Identification of monohydroxy progesterones produced by CYP106A2 using comparative HPLC and electrospray ionisation collision-induced dissociation mass spectrometry

Conformational heterogeneity suggests multiple substrate binding modes in CYP106A2

CYP106A2 (cytochrome P450meg) is a bacterial enzyme originally isolated from B. megaterium, and has been shown to hydroxylate a wide variety of substrates, including steroids. The regio- and stereochemistry of CYP106A2 hydroxylation has been shown to be dependent on a variety of factors, and hydroxylation often occurs at more than one site and/or with lack of stereospecificity for some substrates. Comprehensive backbone 15N, 1H and 13C resonance assignments based on multidimensional nuclear magnetic resonance (NMR) experiments performed with uniform and selective isotopically labeled CYP106A2 samples are reported herein, and broadening and splitting of resonances assigned to regions of the enzyme shown to be affected by substrate binding in other P450 enzymes indicate that substrate binding does not reduce structural heterogeneity as has been observed previously in P450 enzymes CYP101A1 and MycG. Paramagnetic relaxation enhancement (PRE) due to proximity between substrate protons and the heme iron were measured for three different substrates, and the relatively uniform nature of the PREs support the proposal that multiple substrate binding modes are occupied at saturating substrate concentrations.

Structural comparison of the cytochrome P450 enzymes CYP106A1 and CYP106A2 provides insight into their differences in steroid conversion

Understanding the structural basis of the selectivity of steroid hydroxylation requires detailed structural and functional investigations on various steroid hydroxylases with different selectivities, such as the bacterial cytochrome P450 enzymes. Here, the crystal structure of the cytochrome P450 CYP106A1 from Priestia megaterium was solved. CYP106A1 exhibits a rare additional structural motif of a cytochrome P450, a sixth β-sheet. The protein was found in different unusual conformations corresponding to both open and closed forms even when crystallized without any known substrate. The structural comparison of CYP106A1 with the previously investigated CYP106A2, including docking studies for both isoforms with the substrate cortisol, reveals a completely different orientation of the steroid molecule in the active sites. This distinction convincingly explains the experimentally observed differences in substrate conversion and product formation by the two enzymes.

Engineering Regioselectivity of a P450 Monooxygenase Enables the Synthesis of Ursodeoxycholic Acid via 7β-Hydroxylation of Lithocholic Acid

We engineered the cytochrome P450 monooxygenase CYP107D1 (OleP) from Streptomyces antibioticus for the stereo- and regioselective 7β-hydroxylation of lithocholic acid (LCA) to yield ursodeoxycholic acid (UDCA). OleP was previously shown to hydroxylate testosterone at the 7β-position but LCA is exclusively hydroxylated at the 6β-position, forming murideoxycholic acid (MDCA). Structural and 3DM analysis, and molecular docking were used to identify amino acid residues F84, S240, and V291 as specificity-determining residues. Alanine scanning identified S240A as a UDCA-producing variant. A synthetic "small but smart" library based on these positions was screened using a colorimetric assay for UDCA. We identified a nearly perfectly regio- and stereoselective triple mutant (F84Q/S240A/V291G) that produces 10-fold higher levels of UDCA than the S240A variant. This biocatalyst opens up new possibilities for the environmentally friendly synthesis of UDCA from the biological waste product LCA.

Natural Compounds as Pharmaceuticals: The Key Role of Cytochromes P450 Reactivity

The design of drugs from natural products is a re-emerging area due to the need for bioactive compounds. The exploitation of natural products and their derivatives obtained by biocatalysis is in line with the higher attention given today to new sustainable technologies that better preserve the environment (green chemistry). The research field of cytochromes P450 (CYPs) is continuously providing new enzymes and mutants that produce metabolites suitable for late-stage functionalization for new potential drugs. This review provides an overview of the exploitation of CYPs as biocatalysts in drug synthesis. Additionally, recent progress in protein and metabolic engineering is provided to show how these enzymes offer a toolbox that can be combined with other biocatalytic or chemical processes to build new platforms for the green production of new drugs.

