU29429 encoded by ATEG_09980, not in those with EAU39348 encoded by ATEG_00702. Product isolation and structure elucidation by NMR and MS analyses led to identification of N1-reversely and C2-regularly monoprenylated derivatives, as well as N1',N1''reversely, N1'-reversely, C2''-regularly diprenylated derivatives. This proved that EAU29429 functions as an asterriquinone prenyltransferase (AstPT) and indicated the involvement of EAU29429 rather than EAU39348 in the biosynthesis of methylated asterriquinones. Furthermore, incubation of monoprenylated enzyme products with AstPT resulted in the formation of the diprenylated derivatives.

Dual structure-activity relationship of osteoclastogenesis inhibitor methyl gerfelin based on TEG scanning

Methyl gerfelin derivatives, each having an amine-terminated tri(ethylene glycol) linker at the peripheral position, were designed and systematically synthesized. These "TEGylated" derivatives were then subjected to a structure-activity relationship (SAR) study to examine their glyoxalase 1-inhibition activity and binding affinity toward the three binding proteins identified. Among the derivatives synthesized, that with a NH(2)-TEG linker at the C6-methyl group showed the most potent glyoxalase 1-inhibiting activity and glyoxalase 1 selectivity. These results indicated that derivatization at the C6-methyl group would be suitable for the further development of selective glyoxalase 1 inhibitors.

Use of phage display technology for the determination of the targets for small-molecule therapeutics

IMPORTANCE OF THE FIELD

Target discovery of drug-like small-molecules contributes to our understanding of biological phenomena at the molecular level as well as elucidating the mode of action of bioactive compounds. Research in this field is of high value because, in addition to basic observations, the data can be used to directly identify molecular targets or investigate pharmacokinetic characteristics of drugs in clinical use.

AREAS COVERED IN THIS REVIEW

In addition to providing a brief overview of phage display (PD) technology, we discuss screening platforms, different types of phage libraries and the application of this method to the determination of targets for small-molecule therapeutics over the past decade.

WHAT THE READER WILL GAIN

Readers will gain an understanding of the basis of PD technology through successful examples of the use of this method for the determination of targets for small-molecule therapeutics. They will learn what kinds of small-molecules were used to identify their binding partner, what characteristics and drawbacks are present in the use of small-molecule as bait, and what kinds of approaches were introduced in order to improve the technique to overcome the limitations of conventional strategies.

TAKE HOME MESSAGE

A suitable combination of diverse technologies from various different fields can act synergistically to increase throughput and enhance the efficiency of PD technology for the determination of targets for small-molecule therapeutics. The most suitable method for successful target identification of small-molecules of interest using PD technology can often be determined by referring to past examples.

Laccase-catalyzed domino reactions between hydroquinones and cyclic 1,3-dicarbonyls for the regioselective synthesis of substituted p-benzoquinones

Highly substituted p-benzoquinones were obtained in yields ranging from 39% to 98% by laccase-catalyzed domino reactions between hydroquinones and cyclic 1,3-dicarbonyls using aerial oxygen as the oxidant. In almost all reactions bis-adducts with two adjacent 1,3-dicarbonyl substituents on the quinone moiety were formed selectively. The transformations can be regarded as domino oxidation/1,4-addition/oxidation/1,4-addition/oxidation processes. With unsubstituted hydroquinone as the substrate 2,3-disubstituted p-benzoquinones were isolated. Bis-adducts were also formed exclusively upon reaction with monosubstituted hydroquinones. In almost all cases the 2,3,5-trisubstituted p-benzoquinones were obtained. When 2,3-disubstituted hydroquinones were employed as starting materials the 2,3,5,6-tetrasubstituted p-benzoquinones were isolated. The unambiguous structure elucidation of all products has been achieved by NMR spectroscopic methods including spin pattern analysis of the long-range coupled C═O carbons and (13)C satellites analysis in (1)H NMR spectra.

Synthesis and evaluation of arylquinones as BACE1 inhibitors, β-amyloid peptide aggregation inhibitors, and destabilizers of preformed β-amyloid fibrils

BACE1 activity, inhibition of Aβ aggregation, and disaggregation of preformed Aβ fibrils constitute the three major targets in the development of small-molecule lipophilic new drugs for the treatment of Alzheimer's disease (AD). Quinones are widely distributed among natural products and possess relevant and varied biological activities including antitumor and antibiotic, inhibition of HIV-1 reverse transcriptase, antidiabetic, or COX-inhibition, among others. We report herein the interaction of several arylquinones and their derivatives with the amyloidogenic pathway of the amyloid precursor protein processing. Our studies put forward that these compounds are promising candidates in the development of new drugs which are effective simultaneously towards the three major targets of AD.

Regioselective alkylation of heteroaromatic compounds with 3-methyl-2-quinonyl boronic acids

Reactions of heteroaromatic compounds with 3-methyl substituted 2-quinonyl boronic acids proceeded by 1,4-addition followed by spontaneous protodeboronation, leading directly to the Friedel-Crafts alkylation products instead of the commonly observed alkenylation derivatives resulting from quinones. The boronic acid acts as a temporary regiocontroller, making the system a highly reactive quinone equivalent and opening a direct access to 5,5-disubstituted cyclohexene-1,4-diones.

