Biocatalytic conversion of avermectin to 4"-oxo-avermectin: characterization of biocatalytically active bacterial strains and of cytochrome p450 monooxygenase enzymes and their genes

Modular polyketide synthase-derived insecticidal agents: from biosynthesis and metabolic engineering to combinatorial biosynthesis for their production

Covering: up to 2022Polyketides derived from actinomycetes are a valuable source of eco-friendly biochemical insecticides. The development of new insecticides is urgently required, as the number of insects resistant to more than one drug is rapidly increasing. Moreover, significant enhancement of the production of such biochemical insecticides is required for economical production. There has been considerable improvement in polyketide insecticidal agent production and development of new insecticides. However, most commercially important biochemical insecticides are synthesized by modular type I polyketide synthases (PKSs), and their structural complexities make chemical modification challenging. A detailed understanding of the biosynthetic mechanisms of potent polyketide insecticides and the structure-activity relationships of their analogs will provide insight into the comprehensive design of new insecticides with improved efficacies. Further metabolic engineering and combinatorial biosynthesis efforts, reinvigorated by synthetic biology, can eventually produce designed analogs in large quantities. This highlight reviews the biosynthesis of representative insecticides produced by modular type I PKSs, such as avermectin, spinosyn, and spectinabilin, and their insecticidal properties. Metabolic engineering and combinatorial biosynthetic strategies for the development of high-yield strains and analogs with insecticidal activities are emphasized, proposing a way to develop a next-generation insecticide.

Reviewing a plethora of oxidative-type reactions catalyzed by whole cells of Streptomyces species

Selective oxidation reactions represent a challenging task for conventional organic chemistry. Whole-cell biocatalysis provides a very convenient, easy to apply method to carry out different selective oxidation reactions including chemo-, regio-, and enantio-selective reactions. Streptomyces species are important biocatalysts as they can catalyze these selective reactions very efficiently owing to the wide diversity of enzymes and enzymatic cascades in their cell niche. In this review, we present and analyze most of the examples reported to date of oxidative reactions catalyzed by Streptomyces species as whole-cell biocatalysts. We discuss 33 different Streptomyces species and strains and the role they play in different oxidative reactions over the past five decades. The oxidative reactions have been classified into seven categories that include: hydroxylation of steroids/non-steroids, asymmetric sulfoxidations, oxidation of aldehydes, multi-step oxidations, oxidative cleavage, and N-oxidations. The role played by Streptomyces species as recombinant hosts catalyzing bio-oxidations has also been highlighted.

Why are Drosophila larvae more sensitive to avermectin than adults?

The insects have different physiological and morphological characteristics in various developmental stages. The difference in the characteristics may be related to the different sensitivity of insects to insecticides. In avermectin resistant strain screening assay, we found that the Drosophila larvae displayed a higher sensitivity to the insecticidal effect of avermectin, compared with adults. In this study, we found that the Drosophila larvae have relatively thicker chitin layer, faster avermectin metabolism and lower P-glycoprotein (P-gp) level, when compared with the adults. Besides, the expression levels of the molecular targets of avermectin, glutamate-gated chloride channel and γ-aminobutyric acid (GABA)-gated chloride channel, are lower in the larval stage than the adult. These results suggested that lower P-gp level in the body especially in brain may be the major reason for the higher sensitivity of Drosophila larvae to the insecticide. In summary, these results shed new light on the concept that different developmental stages of insects display different sensitivity to the same insecticide, which also provided a physiological explanation of the relevant mechanism of the difference of sensitivity of insect at its larval and adult stages to insecticide.

