Sequential action of two flavoenzymes, PgaE and PgaM, in angucycline biosynthesis: chemoenzymatic synthesis of gaudimycin C

The Chemistry of Angucyclines

Angucyclines and angucyclinones represent a class of natural compounds that belong to the group of aromatic polyketides. They exhibit a wide array of biological properties, such as antimicrobial, antiviral, and cytotoxic. Their considerable therapeutic potential and diverse scaffolds have attracted many synthetic chemists to devise novel strategies to construct their intricate molecular architecture. Over 300 class members have been isolated from natural sources, mainly from bacterial strains of Streptomyces species. This review highlights recent advancements in their synthesis, such as oxidative cyclization, photooxidation, and metal-catalyzed [4+2]-cycloaddition, which has facilitated the efficient and practical total syntheses of various angucycline and angucyclinone natural products.

Biosynthesis of Atypical Angucyclines Unveils New Ring Rearrangement Reactions Catalyzed by Flavoprotein Monooxygenases

Six new angucycline structures, including spirocyclione A (1), which contains an unusual oxaspiro[5.5]undecane architecture, and its ring-A-cleaved product spirocyclione B (2), were discovered by heterologous expression of a type II polyketide biosynthetic gene cluster captured from a marine actinomycete strain Streptomyces sp. HDN155000. Three flavoprotein monooxygenases are confirmed to be responsible for the oxidative carbon skeleton rearrangements in the biosynthesis of compounds 1 and 2. The obtained compounds showed promising cytotoxicity against different types of cancer cells.

Divergence of Classical and C-Ring-Cleaved Angucyclines: Elucidation of Early Tailoring Steps in Lugdunomycin and Thioangucycline Biosynthesis

Angucyclines are an important group of microbial natural products that display tremendous chemical diversity. Classical angucyclines are composed of a tetracyclic benz[a]anthracene scaffold with one ring attached at an angular orientation. However, in atypical angucyclines, the polyaromatic aglycone is cleaved at A-, B-, or C-rings, leading to structural rearrangements and enabling further chemical variety. Here, we have elucidated the branching points in angucycline biosynthesis leading toward cleavage of the C-ring in lugdunomycin and thioangucycline biosynthesis. We showed that 12-hydroxylation and 6-ketoreduction of UWM6 are shared steps in classical and C-ring-cleaved angucycline pathways, although the bifunctional 6-ketoreductase LugOIIred harbors additional unique 1-ketoreductase activity. We identified formation of the key intermediate 8-O-methyltetrangomycin by the LugN methyltransferase as the branching point toward C-ring-cleaved angucyclines. The final common step in lugdunomycin and thioangucycline biosynthesis is quinone reduction, catalyzed by the 7-ketoreductases LugG and TacO, respectively. In turn, the committing step toward thioangucyclines is 12-ketoreduction catalyzed by TacA, for which no orthologous protein exists on the lugdunomycin pathway. Our results confirm that quinone reductions are early tailoring steps and, therefore, may be mechanistically important for subsequent C-ring cleavage. Finally, many of the tailoring enzymes harbored broad substrate promiscuity, which we utilized in combinatorial enzymatic syntheses to generate the angucyclines SM 196 A and hydranthomycin. We propose that enzyme promiscuity and the competition of many of the enzymes for the same substrates lead to a branching biosynthetic network and formation of numerous shunt products typical for angucyclines rather than a canonical linear metabolic pathway.

Unravelling key enzymatic steps in C-ring cleavage during angucycline biosynthesis

Angucyclines are type II polyketide natural products, often characterized by unusual structural rearrangements through B- or C-ring cleavage of their tetracyclic backbone. While the enzymes involved in B-ring cleavage have been extensively studied, little is known of the enzymes leading to C-ring cleavage. Here, we unravel the function of the oxygenases involved in the biosynthesis of lugdunomycin, a highly rearranged C-ring cleaved angucycline derivative. Targeted deletion of the oxygenase genes, in combination with molecular networking and structural elucidation, showed that LugOI is essential for C12 oxidation and maintaining a keto group at C6 that is reduced by LugOII, resulting in a key intermediate towards C-ring cleavage. An epoxide group is then inserted by LugOIII, and stabilized by the novel enzyme LugOV for the subsequent cleavage. Thus, for the first time we describe the oxidative enzymatic steps that form the basis for a wide range of rearranged angucycline natural products.