Bacterial cytochrome P450-catalyzed regio- and stereoselective steroid hydroxylation enabled by directed evolution and rational design

Steroids are the most widely marketed products by the pharmaceutical industry after antibiotics. Steroid hydroxylation is one of the most important functionalizations because their derivatives enable a higher biological activity compared to their less polar non-hydroxylated analogs. Bacterial cytochrome P450s constitute promising biocatalysts for steroid hydroxylation due to their high expression level in common workhorses like Escherichia coli. However, they often suffer from wrong or insufficient regio- and/or stereoselectivity, low activity, narrow substrate range as well as insufficient thermostability, which hampers their industrial application. Fortunately, these problems can be generally solved by protein engineering based on directed evolution and rational design. In this work, an overview of recent developments on the engineering of bacterial cytochrome P450s for steroid hydroxylation is presented.

Bacterial steroid hydroxylases: enzyme classes, their functions and comparison of their catalytic mechanisms

The steroid superfamily includes a wide range of compounds that are essential for living organisms of the animal and plant kingdoms. Structural modifications of steroids highly affect their biological activity. In this review, we focus on hydroxylation of steroids by bacterial hydroxylases, which take part in steroid catabolic pathways and play an important role in steroid degradation. We compare three distinct classes of metalloenzymes responsible for aerobic or anaerobic hydroxylation of steroids, namely: cytochrome P450, Rieske-type monooxygenase 3-ketosteroid 9α-hydroxylase, and molybdenum-containing steroid C25 dehydrogenases. We analyze the available literature data on reactivity, regioselectivity, and potential application of these enzymes in organic synthesis of hydroxysteroids. Moreover, we describe mechanistic hypotheses proposed for all three classes of enzymes along with experimental and theoretical evidences, which have provided grounds for their formulation. In case of the 3-ketosteroid 9α-hydroxylase, such a mechanistic hypothesis is formulated for the first time in the literature based on studies conducted for other Rieske monooxygenases. Finally, we provide comparative analysis of similarities and differences in the reaction mechanisms utilized by bacterial steroid hydroxylases.

Binding modes of CYP106A2 redox partners determine differences in progesterone hydroxylation product patterns

Natural redox partners of bacterial cytochrome P450s (P450s) are mostly unknown. Therefore, substrate conversions are performed with heterologous redox partners; in the case of CYP106A2 from Bacillus megaterium ATCC 13368, bovine adrenodoxin (Adx) and adrenodoxin reductase (AdR). Our aim was to optimize the redox system for CYP106A2 for improved product formation by testing 11 different combinations of redox partners. We found that electron transfer protein 1(516–618) showed the highest yield of the main product, 15β-hydroxyprogesterone, and, furthermore, produced a reduced amount of unwanted polyhydroxylated side products. Molecular protein–protein docking indicated that this is caused by subtle structural changes leading to alternative binding modes of both redox enzymes. Stopped-flow measurements analyzing the CYP106A2 reduction and showing substantial differences in the apparent rate constants supported this conclusion. The study provides for the first time to our knowledge rational explanations for differences in product patterns of a cytochrome P450 caused by difference in the binding mode of the redox partners.

Tanja Sagadin et al. show that different redox systems can be used to tune the rate selectivity and yield of progesterone conversion by the cytochrome P450 CYP106A2. They screen 11 redox partner combinations and identify specific combinations that may be used to improve biotechnological production of mono- and polyhydroxylated products.

CYP260B1 acts as 9α-hydroxylase for 11-deoxycorticosterone

Steroids and their oxyfunctionalized counterparts are valuable compounds for the pharmaceutical industry; however, the regio- and stereoselective introduction of oxygen is a challenging task for the synthetic chemistry. Thus, cytochromes P450 play an important role for the functionalization of steroidal compounds. In this study, we elucidated the main product of 11-deoxycorticosterone conversion formed by CYP260B1 from Sorangium cellulosum So ce56 as 9α-OH 11-deoxycorticosterone by NMR spectroscopy. This is, to the best of our knowledge, the first identification of a 9α-hydroxylase for this substrate. In addition, the major side product was identified as 21-OH pregna-1,4-diene-3,20-dione. Studies using 1α-OH 11-deoxycorticosterone as substrate suggested that the major side product is formed via dehydrogenation reaction. This side reaction was considerably decreased by employing the CYP260B1-T224A mutant, which showed an increased selectivity of about 75% compared to the 60% of the wild type for the 9α-hydroxylation. To scale up the production, an E. coli based whole-cell system harboring the CYP260B1-T224A variant as well as two heterologous redox partners was used. Employing growing cells in minimal medium led to a productivity of about 0.25g/l/d at a 50ml scale showing the biotechnological potential of this system.