More P450s Are Involved in Secondary Metabolite Biosynthesis in Streptomyces Compared to Bacillus, Cyanobacteria, and Mycobacterium

Unraveling the role of cytochrome P450 monooxygenases (CYPs/P450s), heme-thiolate proteins present in living and non-living entities, in secondary metabolite synthesis is gaining momentum. In this direction, in this study, we analyzed the genomes of 203 Streptomyces species for P450s and unraveled their association with secondary metabolism. Our analyses revealed the presence of 5460 P450s, grouped into 253 families and 698 subfamilies. The CYP107 family was found to be conserved and highly populated in Streptomyces and Bacillus species, indicating its key role in the synthesis of secondary metabolites. Streptomyces species had a higher number of P450s than Bacillus and cyanobacterial species. The average number of secondary metabolite biosynthetic gene clusters (BGCs) and the number of P450s located in BGCs were higher in Streptomyces species than in Bacillus, mycobacterial, and cyanobacterial species, corroborating the superior capacity of Streptomyces species for generating diverse secondary metabolites. Functional analysis via data mining confirmed that many Streptomyces P450s are involved in the biosynthesis of secondary metabolites. This study was the first of its kind to conduct a comparative analysis of P450s in such a large number (203) of Streptomyces species, revealing the P450s’ association with secondary metabolite synthesis in Streptomyces species. Future studies should include the selection of Streptomyces species with a higher number of P450s and BGCs and explore the biotechnological value of secondary metabolites they produce.

Ketonization of Proline Residues in the Peptide Chains of Actinomycins by a 4-Oxoproline Synthase

X-type actinomycins (Acms) contain 4-hydroxyproline (Acm X₀ ) or 4-oxoproline (Acm X₂ ) in their β-pentapeptide lactone rings, whereas their α ring contains proline. We demonstrate that these Acms are formed through asymmetric condensation of Acm half molecules (Acm halves) containing proline with 4-hydroxyproline- or 4-oxoproline-containing Acm halves. In turn, we show-using an artificial Acm half analogue (PPL 1) with proline in its peptide chain-their conversion into the 4-hydroxyproline- and 4-oxoproline-containing Acm halves, PPL 0 and PPL 2, in mycelial suspensions of Streptomyces antibioticus. Two responsible genes of the Acm X biosynthetic gene cluster of S. antibioticus, saacmM and saacmN, encoding a cytochrome P450 monooxygenase (Cyp) and a ferredoxin were identified. After coexpression in Escherichia coli, their gene products converted PPL 1 into PPL 0 and PPL 2 in vivo as well as in situ in permeabilized cell of the transformed E. coli strain in conjunction with the host-encoded ferredoxin reductase in a NADH (NADPH)-dependent manner. saAcmM has high sequence similarity to the Cyp107Z (Ema) family of Cyps, which can convert avermectin B1 into its keto derivative, 4''-oxoavermectin B1. Determination of the structure of saAcmM reveals high similarity to the Ema structure but with significant differences in residues decorating their active sites, which defines saAcmM and its orthologues as a distinct new family of peptidylprolineketonizing Cyp.

Cytochromes P450 for natural product biosynthesis in Streptomyces: sequence, structure, and function

Covering: up to January 2017Cytochrome P450 enzymes (P450s) are some of the most exquisite and versatile biocatalysts found in nature. In addition to their well-known roles in steroid biosynthesis and drug metabolism in humans, P450s are key players in natural product biosynthetic pathways. Natural products, the most chemically and structurally diverse small molecules known, require an extensive collection of P450s to accept and functionalize their unique scaffolds. In this review, we survey the current catalytic landscape of P450s within the Streptomyces genus, one of the most prolific producers of natural products, and comprehensively summarize the functionally characterized P450s from Streptomyces. A sequence similarity network of >8500 P450s revealed insights into the sequence-function relationships of these oxygen-dependent metalloenzymes. Although only ∼2.4% and <0.4% of streptomycete P450s have been functionally and structurally characterized, respectively, the study of streptomycete P450s involved in the biosynthesis of natural products has revealed their diverse roles in nature, expanded their catalytic repertoire, created structural and mechanistic paradigms, and exposed their potential for biomedical and biotechnological applications. Continued study of these remarkable enzymes will undoubtedly expose their true complement of chemical and biological capabilities.

Target-specific identification and characterization of the putative gene cluster for brasilinolide biosynthesis revealing the mechanistic insights and combinatorial synthetic utility of 2-deoxy-l-fucose biosynthetic enzymes

From Nocardia was cloned and functionally characterized a giant gene cluster for biosyntheses of brasilinolides as potent immunosuppressive and anticancer agents.