Genome-Guided Discovery of Highly Oxygenated Aromatic Polyketides, Saccharothrixins D-M, from the Rare Marine Actinomycete Saccharothrix sp. D09

Angucyclines and angucyclinones are aromatic polyketides with intriguing structures and therapeutic value. Genome mining of the rare marine actinomycete Saccharothrix sp. D09 led to the identification of a type II polyketide synthase biosynthetic gene cluster, sxn, which encodes several distinct subclasses of oxidoreductases, implying that this strain has the potential to produce novel polycyclic aromatic polyketides with unusual redox modifications. The "one strain-many compounds" (OSMAC) strategy and comparative metabolite analysis facilitated the discovery of 20 angucycline derivatives from the D09 strain, including six new highly oxygenated saccharothrixins D-I (1-6), four new glycosylated saccharothrixins J-M (7-10), and 10 known analogues (11-20). Their structures were elucidated based on detailed HRESIMS, NMR spectroscopic, and X-ray crystallographic analysis. With the help of gene disruption and heterologous expression, we proposed their plausible biosynthetic pathways. In addition, compounds 3, 4, and 8 showed antibacterial activity against Helicobacter pylori with MIC values ranging from 16 to 32 μg/mL. Compound 3 also revealed anti-inflammatory activity by inhibiting the production of NO with an IC₅₀ value of 28 μM.

Adaptation to Endophytic Lifestyle Through Genome Reduction by Kitasatospora sp. SUK42

Endophytic actinobacteria offer great potential as a source of novel bioactive compounds. In order to investigate the potential for the production of secondary metabolites by endophytes, we recovered a filamentous microorgansism from the tree Antidesma neurocarpum Miq. After phenotypic analysis and whole genome sequencing we demonstrated that this organism, SUK42 was a member of the actinobacterial genus Kitasatospora. This strain has a small genome in comparison with other type strains of this genus and has lost metabolic pathways associated with Stress Response, Nitrogen Metabolism and Secondary Metabolism. Despite this SUK42 can grow well in a laboratory environment and encodes a core genome that is consistent with other members of the genus. Finally, in contrast to other members of Kitasatospora, SUK42 encodes saccharide secondary metabolite biosynthetic gene clusters, one of which with similarity to the acarviostatin cluster, the product of which displays α-amylase inhibitory activity. As extracts of the host plant demonstrate this inhibitory activity, it suggests that the potential medicinal properties of A. neurocarpum Miq might be provided by the endophytic partner and illustrate the potential for exploitation of endophytes for clinical or industrial uses.

Genome mining of novel rubiginones from Streptomyces sp. CB02414 and characterization of the post-PKS modification steps in rubiginone biosynthesis

Background

Rubiginones belong to the angucycline family of aromatic polyketides, and they have been shown to potentiate the vincristine (VCR)-induced cytotoxicity against VCR-resistant cancer cell lines. However, the biosynthetic gene clusters (BGCs) and biosynthetic pathways for rubiginones have not been reported yet.

Results

In this study, based on bioinformatics analysis of the genome of Streptomyces sp. CB02414, we predicted the functions of the two type II polyketide synthases (PKSs) BGCs. The rub gene cluster was predicted to encode metabolites of the angucycline family. Scale-up fermentation of the CB02414 wild-type strain led to the discovery of eight rubiginones, including five new ones (rubiginones J, K, L, M, and N). Rubiginone J was proposed to be the final product of the rub gene cluster, which features extensive oxidation on the A-ring of the angucycline skeleton. Based on the production profiles of the CB02414 wild-type and the mutant strains, we proposed a biosynthetic pathway for the rubiginones in CB02414.