CYP109E1 is a novel versatile statin and terpene oxidase from Bacillus megaterium

CYP109E1 is a cytochrome P450 monooxygenase from Bacillus megaterium with a hydroxylation activity for testosterone and vitamin D3. This study reports the screening of a focused library of statins, terpene-derived and steroidal compounds to explore the substrate spectrum of this enzyme. Catalytic activity of CYP109E1 towards the statin drug-precursor compactin and the prodrugs lovastatin and simvastatin as well as biotechnologically relevant terpene compounds including ionones, nootkatone, isolongifolen-9-one, damascones, and β-damascenone was found in vitro. The novel substrates induced a type I spin-shift upon binding to P450 and thus permitted to determine dissociation constants. For the identification of conversion products by NMR spectroscopy, a B. megaterium whole-cell system was applied. NMR analysis revealed for the first time the ability of CYP109E1 to catalyze an industrially highly important reaction, the production of pravastatin from compactin, as well as regioselective oxidations generating drug metabolites (6'β-hydroxy-lovastatin, 3'α-hydroxy-simvastatin, and 4″-hydroxy-simvastatin) and valuable terpene derivatives (3-hydroxy-α-ionone, 4-hydroxy-β-ionone, 11,12-epoxy-nootkatone, 4(R)-hydroxy-isolongifolen-9-one, 3-hydroxy-α-damascone, 4-hydroxy-β-damascone, and 3,4-epoxy-β-damascone). Besides that, a novel compound, 2-hydroxy-β-damascenone, produced by CYP109E1 was identified. Docking calculations using the crystal structure of CYP109E1 rationalized the experimentally observed regioselective hydroxylation and identified important amino acid residues for statin and terpene binding.

Engineering of versatile redox partner fusions that support monooxygenase activity of functionally diverse cytochrome P450s

Most bacterial cytochrome P450 monooxygenases (P450s or CYPs) require two redox partner proteins for activity. To reduce complexity of the redox chain, the Bacillus subtilis flavodoxin YkuN (Y) was fused to the Escherichia coli flavodoxin reductase Fpr (R), and activity was tuned by placing flexible (GGGGS)_n or rigid ([E/L]PPPP)_n linkers (n = 1–5) in between. P-linker constructs typically outperformed their G-linker counterparts, with superior performance of YR-P5, which carries linker ([E/L]PPPP)₅. Molecular dynamics simulations demonstrated that ([E/L]PPPP)_n linkers are intrinsically rigid, whereas (GGGGS)_n linkers are highly flexible and biochemical experiments suggest a higher degree of separation between the fusion partners in case of long rigid P-linkers. The catalytic properties of the individual redox partners were best preserved in the YR-P5 construct. In comparison to the separate redox partners, YR-P5 exhibited attenuated rates of NADPH oxidation and heme iron (III) reduction, while coupling efficiency was improved (28% vs. 49% coupling with B. subtilis CYP109B1, and 44% vs. 50% with Thermobifida fusca CYP154E1). In addition, YR-P5 supported monooxygenase activity of the CYP106A2 from Bacillus megaterium and bovine CYP21A2. The versatile YR-P5 may serve as a non-physiological electron transfer system for exploitation of the catalytic potential of other P450s.

CYP106A2-A versatile biocatalyst with high potential for biotechnological production of selectively hydroxylated steroid and terpenoid compounds