Indole prenylation in alkaloid synthesis

Important biologically active indole alkaloids are decorated with prenyl (3,3-dimethylallyl) and tert-prenyl (1,1-dimethylallyl) groups. Covering the literature until the end of 2010, this review article comprehensively summarises and discusses the currently available technologies of prenylation and tert-prenylation of indoles, which have been applied in natural products total syntheses or could be applied there in the near future. We focus on those procedures which introduce the C(5) units in one step, organised according to the indole position to be functionalised. Key strategies include electrophilic and nucleophilic prenylation and tert-prenylation, prenyl and tert-prenyl rearrangements, transition metal-mediated reactions and enzymatic methods.

Inhibition of heat shock induction of heat shock protein 70 and enhancement of heat shock protein 27 phosphorylation by quercetin derivatives

Inhibitors of heat-induced heat shock protein 70 (HSP70) expression have the potential to enhance the therapeutic effectiveness of heat-induced radiosensitization of tumors. Among known small molecule inhibitors, quercetin has the advantage of being easily modified for structure-activity studies. Herein, we report the ability of five monomethyl and five carbomethoxymethyl derivatives of quercetin to inhibit heat-induced HSP70 expression and enhance HSP27 phosphorylation in human cells. While quercetin and several derivatives inhibit HSP70 induction and enhance HSP27 phosphorylation at Ser78, other analogues selectively inhibit HSP70 induction without enhancing HSP27 phosphorylation that would otherwise aid in cell survival. We also show that good inhibitors of HSP70 induction are also good inhibitors of both CK2 and CamKII, kinases that are known to activate HSP70 expression by phosphorylation of heat shock transcription factor 1. Derivatives that show poor inhibition of either or both kinases are not good inhibitors of HSP70 induction, suggesting that quercetin's effectiveness is due to its ability to inhibit both kinases.

Identification of novel orally available small molecule insulin mimetics

Oral hypoglycemic agents have great potential for the treatment of both type 1 and type 2 diabetes. Here we report the identification of novel, small-molecule, insulin mimetics that activate the insulin receptor (IR) in vivo and in vitro, stimulate the Akt and extracellular signal-regulated kinase pathways downstream of the IR, and mimic the ability of insulin to stimulate glucose uptake, glycogen synthesis, and lipid synthesis in 3T3-L1 adipocytes. However, the compounds do not mimic the mitogenic effect of insulin. In animals, these compounds have oral hypoglycemic effects in both normal C57BL6 mice and diabetic db/db mice. Quantitative structure activity relationship modeling on data from a library of 60 compounds has highlighted structural features that are important for IR agonist activity that can be used to guide design of second and third generation compounds with greater potency and specificity.

Glyceraldehyde 3-phosphate dehydrogenase is a cellular target of the insulin mimic demethylasterriquinone B1

This study was undertaken to identify cellular proteins that bind an orally active natural product insulin mimic. Phage display cloning was used with a biotinylated derivative of this molecule as bait. Among the proteins identified was glyceraldehyde 3-phosphate dehydrogenase (GAPDH), which has recently been shown to affect insulin receptor signaling. Binding data support a role for human GAPDH as another target of the insulin mimic, which could explain its action as a selective insulin receptor modulator.

Parallel synthesis of indolylquinones and their cell-based insulin mimicry

A synthetic route to bis-indolyldihydroxybenzoquinones was adapted for parallel organic synthesis. The route involves selective conjugate addition of an indole to dichlorobenzoquinone promoted by Brønsted acid, followed by a Lewis acid-promoted conjugate addition of a second indole and a final hydrolysis. Methods for high-throughput purification of the products of this synthesis were also developed. Using these methods, we prepared a library whose structures are based on asterriquinone natural products, which have a wide range of biological activities. In this report, the activities of the library members in activation of the insulin receptor on mammalian cells were examined. Novel compounds were discovered that fall outside earlier developed structure-activity relationships for insulin mimics, supporting the value of systematic investigation (inspired by Nature) for the discovery of novel biologically active molecules.

Practical synthesis of 7-prenylindole

7-Prenylindole is a useful building block for natural product and natural product analogue synthesis. While there have been several past syntheses of 7-prenylindole, none of them is very practical for its preparation on scale. Using an aza-Claisen rearrangement as the key step, 7-prenylindole has been prepared in four steps from indoline in 62% overall yield.

A One-Pot Chemoenzymatic Synthesis for the Universal Precursor of Antidiabetes and Antiviral Bis-Indolylquinones

Bis-indolylquinones represent a class of fungal natural products that display antiretroviral, antidiabetes, or cytotoxic bioactivities. Recent advances in Aspergillus genomic mining efforts have led to the discovery of the tdiA-E-gene cluster, which is the first genetic locus dedicated to bis-indolylquinone biosynthesis. We have now genetically and biochemically characterized the enzymes TdiA (bis-indolylquinone synthetase) and TdiD (L-tryptophan:phenylpyruvate aminotransferase), which, together, confer biosynthetic abilities for didemethylasterriquinone D to Aspergillus nidulans. This compound is the universal intermediate for all bis-indolylquinones. In this biochemical study of a bis-indolylquinone synthetase and a fungal natural product transaminase, we present a one-pot chemoenzymatic protocol to generate didemethylasterriquinone D in vitro. As TdiA resembles a nonribosomal peptide synthetase, yet catalyzes carbon-carbon-bond formation, we discuss the implications for peptide synthetase chemistry.