Conclusions

A genome mining strategy enabled the efficient discovery of new rubiginones from Streptomyces sp. CB02414. Based on the isolated biosynthetic intermediates, a plausible biosynthetic pathway for the rubiginones was proposed. Our research lays the foundation for further studies on the mechanism of the cytochrome P450-catalyzed oxidation of angucyclines and for the generation of novel angucyclines using combinatorial biosynthesis strategies.

Supplementary Information

The online version contains supplementary material available at 10.1186/s12934-021-01681-5.

Functional and Structural Insights into a Novel Promiscuous Ketoreductase of the Lugdunomycin Biosynthetic Pathway

Angucyclines are a structurally diverse class of actinobacterial natural products defined by their varied polycyclic ring systems, which display a wide range of biological activities. We recently discovered lugdunomycin (1), a highly rearranged polyketide antibiotic derived from the angucycline backbone that is synthesized via several yet unexplained enzymatic reactions. Here, we show via in vivo, in vitro, and structural analysis that the promiscuous reductase LugOII catalyzes both a C6 and an unprecedented C1 ketoreduction. This then sets the stage for the subsequent C-ring cleavage that is key to the rearranged scaffolds of 1. The 1.1 Å structures of LugOII in complex with either ligand 8-O-Methylrabelomycin (4) or 8-O-Methyltetrangomycin (5) and of apoenzyme were resolved, which revealed a canonical Rossman fold and a remarkable conformational change during substrate capture and release. Mutational analysis uncovered key residues for substrate access, position, and catalysis as well as specific determinants that control its dual functionality. The insights obtained in this work hold promise for the discovery and engineering of other promiscuous reductases that may be harnessed for the generation of novel biocatalysts for chemoenzymatic applications.

Discovery of Three 22-Membered Macrolides by Deciphering the Streamlined Genome of Mangrove-Derived Streptomyces sp. HM190

Strain HM190, a moderate halophile, was isolated from rhizosphere soil of the mangrove Kandelia obovata in Fugong village, China. The 16S ribosomal RNA (rRNA) gene sequence and the results of phylogenetic analysis revealed that strain HM190 belonged to the genus Streptomyces and had the highest sequence similarity of 99.79% to Streptomyces heilongjiangensis NEAU-W2^T. The complete genome of strain HM190 comprised 7,762,826 bp in a linear chromosome with 71.97% G + C content. According to antiSMASH analysis, a total of 30 biosynthetic gene clusters (BGCs) were predicted to be involved in secondary metabolism, 12 of which were responsible for the production of polyketide- and non-ribosomal peptide-derived secondary metabolites. Gene cluster 5 was responsible for macrolide biosynthesis in a strain-specific 126,331-bp genomic island belonging to the left-arm region. Combined genomics–metabolomics analysis led to the discovery of three 22-membered macrolides (compounds 1–3). Their structures were elucidated by using spectroscopic techniques including high-resolution electrospray ionization mass spectroscopy (HRESIMS) and nuclear magnetic resonance (NMR). The absolute configurations of compounds 1–3 were determined by the X-ray single crystal diffraction and NMR data analysis. All three compounds displayed moderate cytotoxic activities toward tumor cell lines HepG2, A549, and HCT116.

Discovery of "heat shock metabolites" produced by thermotolerant actinomycetes in high-temperature culture

In actinomycetes, many secondary metabolite biosynthetic genes are not expressed under typical laboratory culture conditions and various efforts have been made to activate these dormant genes. In this study, we focused on high-temperature culture. First, we examined the thermotolerance of 3160 actinomycete strains from our laboratory culture collection and selected 57 thermotolerant actinomycetes that grew well at 45 °C. These 57 thermotolerant actinomycetes were cultured for 5 days in liquid medium at both 30 °C and 45 °C. Culture broths were extracted with 1-butanol, and each extract was subjected to LC/MS analysis. The metabolic profiles of each strain were compared between the 30 °C and 45 °C cultures. We found that almost half of these thermotolerant actinomycetes produced secondary metabolites that were detected only in the 45 °C culture. This result suggests that high-temperature culture induces the production of dormant secondary metabolites. These compounds were named "heat shock metabolites (HSMs)." To examine HSM production in more detail, 18 strains were selected at random from the initial 57 strains and cultivated in six different media at 30 °C and 45 °C; as before, metabolic profiles of each strain in each medium were compared between the 30 °C and 45 °C cultures. From this analysis, we found a total of 131 HSMs. We identified several angucycline-related compounds as HSMs from two thermotolerant Streptomyces species. Furthermore, we discovered a new compound, murecholamide, as an HSM from thermotolerant Streptomyces sp. AY2. We propose that high-temperature culture of actinomycetes is a convenient method for activating dormant secondary metabolite biosynthetic genes.

