Cytochrome P450: taming a wild type enzyme

Comprehensive assessment of Cytochrome P450 reactions: A multiplex approach using real-time ESI-MS

BACKGROUND

The detailed analysis of Cytochrome P450 (CYP) catalyzed reactions is of great interest, since those are of importance for biotechnical applications, drug interaction studies and environmental research. Often cocktail approaches are carried out in order to monitor several CYP activities in a single experiment. Commonly in these approaches product formation is detected and IC50 values are determined.

METHODS

In the present work, the reactions of two different CYP isoforms were monitored using real-time electrospray ionization mass spectrometry. Multiplex experiments using the highly specific CYP2A6 with its corresponding substrate coumarin as well as the highly promiscuous CYP3A4 with testosterone were conducted. Product formation and substrate depletion were simultaneously monitored and compared to the single CYP experiments. The diffusion-controlled rate of reaction and conversion rates that are used as parameters to assess the enzymatic activity were calculated for all measurements conducted.

RESULTS

Differences in conversion rates and the theoretical rate of reaction that were observed for single CYP and multiplex experiments, respectively, reveal the complexity of the underlying mechanisms. Findings of this study imply that there might be distinct deviations between product formation and substrate degradation when mixtures are used.

CONCLUSIONS

Detailed results indicate that for a comprehensive assessment of these enzymatic reactions both product and substrate should be considered.

GENERAL SIGNIFICANCE

The direct hyphenation of enzymatic reactions to mass spectrometry allows for a comprehensive assessment of enzymatic behavior. Due to the benefits of this technique, the entire system which includes substrate, product and intermediates can be investigated. Thus, besides IC50 values further information regarding the enzymatic behavior offers the opportunity for a more detailed insight.

Drug Oxidation by Cytochrome P450BM3 : Metabolite Synthesis and Discovering New P450 Reaction Types

There is intense interest in late-stage catalytic C-H bond functionalization as an integral part of synthesis. Effective catalysts must have a broad substrate range and tolerate diverse functional groups. Drug molecules provide a good test of these attributes of a catalyst. A library of P450BM3 mutants developed from four base mutants with high activity for hydrocarbon oxidation produced human metabolites of a panel of drugs that included neutral (chlorzoxazone, testosterone), cationic (amitriptyline, lidocaine) and anionic (diclofenac, naproxen) compounds. No single mutant was active for all the tested drugs but multiple variants in the library showed high activity with each compound. The high conversions enabled full product characterization that led to the discovery of the new P450 reaction type of oxidative decarboxylation of an α-hydroxy carboxylic acid and the formation a protected imine from an amine, offering a novel route to α-functionalization of amines. The substrate range and varied product profiles suggest that this library of enzymes is a good basis for developing late-stage C-H activation catalysts.

Enzymatic hydroxylation of an unactivated methylene C-H bond guided by molecular dynamics simulations

The hallmark of enzymes from secondary metabolic pathways is the pairing of powerful reactivity with exquisite site selectivity. The application of these biocatalytic tools in organic synthesis, however, remains under-utilized due to limitations in substrate scope and scalability. Here, we report how the reactivity of a monooxygenase (PikC) from the pikromycin pathway is modified through computationally guided protein and substrate engineering, and applied to the oxidation of unactivated methylene C-H bonds. Molecular dynamics and quantum mechanical calculations were used to develop a predictive model for substrate scope, site selectivity and stereoselectivity of PikC-mediated C-H oxidation. A suite of menthol derivatives was screened computationally and evaluated through in vitro reactions, where each substrate adhered to the predicted models for selectivity and conversion to product. This platform was also expanded beyond menthol-based substrates to the selective hydroxylation of a variety of substrate cores ranging from cyclic to fused bicyclic and bridged bicyclic compounds.

Electrocatalysis by subcellular liver fractions bound to carbon nanostructures for stereoselective green drug metabolite synthesis

A novel, reusable, cofactor-free, and mediator-free human liver microsomal bioreactor constructed on carbon nanostructure electrodes for stereoselective green syntheses of drug metabolites and specialty chemicals is reported here for the first time. Drug metabolites are useful for examining pharmaceutical and pharmacological properties of new drugs under development.

The taming of oxygen: biocatalytic oxyfunctionalisations

The scope and limitations of oxygenases as catalysts for preparative organic synthesis is discussed.