Biosynthesis of the anti-infective marformycins featuring pre-NRPS assembly line N-formylation and O-methylation and post-assembly line C-hydroxylation chemistries

The biosynthetic gene cluster governing production of anti-infective marformycins was identified from deep sea-derived Streptomyces drozdowiczii SCSIO 10141. The putative mfn gene cluster (45 kb, 20 orfs) was found to encode six NRPSs and related proteins for cyclodepsipeptide core construction (mfnCDEFKL), a methionyl-tRNA formyltransferase (mfnA), a SAM-dependent methyltransferase (mfnG), and a cytochrome P450 monooxygenase for piperazic acid moiety hydroxylation (mfnN); notably, only MfnN uses an intact cyclodepsipeptide intermediate as its substrate.

Whole cell bioconversion of vitamin D3 to calcitriol using Pseudonocardia sp. KCTC 1029BP

Calcitriol is an important drug used for treating osteoporosis, which can be produced from vitamin D3. The current method of producing calcitriol from vitamin D3 during cultivation of microbial cells results in low yields of calcitriol and high purification costs. Therefore, in this study, the steps of cell cultivation and bioconversion of vitamin D3 to calcitriol were separated. Cells of Pseudonocardia sp. KCTC 1029BP were utilized as a whole cell catalyst to produce a high level and yield of calcitriol from vitamin D3. In addition, the effects of bioconversion buffers, cyclodextrins, and metal salts on the production of calcitriol were comparatively examined and selected for incorporation in the bioconversion medium, and their compositions were statistically optimized. The optimal bioconversion medium was determined as consisting of 15 mM Trizma base, 25 mM sodium succinate, 2 mM MgSO4, 0.08% β-cyclodextrin, 0.1% NaCl, 0.2% K2HPO4, and 0.03% MnCl2. Using this optimal bioconversion medium, 61.87 mg/L of calcitriol, corresponding to a 30.94% mass yield from vitamin D3, was produced in a 75-L fermentor after 9 days. This calcitriol yield was 3.6 times higher than that obtained using a bioconversion medium lacking β-cyclodextrin, NaCl, K2HPO4, and MnCl2. In conclusion, utilizing whole cells of Pseudonocardia sp. KCTC 1029BP together with the optimal bioconversion medium markedly enhanced the production of calcitriol from vitamin D3.

Comparative characterization of the lactimidomycin and iso-migrastatin biosynthetic machineries revealing unusual features for acyltransferase-less type I polyketide synthases and providing an opportunity to engineer new analogues

Lactimidomycin (LTM, 1) and iso-migrastatin (iso-MGS, 2) belong to the glutarimide-containing polyketide family of natural products. We previously cloned and characterized the mgs biosynthetic gene cluster from Streptomyces platensis NRRL 18993. The iso-MGS biosynthetic machinery featured an acyltransferase (AT)-less type I polyketide synthase (PKS) and three tailoring enzymes (MgsIJK). We now report cloning and characterization of the ltm biosynthetic gene cluster from Streptomyces amphibiosporus ATCC 53964, which consists of nine genes that encode an AT-less type I PKS (LtmBCDEFGHL) and one tailoring enzyme (LtmK). Inactivation of ltmE or ltmH afforded the mutant strain SB15001 or SB15002, respectively, that abolished the production of 1, as well as the three cometabolites 8,9-dihydro-LTM (14), 8,9-dihydro-8S-hydroxy-LTM (15), and 8,9-dihydro-9R-hydroxy-LTM (13). Inactivation of ltmK yielded the mutant strain SB15003 that abolished the production of 1, 13, and 15 but led to the accumulation of 14. Complementation of the ΔltmK mutation in SB15003 by expressing ltmK in trans restored the production of 1, as well as that of 13 and 15. These results support the model for 1 biosynthesis, featuring an AT-less type I PKS that synthesizes 14 as the nascent polyketide intermediate and a cytochrome P450 desaturase that converts 14 to 1, with 13 and 15 as minor cometabolites. Comparative analysis of the LTM and iso-MGS AT-less type I PKSs revealed several unusual features that deviate from those of the collinear type I PKS model. Exploitation of the tailoring enzymes for 1 and 2 biosynthesis afforded two analogues, 8,9-dihydro-8R-hydroxy-LTM (16) and 8,9-dihydro-8R-methoxy-LTM (17), that provided new insights into the structure-activity relationship of 1 and 2. While 12-membered macrolides, featuring a combination of a hydroxyl group at C-17 and a double bond at C-8 and C-9 as found in 1, exhibit the most potent activity, analogues with a single hydroxyl or methoxy group at C-8 or C-9 retain most of the activity whereas analogues with double substitutions at C-8 and C-9 lose significant activity.