Cloning and identification of the Frigocyclinone biosynthetic gene cluster from Streptomyces griseus strain NTK 97

Frigocyclinone is a novel antibiotic with antibacterial and anticancer activities. It is produced by both Antarctica-derived Streptomyces griseus NTK 97 and marine sponge-associated Streptomyces sp. M7_15. Here, we first report the biosynthetic gene cluster of frigocyclinone in the S. griseus NTK 97. The frigocyclinone gene cluster spans a DNA region of 33-kb which consists of 30 open reading frames (ORFs), encoding minimal type II polyketide synthase, aromatase and cyclase, redox tailoring enzymes, sugar biosynthesis-related enzymes, C-glycosyltransferase, a resistance protein, and three regulatory proteins. Based on the bioinformatic analysis, a biosynthetic pathway for frigocyclinone was proposed. Second, to verify the cloned gene cluster, CRISPR-Cpf1 mediated gene disruption was conducted. Mutant with the disruption of beta-ketoacyl synthase encoding gene frig20 fully loses the ability of producing frigocyclinone, while inactivating the glycosyltransferase gene frig1 leads to the production of key intermediate of anti-MRSA anthraquinone tetrangomycin.

Native roles of Baeyer–Villiger monooxygenases in the microbial metabolism of natural compounds

Baeyer–Villiger monooxygenases function in the primary metabolism of atypical carbon sources, as well as the synthesis of complex microbial metabolites.

Insights into Complex Oxidation during BE-7585A Biosynthesis: Structural Determination and Analysis of the Polyketide Monooxygenase BexE

Cores of aromatic polyketides are essential for their biological activities. Most type II polyketide synthases (PKSs) biosynthesize these core structures involving the minimal PKS, a PKS-associated ketoreductase (KR) and aromatases/cyclases (ARO/CYCs). Oxygenases (OXYs) are rarely involved. BE-7585A is an anticancer polyketide with an angucyclic core. (13)C isotope labeling experiments suggest that its angucyclic core may arise from an oxidative rearrangement of a linear anthracyclinone. Here, we present the crystal structure and functional analysis of BexE, the oxygenase proposed to catalyze this key oxidative rearrangement step that generates the angucyclinone framework. Biochemical assays using various linear anthracyclinone model compounds combined with docking simulations narrowed down the substrate of BexE to be an immediate precursor of aklaviketone, possibly 12-deoxy-aklaviketone. The structural analysis, docking simulations, and biochemical assays provide insights into the role of BexE in BE-7585A biosynthesis and lay the groundwork for engineering such framework-modifying enzymes in type II PKSs.

Targeted activation of silent natural product biosynthesis pathways by reporter-guided mutant selection

The continuously increasing genome sequencing data has revealed numerous cryptic pathways, which might encode novel secondary metabolites with interesting biological activities. However, utilization of this hidden potential has been hindered by the observation that many of these gene clusters remain silent (or poorly expressed) under laboratory conditions. Here we present reporter-guided mutant selection (RGMS) as an effective and widely applicable method for targeted activation of silent gene clusters in the native producers. The strategy takes advantage of genome-scale random mutagenesis for generation of genetic diversity and a reporter-guided selection system for the identification of the desired target-activated mutants. It was first validated in the re-activation of jadomycin biosynthesis in Streptomyces venezuelae ISP5230, where high efficiency of activation was achieved. The same strategy was then applied to a hitherto unactivable pga gene cluster in Streptomyces sp. PGA64 leading to the identification of two new anthraquinone aminoglycosides, gaudimycin D and E.