Directed evolution of cytochrome P450 enzymes for biocatalysis: exploiting the catalytic versatility of enzymes with relaxed substrate specificity

Cytochrome P450 enzymes are renowned for their ability to insert oxygen into an enormous variety of compounds with a high degree of chemo- and regio-selectivity under mild conditions. This property has been exploited in Nature for an enormous variety of physiological functions, and representatives of this ancient enzyme family have been identified in all kingdoms of life. The catalytic versatility of P450s makes them well suited for repurposing for the synthesis of fine chemicals such as drugs. Although these enzymes have not evolved in Nature to perform the reactions required for modern chemical industries, many P450s show relaxed substrate specificity and exhibit some degree of activity towards non-natural substrates of relevance to applications such as drug development. Directed evolution and other protein engineering methods can be used to improve upon this low level of activity and convert these promiscuous generalist enzymes into specialists capable of mediating reactions of interest with exquisite regio- and stereo-selectivity. Although there are some notable successes in exploiting P450s from natural sources in metabolic engineering, and P450s have been proven repeatedly to be excellent material for engineering, there are few examples to date of practical application of engineered P450s. The purpose of the present review is to illustrate the progress that has been made in altering properties of P450s such as substrate range, cofactor preference and stability, and outline some of the remaining challenges that must be overcome for industrial application of these powerful biocatalysts.

Synthetic biology for the directed evolution of protein biocatalysts: navigating sequence space intelligently

The amino acid sequence of a protein affects both its structure and its function. Thus, the ability to modify the sequence, and hence the structure and activity, of individual proteins in a systematic way, opens up many opportunities, both scientifically and (as we focus on here) for exploitation in biocatalysis. Modern methods of synthetic biology, whereby increasingly large sequences of DNA can be synthesised de novo, allow an unprecedented ability to engineer proteins with novel functions. However, the number of possible proteins is far too large to test individually, so we need means for navigating the 'search space' of possible protein sequences efficiently and reliably in order to find desirable activities and other properties. Enzymologists distinguish binding (Kd) and catalytic (kcat) steps. In a similar way, judicious strategies have blended design (for binding, specificity and active site modelling) with the more empirical methods of classical directed evolution (DE) for improving kcat (where natural evolution rarely seeks the highest values), especially with regard to residues distant from the active site and where the functional linkages underpinning enzyme dynamics are both unknown and hard to predict. Epistasis (where the 'best' amino acid at one site depends on that or those at others) is a notable feature of directed evolution. The aim of this review is to highlight some of the approaches that are being developed to allow us to use directed evolution to improve enzyme properties, often dramatically. We note that directed evolution differs in a number of ways from natural evolution, including in particular the available mechanisms and the likely selection pressures. Thus, we stress the opportunities afforded by techniques that enable one to map sequence to (structure and) activity in silico, as an effective means of modelling and exploring protein landscapes. Because known landscapes may be assessed and reasoned about as a whole, simultaneously, this offers opportunities for protein improvement not readily available to natural evolution on rapid timescales. Intelligent landscape navigation, informed by sequence-activity relationships and coupled to the emerging methods of synthetic biology, offers scope for the development of novel biocatalysts that are both highly active and robust.

Supramolecular protein assembly supports immobilization of a cytochrome P450 monooxygenase system as water-insoluble gel

Diverse applications of the versatile bacterial cytochrome P450 enzymes (P450s) are hampered by their requirement for the auxiliary proteins, ferredoxin reductases and ferredoxins, that transfer electrons to P450s. Notably, this limits the use of P450s as immobilized enzymes for industrial purposes. Herein, we demonstrate the immobilization of a bacterial P450 and its redox protein partners by supramolecular complex formation using a self-assembled heterotrimeric protein. Employment of homodimeric phosphite dehydrogenase (PTDH) for cross-linking “proliferating cell nuclear antigen-utilized protein complex of P450 and its two electron transfer-related proteins” (PUPPET) yielded a gelling PUPPET-PTDH system capable of regenerating NADH for electron supply owing to its phosphite oxidation activity. The protein gel catalyzed monooxygenation in the presence of phosphite and NAD⁺. The gel was completely water-insoluble and could be reused. This concept of oligomeric protein-insolubilized enzymes can be widely applied to various multienzymatic reactions such as cascade reactions and coupling reactions.