CYP106A2 from Bacillus megaterium ATCC13368, was identified in the 1970s as one of the first bacterial steroid hydroxylases responsible for the conversion of progesterone to 15β-hydroxyprogesterone. Later on it has been proven to be a potent hydroxylase of numerous 3-oxo-Δ⁴ as well as 3-hydroxy-Δ⁵-steroids and has recently also been characterized as a regioselective allylic bacterial diterpene hydroxylase. The main hydroxylation position of CYP106A2 is thought to be influenced by the functional groups at C3 position in the steroid core leading to a favored 15β-hydroxylation of 3-oxo-Δ⁴-steroids and 7β-hydroxylation of 3-hydroxy-Δ⁵-steroids. However, in some cases the hydroxylation is not strictly selective, resulting in the formation of undesired side-products. To overcome the unspecific hydroxylations or, on the contrary, to gain more of these products in case they are of industrial interest, rational protein design and directed evolution have been successfully performed to shift the stereoselectivity of hydroxylation by CYP106A2. The subsequently obtained hydroxylated steroid and terpene derivatives are especially useful as drug metabolites and drug precursors for the pharmaceutical industry, due to their diverse biological properties and hardship of their chemical synthesis. As a soluble prokaryotic P450 with broad substrate spectrum and hydroxylating capacity, CYP106A2 is an outstanding candidate to establish bioconversion processes. It has been expressed with respectable yields in Escherichia coli and Bacillus megaterium and was applied for the preparative hydroxylation of several steroids and terpenes. Recently, the application of the enzyme was assessed under process conditions as well, depicting a successfully optimized process development and getting us closer to industrial scale process requirements and a future large scale application. This article is part of a Special Issue entitled: Cytochrome P450 biodiversity and biotechnology, edited by Erika Plettner, Gianfranco Gilardi, Luet Wong, Vlada Urlacher, Jared Goldstone.

Engineering of CYP106A2 for steroid 9α- and 6β-hydroxylation

CYP 106A2 from Bacillus megaterium ATCC 13368 has been described as a 15β-hydroxylase showing also minor 11α-, 9α- and 6β-hydroxylase activity for progesterone conversion. Previously, mutant proteins with a changed selectivity towards 11α-OH-progesterone have already been produced. The challenge of this work was to create mutant proteins with a higher regioselectivity towards hydroxylation at positions 9 and 6 of the steroid molecule. 9α-hydroxyprogesterone exhibits pharmaceutical importance, because it is a useful intermediate in the production of physiologically active substances which possess progestational activity. Sixteen mutant proteins were selected from a library containing mutated proteins created by a combination of site-directed and saturation mutagenesis of active site residues. Four mutant proteins out of these catalyzed the conversion of progesterone to 9α-OH-progesterone as a main product. For further optimization site-directed mutagenesis was performed. The introduction of seven mutations (D217V, A243V, A106T, F165L, T89N, T247V or T247W) into these four mutant proteins led to 28 new variants, which were also used for an in vivo conversion of progesterone. The best mutant protein, F165L/A395E/G397V, showed a ten-fold increase in the selectivity towards progesterone 9α-hydroxylation compared with the wild type CYP106A2. Also 6β-OH-progesterone is a pharmaceutically important compound, especially as intermediate for the production of drugs against breast cancer. For the rational design of mutant proteins with 6β-selectivity, docking of the 3D-structure of CYP106A2 with progesterone was performed. The introduction of three mutations (T247A, A243S, F173A) led to seven new mutant proteins. Clone A243S showed the greatest improvement in 6β-selectivity being more than ten-fold. Finally, an in vivo conversion of 11-deoxycorticosterone (DOC), testosterone and cortisol with the best five mutant proteins displaying 9α- or 6β-hydroxylation, respectively, of progesterone was performed to investigate whether the introduced mutations also effected the conversion of other substrates.

A Fluorescent Readout for the Oxidation State of Electron Transporting Proteins in Cell Free Settings

Pathways involving sequential electron transfer between multiple proteins are ubiquitous in nature. Here, we demonstrate a new class of fluorescent protein-based reporters for monitoring electron transport through such multistage cascades, specifically those involving ferredoxin-like electron transporters. We created protein fusions between mammalian Adrenodoxin (Adx) and plant Ferredoxin (Fdx) with fluorescent proteins of different colors and found that the fluorescence of such fusions is highly sensitive to the redox state of the electron transporter. The increase in fluorescence from the oxidized to the reduced state was inversely proportional to the linker length between the fusion partners. We first used our approach to quantitatively characterize electron transfer from NADPH through Adrenodoxin Reductase (AdR) to Adrenodoxin (Adx). Our data allowed us to build a detailed mathematical model of this mitochondrial electron transfer chain and validate previously proposed mechanisms. Then, we showed that an Adx-GFP fusion could serve as a sensor for the activity of bacterial Type I Cytochrome P450s (CYPs), a very large class of enzymes with important roles in biotechnology. We further showed that fluorescence of a direct fusion between CYP and GFP was sensitive to CYP activity, suggesting that our approach is applicable to an even broader class of proteins, which undergo a redox state change during their work cycle.