Angucyclines as signals modulate the behaviors of Streptomyces coelicolor

The angucycline antibiotic jadomycin B (JdB) produced by Streptomyces venezuelae has been found here to induce complex survival responses in Streptomyces coelicolor at subinhibitory concentration. The receptor for JdB was identified as a "pseudo" gamma-butyrolactone receptor, ScbR2, which was shown to bind two previously unidentified target promoters, those of redD (redDp) and adpA (adpAp), thus directly regulating undecylprodigiosin (Red) production and morphological differentiation, respectively. Because AdpA also directly regulates the expression of redD, ScbR2, AdpA, and RedD together form a feed-forward loop controlling both differentiation and Red production phenotypes. Different signal strengths (i.e., JdB concentrations) were shown to induce the two different phenotypes by modulating the relative transcription levels of adpA vs. redD. The induction of morphological differentiation and endogenous antibiotic production by exogenous antibiotic exemplifies an important survival strategy more sophisticated than the induction of antibiotic resistance.

Flavoenzymes: versatile catalysts in biosynthetic pathways

Riboflavin-based coenzymes, tightly bound to enzymes catalyzing substrate oxidations and reductions, enable an enormous range of chemical transformations in biosynthetic pathways. Flavoenzymes catalyze substrate oxidations involving amine and alcohol oxidations and desaturations to olefins, the latter setting up Diels-Alder cyclizations in lovastatin and solanapyrone biosyntheses. Both C(4a) and N(5) of the flavin coenzymes are sites for covalent adduct formation. For example, the reactivity of dihydroflavins with molecular oxygen leads to flavin-4a-OOH adducts which then carry out a diverse range of oxygen transfers, including Baeyer-Villiger type ring expansions, olefin epoxidations, halogenations via transient HOCl generation, and an oxidative Favorskii rerrangement during enterocin assembly.

Tailoring enzymes involved in the biosynthesis of angucyclines contain latent context-dependent catalytic activities

Comparison of homologous angucycline modification enzymes from five closely related Streptomyces pathways (pga, cab, jad, urd, lan) allowed us to deduce the biosynthetic steps responsible for the three alternative outcomes: gaudimycin C, dehydrorabelomycin, and 11-deoxylandomycinone. The C-12b-hydroxylated urdamycin and gaudimycin metabolites appear to be the ancestral representatives from which landomycins and jadomysins have evolved as a result of functional divergence of the ketoreductase LanV and hydroxylase JadH, respectively. Specifically, LanV has acquired affinity for an earlier biosynthetic intermediate resulting in a switch in biosynthetic order and lack of hydroxyls at C-4a and C-12b, whereas in JadH, C-4a/C-12b dehydration has evolved into an independent secondary function replacing C-12b hydroxylation. Importantly, the study reveals that many of the modification enzymes carry several alternative, hidden, or ancestral catalytic functions, which are strictly dependent on the biosynthetic context.

Delineation of gilvocarcin, jadomycin, and landomycin pathways through combinatorial biosynthetic enzymology

The exact sequence of events in biosyntheses of natural products is essential not only to understand and learn from nature's strategies and tricks to assemble complex natural products, but also for yield optimization of desired natural products, and for pathway engineering and muta-synthetic preparation of analogues of bioactive natural products. Biosyntheses of natural products were classically studied applying in vivo experiments, usually by combining incorporation experiments with stable-isotope labeled precursors with cross-feeding experiments of putative intermediates. Later genetic studies were dominant, which consist of gene cluster determination and analysis of gene inactivation experiments. From such studies various biosynthetic pathways were proposed, to a large extent just through in silico analyses of the biosynthetic gene clusters after DNA sequencing. Investigations of the complex biosyntheses of the angucycline group anticancer drugs landomycin, jadomycin and gilvocarcin revealed that in vivo and in silico studies were insufficient to delineate the true biosynthetic sequence of events. Neither was it possible to unambiguously assign enzyme activities, especially where multiple functional enzymes were involved. However, many of the intriguing ambiguities could be solved after in vitro reconstitution of major segments of these pathways, and subsequent systematic variations of the used enzyme mixtures. This method has been recently termed 'combinatorial biosynthetic enzymology'.

