Development of a fed-batch process for the production of the cytochrome P450 monooxygenase CYP102A1 from Bacillus megaterium in E. coli

Effect of chromosomal integration on catalytic performance of a multi-component P450 system in Escherichia coli

Cytochromes P450 are useful biocatalysts in synthetic chemistry and important bio-bricks in synthetic biology. Almost all bacterial P450s require separate redox partners for their activity, which are often expressed in recombinant Escherichia coli using multiple plasmids. However, the application of CRISPR/Cas recombineering facilitated chromosomal integration of heterologous genes which enables more stable and tunable expression of multi-component P450 systems for whole-cell biotransformations. Herein, we compared three E. coli strains W3110, JM109, and BL21(DE3) harboring three heterologous genes encoding a P450 and two redox partners either on plasmids or after chromosomal integration in two genomic loci. Both loci proved to be reliable and comparable for the model regio- and stereoselective two-step oxidation of (S)-ketamine. Furthermore, the CRISPR/Cas-assisted integration of the T7 RNA polymerase gene enabled an easy extension of T7 expression strains. Higher titers of soluble active P450 were achieved in E. coli harboring a single chromosomal copy of the P450 gene compared to E. coli carrying a medium copy pET plasmid. In addition, improved expression of both redox partners after chromosomal integration resulted in up to 80% higher (S)-ketamine conversion and more than fourfold increase in total turnover numbers.

Oxygenating Biocatalysts for Hydroxyl Functionalisation in Drug Discovery and Development

C-H oxyfunctionalisation remains a distinct challenge for synthetic organic chemists. Oxygenases and peroxygenases (grouped here as "oxygenating biocatalysts") catalyse the oxidation of a substrate with molecular oxygen or hydrogen peroxide as oxidant. The application of oxygenating biocatalysts in organic synthesis has dramatically increased over the last decade, producing complex compounds with potential uses in the pharmaceutical industry. This review will focus on hydroxyl functionalisation using oxygenating biocatalysts as a tool for drug discovery and development. Established oxygenating biocatalysts, such as cytochrome P450s and flavin-dependent monooxygenases, have widely been adopted for this purpose, but can suffer from low activity, instability or limited substrate scope. Therefore, emerging oxygenating biocatalysts which offer an alternative will also be covered, as well as considering the ways in which these hydroxylation biotransformations can be applied in drug discovery and development, such as late-stage functionalisation (LSF) and in biocatalytic cascades.

Optimization and Engineering of a Self-Sufficient CYP102 Enzyme from Bacillus amyloliquefaciens towards Synthesis of In-Chain Hydroxy Fatty Acids

Cytochrome P450 (CYP) mediated enzymatic hydroxylation of fatty acids present a green alternative to chemical synthesis of hydroxy fatty acids (HFAs), which are high-value oleochemicals with various uses in materials industry and medical field. Although many CYPs require the presence of additional reductase proteins for catalytic activity, self-sufficient CYPs have their reductase partner naturally fused into their catalytic domain, leading to a greatly simplified biotransformation process. A recently discovered self-sufficient CYP, BAMF2522 from Bacillus amyloliquefaciens DSM 7, exhibits novel regioselectivity by hydroxylating in-chain positions of palmitic acid generating ω-1 to ω-7 HFAs, a rare regiodiversity profile among CYPs. Besides, F89I mutant of BAMF2522 expanded hydroxylation up to ω-9 position of palmitic acid. Here, we further characterize this enzyme by determining optimum temperature and pH as well as thermal stability. Moreover, using extensive site-directed and site-saturation mutagenesis, we obtained BAMF2522 variants that demonstrate greatly increased regioselectivity for in-chain positions (ω-4 to ω-9) of various medium to long chain fatty acids. Remarkably, when a six-residue mutant was reacted with palmitic acid, 84% of total product content was the sum of ω-7, ω-8 and ω-9 HFA products, the highest in-chain selectivity observed to date with a self-sufficient CYP. In short, our study demonstrates the potential of a recently identified CYP and its mutants for green and sustainable production of a variety of in-chain hydroxy enriched HFAs.