Single-step fermentative production of the cholesterol-lowering drug pravastatin via reprogramming of Penicillium chrysogenum

The cholesterol-lowering blockbuster drug pravastatin can be produced by stereoselective hydroxylation of the natural product compactin. We report here the metabolic reprogramming of the antibiotics producer Penicillium chrysogenum toward an industrial pravastatin production process. Following the successful introduction of the compactin pathway into the β-lactam-negative P. chrysogenum DS50662, a new cytochrome P450 (P450 or CYP) from Amycolatopsis orientalis (CYP105AS1) was isolated to catalyze the final compactin hydroxylation step. Structural and biochemical characterization of the WT CYP105AS1 reveals that this CYP is an efficient compactin hydroxylase, but that predominant compactin binding modes lead mainly to the ineffective epimer 6-epi-pravastatin. To avoid costly fractionation of the epimer, the enzyme was evolved to invert stereoselectivity, producing the pharmacologically active pravastatin form. Crystal structures of the optimized mutant P450(Prava) bound to compactin demonstrate how the selected combination of mutations enhance compactin binding and enable positioning of the substrate for stereo-specific oxidation. Expression of P450(Prava) fused to a redox partner in compactin-producing P. chrysogenum yielded more than 6 g/L pravastatin at a pilot production scale, providing an effective new route to industrial scale production of an important drug.

Guidelines for development and implementation of biocatalytic P450 processes

Biocatalytic reactions performed by cytochrome P450 monooxygenases are interesting in pharmaceutical research since they are involved in human drug metabolism. Furthermore, they are potentially interesting as biocatalysts for synthetic chemistry because of the exquisite selectivity of the chemistry they undertake. For example, selective hydroxylation can be undertaken on a highly functionalized molecule without the need for functional group protection. Recent progress in the discovery of novel P450s as well as protein engineering of these enzymes strongly encourages further development of their application, including use in synthetic processes. The biological characteristics of P450s (e.g., cofactor dependence) motivate the use of whole-cell systems for synthetic processes, and those processes implemented in industry are so far dominated by growing cells and native host systems. However, for an economically feasible process, the expression of P450 systems in a heterologous host with sufficient biocatalyst yield (g/g cdw) for non-growing systems or space-time yield (g/L/h) for growing systems remains a major challenge. This review summarizes the opportunities to improve P450 whole-cell processes and strategies in order to apply and implement them in industrial processes, both from a biological and process perspective. Indeed, a combined approach of host selection and cell engineering, integrated with process engineering, is suggested as the most effective route to implementation.

Terpene hydroxylation with microbial cytochrome P450 monooxygenases

Terpenoids comprise a highly diverse group of natural products. In addition to their basic carbon skeleton, they differ from one another in their functional groups. Functional groups attached to the carbon skeleton are the basis of the terpenoids' diverse properties. Further modifications of terpene olefins include the introduction of acyl-, aryl-, or sugar moieties and usually start with oxidations catalyzed by cytochrome P450 monooxygenases (P450s, CYPs). P450s are ubiquitously distributed throughout nature, involved in essential biological pathways such as terpenoid biosynthesis as well as the tailoring of terpenoids and other natural products. Their ability to introduce oxygen into nonactivated C-H bonds is unique and makes P450s very attractive for applications in biotechnology. Especially in the field of terpene oxidation, biotransformation methods emerge as an attractive alternative to classical chemical synthesis. For this reason, microbial P450s depict a highly interesting target for protein engineering approaches in order to increase selectivity and activity, respectively. Microbial P450s have been described to convert industrial and pharmaceutically interesting terpenoids such as ionones, limone, valencene, resin acids, and triterpenes (including steroids) as well as vitamin D3. Highly selective and active mutants have been evolved by applying classical site-directed mutagenesis as well as directed evolution of proteins. As P450s usually depend on electron transfer proteins, mutagenesis has also been applied to improve the interactions between P450s and their respective redox partners. This chapter provides an overview of terpenoid hydroxylation reactions catalyzed by bacterial P450s and highlights the achievements made by protein engineering to establish productive hydroxylation processes.