Process development for the production of 15β-hydroxycyproterone acetate using Bacillus megaterium expressing CYP106A2 as whole-cell biocatalyst

Background

CYP106A2 from Bacillus megaterium ATCC 13368 was first identified as a regio- and stereoselective 15β-hydroxylase of 3-oxo-∆⁴-steroids. Recently, it was shown that besides 3-oxo-∆⁴-steroids, 3-hydroxy-∆⁵-steroids as well as di- and triterpenes can also serve as substrates for this biocatalyst. It is highly selective towards the 15β position, but the 6β, 7α/β, 9α, 11α and 15α positions have also been described as targets for hydroxylation. Based on the broad substrate spectrum and hydroxylating capacity, it is an excellent candidate for the production of human drug metabolites and drug precursors.

Results

In this work, we demonstrate the conversion of a synthetic testosterone derivative, cyproterone acetate, by CYP106A2 under in vitro and in vivo conditions. Using a Bacillus megaterium whole-cell system overexpressing CYP106A2, sufficient amounts of product for structure elucidation by nuclear magnetic resonance spectroscopy were obtained. The product was characterized as 15β-hydroxycyproterone acetate, the main human metabolite. Since the product is of pharmaceutical interest, our aim was to intensify the process by increasing the substrate concentration and to scale-up the reaction from shake flasks to bioreactors to demonstrate an efficient, yet green and cost-effective production. Using a bench-top bioreactor and the recombinant Bacillus megaterium system, both a fermentation and a transformation process were successfully implemented. To improve the yield and product titers for future industrial application, the main bottlenecks of the reaction were addressed. Using 2-hydroxypropyl-β-cyclodextrin, an effective bioconversion of 98% was achieved using 1 mM substrate concentration, corresponding to a product formation of 0.43 g/L, at a 400 mL scale.

Conclusions

Here we describe the successful scale-up of cyproterone acetate conversion from shake flasks to bioreactors, using the CYP106A2 enzyme in a whole-cell system. The substrate was converted to its main human metabolite, 15β-hydroxycyproterone acetate, a highly interesting drug candidate, due to its retained antiandrogen activity but significantly lower progestogen properties than the mother compound. Optimization of the process led to an improvement from 55% to 98% overall conversion, with a product formation of 0.43 g/L, approaching industrial process requirements and a future large-scale application.

Steroid conversion with CYP106A2 - production of pharmaceutically interesting DHEA metabolites

Background

Steroids are lipophilic compounds with a gonane skeleton and play an important role in higher organisms. Due to different functionalizations - mainly hydroxylations - at the steroid molecule, they vary highly in their mode of action. The pharmaceutical industry is, therefore, interested in hydroxysteroids as therapeutic agents. The insertion of hydroxyl groups into a steroid core, however, is hardly accomplishable by classical chemical means; that is because microbial steroid hydroxylations are investigated and applied since decades. CYP106A2 is a cytochrome P450 monooxygenase from Bacillus megaterium ATCC 13368, which was first described in the late 1970s and which is capable to hydroxylate a variety of 3-oxo-delta4 steroids at position 15beta. CYP106A2 is a soluble protein, easy to express and to purify in high amounts, which makes this enzyme an interesting target for biotechnological purposes.

Results

In this work a focused steroid library was screened in vitro for new CYP106A2 substrates using a reconstituted enzyme assay. Five new substrates were identified, including dehydroepiandrosterone and pregnenolone. NMR-spectroscopy revealed that both steroids are mainly hydroxylated at position 7beta. In order to establish a biotechnological system for the preparative scale production of 7beta-hydroxylated dehydroepiandrosterone, whole-cell conversions with growing and resting cells of B. megaterium ATCC1336 the native host of CYP1062 and also with resting cells of a recombinant B. megaterium MS941 strain overexpressing CYP106A2 have been conducted and conversion rates of 400 muM/h (115 mg/l/h) were obtained. Using the B. megaterium MS941 overexpression strain, the selectivity of the reaction was improved from 0.7 to 0.9 for 7beta-OH-DHEA.