CYP105-diverse structures, functions and roles in an intriguing family of enzymes in Streptomyces

The cytochromes P450 (CYP or P450) are a large superfamily of haem-containing enzymes found in all domains of life. They catalyse a variety of complex reactions, predominantly mixed-function oxidations, often displaying highly regio- and/or stereospecific chemistry. In streptomycetes, they are predominantly associated with secondary metabolite biosynthetic pathways or with xenobiotic catabolism. Homologues of one family, CYP105, have been found in all Streptomyces species thus far sequenced. This review looks at the diverse biological functions of CYP105s and the biosynthetic/catabolic pathways they are associated with. Examples are presented showing a range of biotransformative abilities and different contexts. As biocatalysts capable of some remarkable chemistry, CYP105s have great biotechnological potential and merit detailed study. Recent developments in biotechnological applications which utilize CYP105s are described, alongside a brief overview of the benefits and drawbacks of using P450s in commercial applications. The role of CYP105s in vivo is in many cases undefined and provides a rich source for further investigation into the functions these enzymes fulfil and the metabolic pathways they participate in, in the natural environment.

Expression of CYP107Z13 in Streptomyces lividans TK54 catalyzes the oxidation of avermectin to 4″-oxo-avermectin

Streptomyces ahygroscopicus ZB01 has strong catalytic activity for the regiospecific oxidation of 4″-OH of avermectin to form 4″-oxo-avermectin. A cytochrome P450 gene from S. ahygroscopicus ZB01, cyp107z13, was cloned into pKC1139 to generate pKCZ1 and was transformed into Streptomyces lividans TK54, which does not have the ability to catalyze the conversion of avermectin. CYP107Z13, under the control of an ermE* promoter, was actively expressed in the TK54 recombinant strain as determined by a reduced CO difference spectrum analysis of the crude protein. Analysis of whole-cell biocatalytic activity by high-performance liquid chromatography revealed the recombinant to be able to oxidize avermectin regiospecifically to 4″-oxo-avermectin and CYP107Z13 to be a regioselective oxidase of avermectin. In addition, the whole-cell reaction conditions of the recombinant were optimized. Growth on medium ISP-2 at pH 6 was more conducive for the expression of CYP107Z13 than on medium PYG1 or at pH 7, and active cells of the recombinant strain had higher biocatalytic activity than resting cells.

Comparative analysis of the biosynthetic gene clusters and pathways for three structurally related antitumor antibiotics: bleomycin, tallysomycin, and zorbamycin

The biosynthetic gene clusters for the glycopeptide antitumor antibiotics bleomycin (BLM), tallysomycin (TLM), and zorbamycin (ZBM) have been recently cloned and characterized from Streptomyces verticillus ATCC15003, Streptoalloteichus hindustanus E465-94 ATCC31158, and Streptomyces flavoviridis ATCC21892, respectively. The striking similarities and differences among the biosynthetic gene clusters for the three structurally related glycopeptide antitumor antibiotics prompted us to compare and contrast their respective biosynthetic pathways and to investigate various enzymatic elements. The presence of different numbers of isolated nonribosomal peptide synthetase (NRPS) domains in all three clusters does not result in major structural differences of the respective compounds. The seemingly identical domain organization of the NRPS modules responsible for heterocycle formation, on the other hand, is contrasted by the biosynthesis of two different structural entities, bithiazole and thiazolinyl-thiazole, for BLM/TLM and ZBM, respectively. Variations in sugar biosynthesis apparently dictate the glycosylation patterns distinct for each of the BLM, TLM, and ZBM glycopeptide scaffolds. These observations demonstrate nature's ingenuity and flexibility in achieving structural differences and similarities via various mechanisms and will surely inspire combinatorial biosynthesis efforts to expand on natural product structural diversity.