Use of chemical auxiliaries to control p450 enzymes for predictable oxidations at unactivated C-h bonds of substrates

Cytochrome P450 enzymes (P450s) have the ability to oxidize unactivated C-H bonds of substrates with remarkable regio- and stereoselectivity. Comparable selectivity for chemical oxidizing agents is typically difficult to achieve. Hence, there is an interest in exploiting P450s as potential biocatalysts. Despite their impressive attributes, the current use of P450s as biocatalysts is limited. While bacterial P450 enzymes typically show higher activity, they tend to be highly selective for one or a few substrates. On the other hand, mammalian P450s, especially the drug-metabolizing enzymes, display astonishing substrate promiscuity. However, product prediction continues to be challenging. This review discusses the use of small molecules for controlling P450 substrate specificity and product selectivity. The focus will be on two approaches in the area: (1) the use of decoy molecules, and (2) the application of substrate engineering to control oxidation by the enzyme.

Conversions of tricyclic antidepressants and antipsychotics with selected P450s from Sorangium cellulosum So ce56

Human cytochromes P450 (P450s) play a major role in the biotransformation of drugs. The generated metabolites are important for pharmaceutical, medical, and biotechnological applications and can be used for derivatization or toxicological studies. The availability of human drug metabolites is restricted and alternative ways of production are requested. For this, microbial P450s turned out to be a useful tool for the conversion of drugs and related derivatives. Here, we used 10 P450s from the myxobacterium Sorangium cellulosum So ce56, which have been cloned, expressed, and purified. The P450s were investigated concerning the conversion of the antidepressant drugs amitriptyline, clomipramine, imipramine, and promethazine; the antipsychotic drugs carbamazepine, chlorpromazine, and thioridazine, as well as their precursors, iminodibenzyl and phenothiazine. Amitriptyline, chlorpromazine, clomipramine, imipramine, and thioridazine are efficiently converted during the in vitro reaction and were chosen to upscale the production by an Escherichia coli-based whole-cell bioconversion system. Two different approaches, a whole-cell system using M9CA medium and a system using resting cells in buffer, were used for the production of sufficient amounts of metabolites for NMR analysis. Amitriptyline, clomipramine, and imipramine are converted to the corresponding 10-hydroxylated products, whereas the conversion of chlorpromazine and thioridazine leads to a sulfoxidation in position 5. It is shown for the first time that myxobacterial P450s are efficient to produce known human drug metabolites in a milligram scale, revealing their ability to synthesize pharmaceutically important compounds.

New platform for cytochrome p450 reaction combining in situ immobilization on biopolymer

We describe an efficienct chemical conversion platform with in situ immobilization of P450-BM3 on poly(3-hydroxybutyrate) granules. Through fusion with phasin, P450-BM3 is easily immobilized on poly(3-hydroxybutyrate) granules in Escherichia coli. In our work, the immobilized P450 exhibited higher stability and catalytic activity compared to free P450 against changes of pH, temperature, and concentrations of urea and ions. Through quick recovery of immobilized enzyme, the P450-P(3HB) complex successfully catalyzed an O-dealkylation reaction several times with maintained activity. Using the robust P450-P(3HB) complex, we performed a P450-catalyzed reaction on a preparative reactor scale (100 mL) and high-level production (12.3 μM) of 7-hydroxycoumarine from 7-ethoxycoumarin could be achieved.

Controlling substrate specificity and product regio- and stereo-selectivities of P450 enzymes without mutagenesis

P450 enzymes (P450s) are well known for their ability to oxidize unactivated CH bonds with high regio- and stereoselectivity. Hence, there is emerging interest in exploiting P450s as potential biocatalysts. Although bacterial P450s typically show higher activity than their mammalian counterparts, they tend to be more substrate selective. Most drug-metabolizing P450s on the other hand, display remarkable substrate promiscuity, yet product prediction remains challenging. Protein engineering is one established strategy to overcome these issues. A less explored, yet promising alternative involves substrate engineering. This review discusses the use of small molecules for controlling the substrate specificity and product selectivity of P450s. The focus is on two approaches, one taking advantage of non-covalent decoy molecules, and the other involving covalent substrate modifications.