Conclusions

In this work we describe CYP106A2 for the first time as a regio-selective hydroxylase for 3-hydroxy-delta5 steroids. DHEA was shown to be converted to 7beta-OH-DHEA which is a highly interesting human metabolite, supposed to act as neuroprotective, anti-inflammatory and immune-modulatory agent. Optimization of the whole-cell system using different B. megaterium strains lead to a conversion of DHEA with B. megaterium showing high selectivity and conversion rates and displaying a volumetric yield of 103 mg/l/h 7beta-OH-DHEA.

Functional characterization of steroid hydroxylase CYP106A1 derived from Bacillus megaterium

In this study, we examined the catalytic activity of CYP106A1 from the Bacillus megaterium American Type Culture Collection 14581 strain. The CYP106A1 gene was cloned from B. megaterium, heterologously expressed in Escherichia coli, and purified. Potential electron partners and possible bacterial CYP106A1 substrates were identified by examining the oxidative activity toward a set of steroids in the presence of several reductase systems. The activities of CYP106A1 in a reconstituted system could not be achieved using rat NADPH-P450 reductase or a putidaredoxin reductase-putidaredoxin pair. However, the spinach redox proteins, a ferredoxin reductase-ferredoxin pair, were found to be efficient redox partners for CYP106A1. CYP106A1 catalyzes the hydroxylation of a set of steroids including testosterone, progesterone, 17α-hydroxyprogesterone, 11-deoxycorticosterone, corticosterone, and 11-deoxycortisol to produce monohydroxylated products as the major metabolites. These results suggest that CYP106A1 would be useful for the bioconversion of steroid hormones to hydroxylated products that can be used for industrial applications.

Selective steroid oxyfunctionalisation by CYP154C5, a bacterial cytochrome P450

Background

Cytochrome P450 monooxygenases – able to regio- and stereoselectively hydroxylate non-activated carbon atoms – are important enzymes for the synthesis of valuable intermediates in the production of steroid hormones in the pharmaceutical industry. However, up to now only a few bacterial enzymes able to hydroxylate steroids have been reported. CYP154C5 from Nocardia farcinica IFM 10152, a bacterial P450 monooxygenase, was previously shown to convert testosterone to 16α-hydroxytestosterone. Since the hydroxylation at 16α-position is of special interest for the pharmaceutical industry, we have studied this enzyme in more detail to investigate its activity and selectivity in bioconversions of further steroids.

Results

CYP154C5 was coexpressed in Escherichia coli together with putidaredoxin and putidaredoxin reductase from Pseudomonas putida as redox partners for electron transfer and applied in bioconversions of various pregnanes and androstanes [pregnenolone (1), dehydroepiandrosterone (2), progesterone (3), androstenedione (4), testosterone (5) and nandrolone (6)]. Structure elucidation of the formed products revealed an exclusive regio- and stereoselectivity of CYP154C5, always yielding the corresponding 16α-hydroxylated steroids. Application of whole cells expressing the three components, P450, Pdx and PdR, in steroid biotransformations resulted in significantly higher conversions and total turnover numbers (TTN) compared to reactions using cell-free extracts. Additionally, considerably higher substrate loads (up to 15 mM) were tolerated by the whole-cell system. Furthermore, turnover numbers (TON) were determined for the six different steroids using whole cells. Thus, testosterone was found to be the worst substrate with a TON of only 0.8 μmol substrate consumed min^-1 μmol^-1 CYP154C5, while progesterone and pregnenolone were converted the fastest resulting in TON of 3.3 μmol substrate consumed min^-1 μmol^-1 CYP154C5.

Conclusion

CYP154C5 from N. farcinica constitutes a promising catalyst due to its high regio- and stereoselectivity in the hydroxylation of different steroids as well as its efficient expression in E. coli at high yields. Using this enzyme, 16α-hydroxylated steroids, which are important precursors for the synthesis of high value steroidal drugs in the pharmaceutical industry, can be selectively produced on preparative scale with TTN (μmol substrate consumed μmol^-1 CYP154C5) exceeding 2000.