Structural characterization of OxyD, a cytochrome P450 involved in beta-hydroxytyrosine formation in vancomycin biosynthesis

The cytochrome P450 OxyD from the balhimycin glycopeptide antibiotic biosynthetic operon of Amycolatopsis mediterranei is involved in the biosynthesis of the modified amino acid beta-R-hydroxytyrosine, an essential precursor for biosynthesis of the vancomycin-type aglycone. OxyD binds the substrate tyrosine not free in solution, but rather covalently linked to the carrier protein (CP) domain of the non-ribosomal peptide synthase BpsD, exhibiting micromolar binding affinity to a tyrosine-loaded carrier protein construct. The crystal structure of OxyD was determined to 2.1-A resolution, revealing a potential binding site for the carrier protein-bound substrate in a different orientation to that seen with the acyl carrier protein-bound P450(BioI) (Cryle, M. J., and Schlichting, I. (2008) Proc. Natl. Acad. Sci. U.S.A. 105, 15696-15701). A series of residues were identified across known aminoacyl-CP-oxidizing P450s that are highly conserved and cluster in the active site or potential CP binding site of OxyD. These residues appear to be characteristic for aminoacyl-CP-oxidizing P450s, allowing sequence based identification of P450 function for this subgroup of P450s that play vital roles in the biosyntheses of many important natural products in addition to the vancomycin-type antibiotics. The ability to analyze such P450 function based upon sequence data alone should prove an important tool in the analysis and identification of new medicinally relevant biomolecules.

Identification and characterization of a bacterial cytochrome P450 for the metabolism of diclofenac

The bacterium Actinoplanes sp. ATCC 53771 is known to perform drug metabolism of several xenobiotics similarly to humans. We identified a cytochrome P450 enzyme from this strain, CYP107E4, and expressed it in Escherichia coli using the pET101 vector. The purified enzyme showed the characteristic reduced-CO difference spectra with a peak at 450 nm, indicating the protein is produced in the active form with proper heme incorporation. The CYP107E4 enzyme was found to bind the drug diclofenac. Using redox enzymes from spinach, the reconstituted system is able to produce hydroxylated metabolites of diclofenac. Production of the human 4'-hydroxydiclofenac metabolite by CYP107E4 was confirmed, and a second hydroxylated metabolite was also produced.

Purification, characterization, and directed evolution study of a vitamin D3 hydroxylase from Pseudonocardia autotrophica

Vitamin D(3) (VD(3)) is a fat-soluble prohormone that plays a crucial role in bone metabolism, immunity, and control of cell proliferation and cell differentiation in mammals. The actinomycete Pseudonocardia autotrophica is capable of bioconversion of VD(3) into its physiologically active forms, namely, 25(OH)VD(3) or 1alpha,25(OH)(2)VD(3). In this study, we isolated and characterized Vdh (vitamin D(3) hydroxylase), which hydroxylates VD(3) from P. autotrophica NBRC 12743. The vdh gene encodes a protein containing 403 amino acids with a molecular weight of 44,368Da. This hydroxylase was found to be homologous with the P450 belonging to CYP107 family. Vdh had the same ratio of the V(max) values for VD(3) 25-hydroxylation and 25(OH)VD(3) 1alpha-hydroxylation, while other enzymes showed preferential regio-specific hydroxylation on VD(3). We characterized a collection of Vdh mutants obtained by random mutagenesis and obtained a Vdh-K1 mutant by the combination of four amino acid substitutions. Vdh-K1 showed one-order higher VD(3) 25-hydroxylase activity than the wild-type enzyme. Biotransformation of VD(3) into 25(OH)VD(3) was successfully accomplished with a Vdh-expressed recombinant strain of actinobacterium Rhodococcus erythropolis. Vdh may be a useful enzyme for the production of physiologically active forms of VD(3) by a single cytochrome P450.