Non-natural olefin cyclopropanation catalyzed by diverse cytochrome P450s and other hemoproteins

Recent work has shown that engineered variants of cytochrome P450BM3 (CYP102A1) efficiently catalyze non-natural reactions, including carbene and nitrene transfer reactions. Given the broad substrate range of natural P450 enzymes, we set out to explore if this diversity could be leveraged to generate a broad panel of new catalysts for olefin cyclopropanation (i.e., carbene transfer). Here, we took a step towards this goal by characterizing the carbene transfer activities of four new wild-type P450s that have different native substrates. All four were active and exhibited a range of product selectivities in the model reaction: cyclopropanation of styrene by using ethyl diazoacetate (EDA). Previous work on P450BM3 demonstrated that mutation of the axial coordinating cysteine, universally conserved among P450 enzymes, to a serine residue, increased activity for this non-natural reaction. The equivalent mutation in the selected P450s was found to activate carbene transfer chemistry both in vitro and in vivo. Furthermore, serum albumins complexed with hemin were also found to be efficient in vitro cyclopropanation catalysts.

Aglycosylated full-length IgG antibodies: steps toward next-generation immunotherapeutics

Albeit the removal of Asn297 glycans of IgG perturbs the overall conformation and flexibility of the IgG CH2 domain, resulting in the loss of Fc-ligand interactions and therapeutically critical immune effector functions, aglycosylated full-length IgG antibodies are nearly identical to the glycosylated counterparts in terms of antigen binding, stability at physiological or low temperature conditions, pharmacokinetics, and biodistribution. To bypass the drawbacks of glycosylated antibodies that include glycan heterogeneity and requirement of high capital investment for biomanufacturing, aglycosylated antibodies have been developed and several are under clinical trials. Comprehensive cellular and bioprocess engineering has enabled to produce highly complex aglycosylated IgGs in a simple bacterial cultivation with comparable production level as that of mammalian cells. Moreover, extensive engineering of aglycosylated Fc has converted the aglycosylated IgG antibodies into a new class of effector functional human immunotherapeutics.

Cytochrome P450-mediated metabolic engineering: current progress and future challenges

Cytochromes P450 catalyze a broad range of regiospecific, stereospecific and irreversible steps in the biosynthetic routes of plant natural metabolites with important applications in pharmaceutical, cosmetic, fragrance and flavour, or polymer industries. They are consequently essential drivers for the engineered bioproduction of such compounds. Two ground-breaking developments of commercial products driven by the engineering of P450s are the antimalarial drug precursor artemisinic acid and blue roses or carnations. Tedious optimizations were required to generate marketable products. Hurdles encountered in P450 engineering and their potential solutions are summarized here. Together with recent technical developments and novel approaches to metabolic engineering, the lessons from this pioneering work should considerably boost exploitation of the amazing P450 toolkit emerging from accelerated sequencing of plant genomes.

Cytochromes P450 as promising catalysts for biotechnological application: chances and limitations

Cytochromes P450 (CYPs) belong to the superfamily of heme b containing monooxygenases with currently more than 21,000 members. These enzymes accept a vast range of organic molecules and catalyze diverse reactions. These extraordinary capabilities of CYP systems that are unmet by other enzymes make them attractive for biotechnology. However, the complexity of these systems due to the need of electron transfer from nicotinamide adenine dinucleotide (phosphate) (NAD(P)H) via redox partner proteins for the initial hydroxylation step limits a broader technical implementation of CYP enzymes. There have been several reviews during the past years tackling the potential CYPs for synthetic application. The aim of this review is to give a critical overview about possibilities and chances for application of these interesting catalysts as well as to discuss drawbacks and problems related to their use. Solutions to overcome these limitations will be demonstrated, and several selected examples of successful CYP applications under industrial conditions will be reviewed.

Evaluation of coumarin-based fluorogenic P450 BM3 substrates and prospects for competitive inhibition screenings