Application of a new versatile electron transfer system for cytochrome P450-based Escherichia coli whole-cell bioconversions

Cytochromes P450 monooxygenases are highly interesting biocatalysts for biotechnological applications, since they perform a diversity of reactions on a broad range of organic molecules. Nevertheless, the application of cytochromes P450 is limited compared to other enzymes mainly because of the necessity of a functional redox chain to transfer electrons from NAD(P)H to the monooxygenase. In this study, we established a novel robust redox chain based on adrenodoxin, which can deliver electrons to mitochondrial, bacterial and microsomal P450s. The natural membrane-associated reductase of adrenodoxin was replaced by the soluble Escherichia coli reductase. We could demonstrate for the first time that this reductase can transfer electrons to adrenodoxin. In the first step, the electron transfer properties and the potential of this new system were investigated in vitro, and in the second step, an efficient E. coli whole-cell system using CYP264A1 from Sorangium cellulosum So ce56 was developed. It could be demonstrated that this novel redox chain leads to an initial conversion rate of 55 μM/h, which was 52 % higher compared to the 36 μM/h of the redox chain containing adrenodoxin reductase. Moreover, we optimized the whole-cell biotransformation system by a detailed investigation of the effects of different media. Finally, we are able to demonstrate that the new system is generally applicable to other cytochromes P450 by combining it with the biotechnologically important steroid hydroxylase CYP106A2 from Bacillus megaterium.

Autodisplay of functional CYP106A2 in Escherichia coli

Cytochrome P450 enzymes catalyse a wide variety of reactions, including the hydroxylation and epoxidation of CC bonds, and dealkylation reactions. There is high interest in these reactions for biotechnology and pharmaceutical processes. Many P450s require membrane surroundings and have substrates that do not cross biological membranes. To circumvent these obstacles, CYP106A2 from Bacillus megaterium was expressed on the outer membrane of Escherichia coli cells by Autodisplay. Exposure on the surface was confirmed by a protease accessibility test and flow cytometry after immunolabelling. HPLC assays showed that 0.5 ml of cells displaying the enzyme (OD₅₇₈ = 6) converted 9.13 μmol of deoxycorticosterone to 15β-OH-deoxycorticosterone within 1h. Imipramine and abietic acid were also accepted as substrates. The number of active enzyme molecules per cell was calculated to be 20,000. Surprisingly, surface-exposed CYP106A2 was active in E. coli BL21 without the external addition of the heme group. However, when CYP106A2 was expressed on the surface of an E. coli strain lacking the TolC channel protein (JW5503), enzymatic activity was almost completely abolished. The activity of CYP106A2 on the surface of E. coli JW5503 could be restored by the external addition of the heme group. This suggests, as has been reported before, that E. coli uses a TolC-dependent mechanism to export heme into the growth media, where it can be scavenged by a surface-displayed apoenzyme. Our results indicate that Autodisplay enables the functional surface display of P450 enzymes and provides a new platform to access their synthetic potential.

Molecular evolution of a steroid hydroxylating cytochrome P450 using a versatile steroid detection system for screening

Molecular evolution is a powerful tool for improving or changing activities of enzymes for their use in biotechnological processes. Cytochromes P450 are highly interesting enzymes for biotechnological purposes because they are able to hydroxylate a broad variety of substrates with high regio- and stereoselectivity. One promising steroid hydroxylating cytochrome P450 for biotechnological applications is CYP106A2 from Bacillus megaterium ATCC 13368. It is one of a few known bacterial cytochromes P450 able to transform steroids such as progesterone and 11-deoxycortisol. CYP106A2 can be easily expressed in Escherichia coli with a high yield and can be reconstituted using the adrenal redox proteins, adrenodoxin and adrenodoxin reductase. We developed a simple screening assay for this system and performed random mutagenesis of CYP106A2, yielding variants with improved 11-deoxycortisol and progesterone hydroxylation activity. After two generations of directed evolution, we were able to improve the k (cat)/K (m) of the 11-deoxycortisol hydroxylation by a factor of more than four. At the same time progesterone conversion was improved about 1.4-fold. Mapping the mutations identified in catalytically improved CYP106A2 variants into the structure of a CYP106A2 model suggests that these mutations influence the mobility of the F/G loop, and the interaction with the redox partner adrenodoxin. The results show the evolution of a soluble steroid hydroxylase as a potential new catalyst for the production of steroidogenic compounds.