Characterization of P450 FcpC, the enzyme responsible for bioconversion of diosgenone to isonuatigenone in Streptomyces virginiae IBL-14

A new cytochrome P450 monooxygenase, FcpC, from Streptomyces virginiae IBL-14 has been identified. This enzyme is found to be responsible for the bioconversion of a pyrano-spiro steroid (diosgenone) to a rare nuatigenin-type spiro steroid (isonuatigenone), which is a novel C-25-hydroxylated diosgenone derivative. A whole-cell P450 system was developed for the production of isonuatigenone via the expression of the complete three-component electron transfer chain in an Escherichia coli strain.

New ventures in the chemistry of avermectins

An overview is given on recent work towards new avermectin derivatives of extremely high insecticidal and acaricidal activity. These compounds were prepared from commercially available abamectin (avermectin B1) 1. For the synthesis, many novel entries have been opened up, making use of modern synthetic methods and applying them, for the first time, to the chemistry of avermectins. Several types of avermectin derivatives can be regarded as key innovations in the field. These are, in particular, 4''-deoxy-4''-(S)-amino avermectins 3, 4'-O-alkoxyalkyl avermectin monosaccharides 5, 4''-deoxy-4''-C-substituted 4''-amino avermectins 6 and 2''-substituted avermectins 7. 4''-Deoxy-4''-(S)-amino avermectins 3 were obtained by the consecutive application of the Staudinger and Aza-Wittig reaction. 4'-O-Alkoxyalkyl avermectin monosaccharides 5 were prepared by alkoxyalkylation of 5-O-protected avermectin monosaccharide. For the synthesis of 4''-deoxy-4''-C-substituted 4''-amino avermectins 6, several methods were used to construct the fully substituted 4''-carbon centre, such as a modified Strecker synthesis, the addition of organometallics to a 4''-sulfinimine and a modified Ugi approach. In order to prepare 2''-substituted avermectins 7, 5-O-protected avermectin monosaccharide was coupled with carbohydrate building blocks. An alternative synthesis involved the hitherto unknown enol ether chemistry of 4''-oxo-avermectin and the conjugate addition of a cuprate to an avermectin 2'',3''-en-4''-one. In addition, a number of other highly potent derivatives were synthesised. Examples are 4''-O-amino avermectins 8, as well as products arising from intramolecular rhodium catalysed amidations and carbene insertions. A radical cyclisation led to an intriguing rearrangement of the avermectin skeleton. Many of the new avermectins surpassed the activity of abamectin 1 against insects and mites.

The cytochrome P450 enzyme CYP96A15 is the midchain alkane hydroxylase responsible for formation of secondary alcohols and ketones in stem cuticular wax of Arabidopsis

Most aerial surfaces of plants are covered by cuticular wax that is synthesized in epidermal cells. The wax mixture on the inflorescence stems of Arabidopsis (Arabidopsis thaliana) is dominated by alkanes, secondary alcohols, and ketones, all thought to be formed sequentially in the decarbonylation pathway of wax biosynthesis. Here, we used a reverse-genetic approach to identify a cytochrome P450 enzyme (CYP96A15) involved in wax biosynthesis and characterized it as a midchain alkane hydroxylase (MAH1). Stem wax of T-DNA insertional mutant alleles was found to be devoid of secondary alcohols and ketones (mah1-1) or to contain much lower levels of these components (mah1-2 and mah1-3) than wild type. All mutant lines also had increased alkane amounts, partially or fully compensating for the loss of other compound classes. In spite of the chemical variation between mutant and wild-type waxes, there were no discernible differences in the epicuticular wax crystals on the stem surfaces. Mutant stem wax phenotypes could be partially rescued by expression of wild-type MAH1 under the control of the native promoter as well as the cauliflower mosaic virus 35S promoter. Cauliflower mosaic virus 35S-driven overexpression of MAH1 led to ectopic accumulation of secondary alcohols and ketones in Arabidopsis leaf wax, where only traces of these compounds are found in the wild type. The newly formed leaf alcohols and ketones had midchain functional groups on or next to the central carbon, thus matching those compounds in wild-type stem wax. Taken together, mutant analyses and ectopic expression of MAH1 in leaves suggest that this enzyme can catalyze the hydroxylation reaction leading from alkanes to secondary alcohols and possibly also a second hydroxylation leading to the corresponding ketones. MAH1 expression was largely restricted to the expanding regions of the inflorescence stems, specifically to the epidermal pavement cells, but not in trichomes and guard cells. MAH1-green fluorescent protein fusion proteins localized to the endoplasmic reticulum, providing evidence that both intermediate and final products of the decarbonylation pathway are generated in this subcellular compartment and must subsequently be delivered to the plasma membrane for export toward the cuticle.