Elucidation of post-PKS tailoring steps involved in landomycin biosynthesis

The functional roles of all proposed enzymes involved in the post-PKS redox reactions of the biosynthesis of various landomycin aglycones were thoroughly studied, both in vivo and in vitro. The results revealed that LanM2 acts as a dehydratase and is responsible for concomitant release of the last PKS-tethered intermediate to yield prejadomycin (10). Prejadomycin (10) was confirmed to be a general pathway intermediate of the biosynthesis. Oxygenase LanE and the reductase LanV are sufficient to convert 10 into 11-deoxylandomycinone (5) in the presence of NADH. LanZ4 is a reductase providing reduced flavin (FMNH) co-factor to the partner enzyme LanZ5, which controls all remaining steps. LanZ5, a bifunctional oxygenase-dehydratase, is a key enzyme directing landomycin biosynthesis. It catalyzes hydroxylation at the 11-position preferentially only after the first glycosylation step, and requires the presence of LanZ4. In the absence of such a glycosylation, LanZ5 catalyzes C5,6-dehydration, leading to the production of anhydrolandomycinone (8) or tetrangulol (9). The overall results provided a revised pathway for the biosynthesis of the four aglycones that are found in various congeners of the landomycin group.

Angucyclines: Biosynthesis, mode-of-action, new natural products, and synthesis

Covering: 1997 to 2010. The angucycline group is the largest group of type II PKS-engineered natural products, rich in biological activities and chemical scaffolds. This stimulated synthetic creativity and biosynthetic inquisitiveness. The synthetic studies used five different strategies, involving Diels-Alder reactions, nucleophilic additions, electrophilic additions, transition-metal mediated cross-couplings and intramolecular cyclizations to generate the angucycline frames. Biosynthetic studies were particularly intriguing when unusual framework rearrangements by post-PKS tailoring oxidoreductases occurred, or when unusual glycosylation reactions were involved in decorating the benz[a]anthracene-derived cores. This review follows our previous reviews, which were published in 1992 and 1997, and covers new angucycline group antibiotics published between 1997 and 2010. However, in contrast to the previous reviews, the main focus of this article is on new synthetic approaches and biosynthetic investigations, most of which were published between 1997 and 2010, but go beyond, e.g. for some biosyntheses all the way back to the 1980s, to provide the necessary context of information.

Functional annotation and characterization of 3-hydroxybenzoate 6-hydroxylase from Rhodococcus jostii RHA1

The genome of Rhodococcus jostii RHA1 contains an unusually large number of oxygenase encoding genes. Many of these genes have yet an unknown function, implying that a notable part of the biochemical and catabolic biodiversity of this Gram-positive soil actinomycete is still elusive. Here we present a multiple sequence alignment and phylogenetic analysis of putative R. jostii RHA1 flavoprotein hydroxylases. Out of 18 candidate sequences, three hydroxylases are absent in other available Rhodococcus genomes. In addition, we report the biochemical characterization of 3-hydroxybenzoate 6-hydroxylase (3HB6H), a gentisate-producing enzyme originally mis-annotated as salicylate hydroxylase. R. jostii RHA1 3HB6H expressed in Escherichia coli is a homodimer with each 47kDa subunit containing a non-covalently bound FAD cofactor. The enzyme has a pH optimum around pH 8.3 and prefers NADH as external electron donor. 3HB6H is active with a series of 3-hydroxybenzoate analogues, bearing substituents in ortho- or meta-position of the aromatic ring. Gentisate, the physiological product, is a non-substrate effector of 3HB6H. This compound is not hydroxylated but strongly stimulates the NADH oxidase activity of the enzyme.