Fluorescence-based assays for the cytochrome P450 BM3 monooxygenase from Bacillus megaterium address an attractive biotechnological challenge by facilitating enzyme engineering and the identification of potential substrates of this highly promising biocatalyst. In the current study, we used the scarcity of corresponding screening systems as an opportunity to evaluate a novel and continuous high-throughput assay for this unique enzyme. A set of nine catalytically diverse P450 BM3 variants was constructed and tested toward the native substrate-inspired fluorogenic substrate 12-(4-trifluoromethylcoumarin-7-yloxy)dodecanoic acid. Particularly high enzyme-mediated O-dealkylation yielding the fluorescent product 7-hydroxy-4-trifluoromethylcoumarin was observed with mutants containing the F87V substitution, with A74G/F87V showing the highest catalytic efficiency (0.458 min(-1)μM(-1)). To simplify the assay procedure and show its versatility, different modes of application were successfully demonstrated, including (i) the direct use of NADPH or its oxidized form NADP(+) along with diverse NADPH recycling systems for electron supply, (ii) the use of cell-free lysates and whole-cell preparations as the biocatalyst source, and (iii) its use for competitive inhibition screens to identify or characterize substrates and inhibitors. A detailed comparison with known, fluorescence-based P450 BM3 assays finally emphasizes the relevance of our contribution to the ongoing research.

Isolation, bioactivity, and production of ortho-hydroxydaidzein and ortho-hydroxygenistein

Daidzein and genistein are two major components of soy isoflavones. They exist abundantly in plants and possess multiple bioactivities. In contrast, ortho-hydroxydaidzein (OHD) and ortho-hydroxygenistein (OHG), including 6-hydroxydaidzein (6-OHD), 8-hydroxydaidzein (8-OHD), 3'-hydroxydaidzein (3'-OHD), 6-hydroxygenistein (6-OHG), 8-hydroxygenistein (8-OHG), and 3'-hydroxygenistein (3'-OHG), are rarely found in plants. Instead, they are usually isolated from fermented soybean foods or microbial fermentation broth feeding with soybean meal. Accordingly, the bioactivity of OHD and OHG has been investigated less compared to that of soy isoflavones. Recently, OHD and OHG were produced by genetically engineering microorganisms through gene cloning of cytochrome P450 (CYP) enzyme systems. This success opens up bioactivity investigation and industrial applications of OHD and OHG in the future. This article reviews isolation of OHD and OHG from non-synthetic sources and production of the compounds by genetically modified microorganisms. Several bioactivities, such as anticancer and antimelanogenesis-related activities, of OHD and OHG, are also discussed.

Directing group-controlled regioselectivity in an enzymatic C-H bond oxygenation

Highly regioselective remote hydroxylation of a natural product scaffold is demonstrated by exploiting the anchoring mechanism of the biosynthetic P450 monooxygenase PikCD50N-RhFRED. Previous studies have revealed structural and biochemical evidence for the role of a salt bridge between the desosamine N,N-dimethylamino functionality of the natural substrate YC-17 and carboxylate residues within the active site of the enzyme, and selectivity in subsequent C-H bond functionalization. In the present study, a substrate-engineering approach was conducted that involves replacing desosamine with varied synthetic N,N-dimethylamino anchoring groups. We then determined their ability to mediate enzymatic total turnover numbers approaching or exceeding that of the natural sugar, while enabling ready introduction and removal of these amino anchoring groups from the substrate. The data establish that the size, stereochemistry, and rigidity of the anchoring group influence the regioselectivity of enzymatic hydroxylation. The natural anchoring group desosamine affords a 1:1 mixture of regioisomers, while synthetic anchors shift YC-17 analogue C-10/C-12 hydroxylation from 20:1 to 1:4. The work demonstrates the utility of substrate engineering as an orthogonal approach to protein engineering for modulation of regioselective C-H functionalization in biocatalysis.

Cytochrome P450 catalyzed oxidative hydroxylation of achiral organic compounds with simultaneous creation of two chirality centers in a single C-H activation step

Regio- and stereoselective oxidative hydroxylation of achiral or chiral organic compounds mediated by synthetic reagents, catalysts, or enzymes generally leads to the formation of one new chiral center that appears in the respective enantiomeric or diastereomeric alcohols. By contrast, when subjecting appropriate achiral compounds to this type of C-H activation, the simultaneous creation of two chiral centers with a defined relative and absolute configuration may result, provided that control of the regio-, diastereo-, and enantioselectivity is ensured. The present study demonstrates that such control is possible by using wild type or mutant forms of the monooxygenase cytochrome P450 BM3 as catalysts in the oxidative hydroxylation of methylcyclohexane and seven other monosubstituted cyclohexane derivatives.