Biocatalytic conversion of avermectin to 4''-oxo-avermectin: improvement of cytochrome p450 monooxygenase specificity by directed evolution

Discovery of the CYP107Z subfamily of cytochrome P450 oxidases (CYPs) led to an alternative biocatalytic synthesis of 4''-oxo-avermectin, a key intermediate for the commercial production of the semisynthetic insecticide emamectin. However, under industrial process conditions, these wild-type CYPs showed lower yields due to side product formation. Molecular evolution employing GeneReassembly was used to improve the regiospecificity of these enzymes by a combination of random mutagenesis, protein structure-guided site-directed mutagenesis, and recombination of multiple natural and synthetic CYP107Z gene fragments. To assess the specificity of CYP mutants, a miniaturized, whole-cell biocatalytic reaction system that allowed high-throughput screening of large numbers of variants was developed. In an iterative process consisting of four successive rounds of GeneReassembly evolution, enzyme variants with significantly improved specificity for the production of 4''-oxo-avermectin were identified; these variants could be employed for a more economical industrial biocatalytic process to manufacture emamectin.

Biocatalytic conversion of avermectin into 4''-oxo-avermectin: discovery, characterization, heterologous expression and specificity improvement of the cytochrome P450 enzyme

4''-Oxo-avermectin is a key intermediate in the manufacture of the insecticide emamectin benzoate from the natural product avermectin. Seventeen Streptomyces strains with the ability to oxidize avermectin to 4''-oxo-avermectin in a regioselective manner have been discovered, and the enzymes responsible for this reaction were found to be CYPs (cytochrome P450 mono-oxygenases). The genes for these enzymes have been cloned, sequenced and compared to reveal a new subfamily of CYPs. The biocatalytic enzymes have been overexpressed in Escherichia coli, Streptomyces lividans and solvent-tolerant Pseudomonas putida strains using different promoters and vectors. FDs (ferredoxins) and FREs (ferredoxin:NADP(+) reductases) were also cloned from Streptomyces coelicolor and biocatalytic Streptomyces strains, and tested in co-expression systems to optimize the electron transport. Subsequent studies showed that increasing the biocatalytic conversion levels to commercial relevance results in the production of several side products in significant amounts. Chimaeric Ema CYPs were created by sequential rounds of GeneReassembly, a proprietary directed evolution method, and selected for improved substrate specificity by high-throughput screening.

Biocatalytic conversion of avermectin to 4"-oxo-avermectin: heterologous expression of the ema1 cytochrome P450 monooxygenase

The cytochrome P450 monooxygenase Ema1 from Streptomyces tubercidicus R-922 and its homologs from closely related Streptomyces strains are able to catalyze the regioselective oxidation of avermectin into 4"-oxo-avermectin, a key intermediate in the manufacture of the agriculturally important insecticide emamectin benzoate (V. Jungmann, I. Molnár, P. E. Hammer, D. S. Hill, R. Zirkle, T. G. Buckel, D. Buckel, J. M. Ligon, and J. P. Pachlatko, Appl. Environ. Microbiol. 71:6968-6976, 2005). The gene for Ema1 has been expressed in Streptomyces lividans, Streptomyces avermitilis, and solvent-tolerant Pseudomonas putida strains using different promoters and vectors to provide biocatalytically competent cells. Replacing the extremely rare TTA codon with the more frequent CTG codon to encode Leu4 in Ema1 increased the biocatalytic activities of S. lividans strains producing this enzyme. Ferredoxins and ferredoxin reductases were also cloned from Streptomyces coelicolor and biocatalytic Streptomyces strains and tested in ema1 coexpression systems to optimize the electron transport towards Ema1.