Flavoprotein hydroxylase PgaE catalyzes two consecutive oxygen-dependent tailoring reactions in angucycline biosynthesis

A simplified model system composed of a NADPH-dependent flavoprotein hydroxylase PgaE and a short-chain alcohol dehydrogenase/reductase (SDR) CabV was used to dissect a multistep angucycline modification redox cascade into several subreactions in vitro. We demonstrate that the two enzymes are sufficient for the conversion of angucycline substrate 2,3-dehydro-UWM6 to gaudimycin C. The flavoenzyme PgaE is shown to be responsible for two consecutive NADPH- and O(2)-dependent reactions, consistent with the enzyme-catalyzed incorporation of oxygen atoms at C-12 and C-12b in gaudimycin C. The two reactions do not significantly overlap, and the second catalytic cycle is initiated only after the original substrate 2,3-dehydro-UWM6 is nearly depleted. This allowed us to isolate the product of the first reaction at limiting NADPH concentrations and allowed the study of the qualitative and kinetic properties of the separated reactions. Dissection of the reaction cascade also allowed us to establish that the SDR reductase CabV catalyzes the final biosynthetic step, which is closely coupled to the second PgaE reaction. In the absence of CabV, the complete PgaE reaction leads invariably to product degradation, whereas in its presence, the reaction yields the final product, gaudimycin C. The result implies that the C-6 ketoreduction step catalyzed by CabV is required for stabilization of a reactive intermediate. The close relationship between PgaE and CabV would explain previous in vivo observations: why the absence of a reductase gene may result in the lack of C-12b-oxygenated species and, vice versa, why all C-12b-oxygenated angucyclines appear to have undergone reduction at position C-6.

Crystal structure of the cofactor-independent monooxygenase SnoaB from Streptomyces nogalater: implications for the reaction mechanism

SnoaB is a cofactor-independent monooxygenase that catalyzes the conversion of 12-deoxynogalonic acid to nogalonic acid in the biosynthesis of the aromatic polyketide nogalamycin in Streptomyces nogalater. In vitro (18)O(2) experiments establish that the oxygen atom incorporated into the substrate is derived from molecular oxygen. The crystal structure of the enzyme was determined in two different space groups to 1.7 and 1.9 A resolution, respectively. The enzyme displays the ferredoxin fold, with the characteristic beta-strand exchange at the dimer interface. The crystal structures reveal a putative catalytic triad involving two asparagine residues, Asn18 and Asn63, and a water molecule, which may play important roles in the enzymatic reaction. Site-directed mutagenesis experiments, replacing the two asparagines individually by alanine, led to a 100-fold drop in enzymatic activity. Replacement of an invariant tryptophan residue in the active site of the enzyme by phenylalanine also resulted in an enzyme variant with about 1% residual activity. Taken together, our findings are most consistent with a carbanion mechanism where the deprotonated substrate reacts with molecular oxygen via one electron transfer and formation of a caged radical.

Crystal structure of Baeyer-Villiger monooxygenase MtmOIV, the key enzyme of the mithramycin biosynthetic pathway

Baeyer-Villiger monooxygenases (BVMOs), mostly flavoproteins, were shown to be powerful biocatalysts for synthetic organic chemistry applications and were also suggested to play key roles for the biosyntheses of various natural products. Here we present the three-dimensional structure of MtmOIV, a 56 kDa homodimeric FAD- and NADPH-dependent monooxygenase, which catalyzes the key frame-modifying step of the mithramycin biosynthetic pathway and currently the only BVMO proven to react with its natural substrate via a Baeyer-Villiger reaction. MtmOIV's structure was determined by X-ray crystallography using molecular replacement to a resolution of 2.9 A. MtmOIV cleaves a C-C bond, essential for the conversion of the biologically inactive precursor, premithramycin B, into the active drug mithramycin. The MtmOIV structure combined with substrate docking calculations and site-directed mutagenesis experiments identifies several residues that participate in cofactor and substrate binding. Future experimentation aimed at broadening the substrate specificity of the enzyme could facilitate the generation of chemically diverse mithramycin analogues through combinatorial biosynthesis.