Metalloenzyme design and engineering through strategic modifications of native protein scaffolds

Metalloenzymes are among the major targets of protein design and engineering efforts aimed at attaining novel and efficient catalysis for biochemical transformation and biomedical applications, due to the diversity of functions imparted by the metallo-cofactors along with the versatility of the protein environment. Naturally evolved protein scaffolds can often serve as robust foundations for sustaining artificial active sites constructed by rational design, directed evolution, or a combination of the two strategies. Accumulated knowledge of structure-function relationship and advancement of tools such as computational algorithms and unnatural amino acids incorporation all contribute to the design of better metalloenzymes with catalytic properties approaching the needs of practical applications.

Current and emerging options for taxol production

Paclitaxel (trademark "Taxol") is a plant-derived isoprenoid natural product that exhibits potent anticancer activity. Taxol was originally isolated from the Pacific yew tree in 1967 and triggered an intense scientific and engineering venture to provide the compound reliably to cancer patients. The choices available for production include synthetic and biosynthetic routes (and combinations thereof). This chapter focuses on the currently utilized and emerging biosynthetic options for Taxol production. A particular emphasis is placed on the biosynthetic production hosts including macroscopic and unicellular plant species and more recent attempts to elucidate, transfer, and reconstitute the Taxol pathway within technically advanced microbial hosts. In so doing, we provide the reader with relevant background related to Taxol and more general information related to producing valuable, but structurally complex, natural products through biosynthetic strategies.

CH-activating oxidative hydroxylation of 1-tetralones and related compounds with high regio- and stereoselectivity

Mutants of P450-BM3 evolved by directed evolution are excellent catalysts in the CH-activating oxidative hydroxylation of 1-tetralone derivatives and of indanone, with unusually high regio- and enantioselectivity being observed.

FAD C(4a)-hydroxide stabilized in a naturally fused styrene monooxygenase

StyA2B represents a new class of styrene monooxygenases that integrates flavin-reductase and styrene-epoxidase activities into a single polypeptide. This naturally-occurring fusion protein offers new avenues for studying and engineering biotechnologically relevant enantioselective biochemical epoxidation reactions. Stopped-flow kinetic studies of StyA2B reported here identify reaction intermediates similar to those reported for the separate reductase and epoxidase components of related two-component systems. Our studies identify substrate epoxidation and elimination of water from the FAD C(4a)-hydroxide as rate-limiting steps in the styrene epoxidation reaction. Efforts directed at accelerating these reaction steps are expected to greatly increase catalytic efficiency and the value of StyA2B as biocatalyst.

Structural evidence: a single charged residue affects substrate binding in cytochrome P450 BM-3

Cytochrome P450 BM-3 is a bacterial enzyme with sequence similarity to mammalian P450s that catalyzes the hydroxylation of fatty acids with high efficiency. Enzyme-substrate binding and dynamics has been an important topic of study for cytochromes P450 because most of the crystal structures of substrate-bound structures show the complex in an inactive state. We have determined a new crystal structure for cytochrome P450 BM-3 in complex with N-palmitoylglycine (NPG), which unexpectedly showed a direct bidentate ion pair between NPG and arginine 47 (R47). We further explored the role of R47, the only charged residue in the binding pocket in cytochrome P450 BM-3, through mutagenesis and crystallographic studies. The mutations of R47 to glutamine (R47Q), glutamic acid (R47E), and lysine (R47K) were designed to investigate the role of its charge in binding and catalysis. The oppositely charged R47E mutation had the greatest effect on activity and binding. The crystal structure of R47E BMP shows that the glutamic acid side chain is blocking the entrance to the binding pocket, accounting for NPG's low binding affinity and charge repulsion. For R47Q and R47K BM-3, the mutations caused only a slight change in kcat and a large change in Km and Kd, which suggests that R47 mostly is involved in binding and that our crystal structure, 4KPA , represents an initial binding step in the P450 cycle.

Screening for cytochrome P450 reactivity with a reporter enzyme

The identification of novel substrates of cytochrome P450 enzymes by high-throughput screening assays is of utmost importance to further increase the scope of these enzymes for future applications. Most screens are either confined to individual substrate analogues or hampered by low throughput due to elaborate analysis techniques. Here we describe a general high-throughput screening assay that interrogates the activity of P450 enzymes with the aid of catalase as a reporter enzyme.