Function and engineering of the 15beta-hydroxylase CYP106A2

CYP106A2 from Bacillus megaterium ATCC 13368 is a bacterial cytochrome P450 that is capable of transforming steroid hormones. It can be easily expressed in Escherichia coli with a high yield. Its activity in vitro can be achieved by using the adrenal redox proteins adrenodoxin and adrenodoxin reductase. So far, it was not possible to crystallize CYP106A2 because of degradation during the crystallization process. Nevertheless, CYP106A2 is an interesting enzyme for biotechnological use. It hydroxylates pharmaceutically important steroids such as progesterone and 11-deoxycortisol. However, it will be necessary for efficient application of CYP106A2 in biotechnology to improve the hydroxylation activity and manipulate the regiospecificity. The present paper gives an overview of recent developments in protein engineering of CYP106A2.

Design of an Escherichia coli system for whole cell mediated steroid synthesis and molecular evolution of steroid hydroxylases

The 15beta-hydroxylase (CYP106A2) from Bacillus megaterium, one of the few bacterial steroid hydroxylases, which has been isolated and characterized so far, catalyses the 15beta-hydroxylation of a variety of steroids. The enzyme can be supported in its activity with adrenodoxin (Adx) and adrenodoxin reductase (AdR) from bovine adrenals, supplying this enzyme with the reducing equivalents necessary for steroid hydroxylation activity. This three-component electron transfer chain was implemented in Escherichia coli by coexpression of the corresponding coding sequences from two plasmids, containing different selection markers and compatible origins of replication. The cDNAs of AdR and Adx on the first plasmid were separated by a ribosome binding sequence, with the reductase preceding the ferredoxin. The second plasmid for CYP106A2 expression was constructed with all features necessary for a molecular evolution approach. The transformed bacteria show the inducible ability to efficiently convert 11-deoxycorticosterone (DOC) to 15beta-DOC at an average rate of 1 mM/d in culture volumes of 300 ml. The steroid conversion system was downscaled to the microtiter plate format and a robot set-up was developed for a fluorescence-based conversion assay as well as a CO difference spectroscopy assay, which enables the screening for enzyme variants with higher activity and stability.

Current literature in mass spectrometry

In order to keep subscribers up‐to‐date with the latest developments in their field, John Wiley & Sons are providing a current awareness service in each issue of the journal. The bibliography contains newly published material in the field of mass spectrometry. Each bibliography is divided into 11 sections: 1 Books, Reviews & Symposia; 2 Instrumental Techniques & Methods; 3 Gas Phase Ion Chemistry; 4 Biology/Biochemistry: Amino Acids, Peptides & Proteins; Carbohydrates; Lipids; Nucleic Acids; 5 Pharmacology/Toxicology; 6 Natural Products; 7 Analysis of Organic Compounds; 8 Analysis of Inorganics/Organometallics; 9 Surface Analysis; 10 Environmental Analysis; 11 Elemental Analysis. Within each section, articles are listed in alphabetical order with respect to author (5 Weeks journals ‐ Search completed at 8th. Sept. 2004)

Use of high-performance liquid chromatography/electrospray ionization collision-induced dissociation mass spectrometry for structural identification of monohydroxylated progesterones

For the structural identification of monohydroxylated progesterones synthesized by microorganisms, a method was developed using a combination of high-performance liquid chromatography and electrospray ionization collision-induced dissociation mass spectrometry (HPLC/ESI-CIDMS). The retention times and MS/MS spectra of 11 different standards at 30 eV were collected and compared. The identification of D-ring-hydroxylated progesterones (15beta-, 16alpha-, 17alpha- and 21-OH-P) using ESI-CIDMS was not possible. However, they were separated chromatographically using a 65:35 mixture of water and acetonitrile containing 0.5% acetic acid. The other hydroxylated progesterones (2alpha-, 6beta-, 7beta-, 9alpha-, 11alpha-, 11beta-, and 19-OH-P) could be identified by comparison of eight fragments. The complete separation of 11 standards was achieved chromatographically. The developed assay was evaluated by the identification of monohydroxylated progesterones produced by CYP106A2 from Bacillus megaterium ATCC 13368.