Self-Sufficient Class VII Cytochromes P450: From Full-Length Structure to Synthetic Biology Applications

Members of class VII cytochromes P450 are catalytically self-sufficient enzymes containing a phthalate dioxygenase reductase-like domain fused to the P450 catalytic domain. Among these, CYP116B46 is the first enzyme for which the 3D structure of the whole polypeptide chain has been solved, shedding light on the interaction between its domains, which is crucial for catalysis. Most of these enzymes have been isolated from extremophiles or detoxifying bacteria that can carry out regio- and enantioselective oxidation of compounds of biotechnological interest. Protein engineering has generated mutants that can perform challenging organic reactions such as the anti-Markovnikov alkene oxidation. This potential, combined with the detailed 3D structure, forms the basis for further directed evolution studies aimed at widening their biotechnological exploitation.

Production of metabolites of the anti-cancer drug noscapine using a P450BM3 mutant library

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Mutants of P450_BM3 can metabolise noscapine.

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Noscapine is N-demethylated with high selectivity.

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The metabolites produced are of interest for drug development.

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The profile of metabolites generated resembles that of mammalian CYP3A4.

Cytochrome P450 enzymes are a promising tool for the late-stage diversification of lead drug candidates and can provide an alternative route to structural modifications that are difficult to achieve with synthetic chemistry. In this study, a library of P450_BM3 mutants was produced using site-directed mutagenesis and the enzymes screened for metabolism of the opium poppy alkaloid noscapine, a drug with anticancer activity. Of the 18 enzyme mutants screened, 12 showed an ability to metabolise noscapine that was not present in the wild-type enzyme. Five noscapine metabolites were detected by LC-MS/MS, with the major metabolite for all mutants being N-demethylated noscapine. The highest observed regioselectivity for N-demethylation was 88%. Two hydroxylated metabolites, a catechol and two C-C cleavage products were also detected. P450-mediated production of hydroxylated and N-demethylated noscapine structures may be useful for the development of noscapine analogues with improved biological activity. The variation in substrate turnover, coupling efficiency and product distribution between the active mutants was considered alongside in silico docking experiments to gain insight into structural and functional effects of the introduced mutations. Selected mutants were identified as targets for further mutagenesis to improve activity and when coupled with an optimised process may provide a route for the preparative-scale production of noscapine metabolites.

Laboratory-Scale Hydroxylation of Steroids by P450BM3 Variants

This chapter describes the asymmetric hydroxylation of steroids on laboratory preparative scale, using engineered variants of P450_BM3 (CYP102A1) as enzyme catalyst. The following protocol covers the creation of an Escherichia coli BL21-Gold (DE3) expression strain, including necessary control experiments like plasmid preparation, test expression, and creation of storage cultures, to verify successful experimental access to recombinant expressed P450_BM3 variants. The recombinant expressed P450_BM3 variants are obtained as cleared cell lysate and used in a biotransformation setup to hydroxylate 2.8 mg and up to 15 mg testosterone in the presented protocol. Since P450_BM3 depends on NADPH as an electron source for the reaction, a glucose and glucose dehydrogenate based recycling system is added to the reaction. The protocol further includes liquid-liquid extraction of hydroxytestosterone and directs the experimenter to compound purification via column chromatography.

Cloning, expression and characterisation of P450-Hal1 (CYP116B62) from Halomonas sp. NCIMB 172: A self-sufficient P450 with high expression and diverse substrate scope

Cytochrome P450 monooxygenases are able to catalyse a range of synthetically challenging reactions ranging from hydroxylation and demethylation to sulfoxidation and epoxidation. As such they have great potential for biocatalytic applications but are underutilised due to often-poor expression, stability and solubility in recombinant bacterial hosts. The use of self-sufficient P450 s with fused haem and reductase domains has already contributed heavily to improving catalytic efficiency and simplifying an otherwise more complex multi-component system of P450 and redox partners. Herein, we present a new addition to the class VII family with the cloning, sequencing and characterisation of the self-sufficient CYP116B62 Hal1 from Halomonas sp. NCIMB 172, the genome of which has not yet been sequenced. Hal1 exhibits high levels of expression in a recombinant E. coli host and can be utilised from cell lysate or used in purified form. Hal1 favours NADPH as electron donor and displays a diverse range of activities including hydroxylation, demethylation and sulfoxidation. These properties make Hal1 suitable for future biocatalytic applications or as a template for optimisation through engineering.

Constrained azeotropic optimization of extraction system components for the safe and efficient recovery of a desired metabolite (e.g., 3-demethylated colchicine)

Constrained azeotropic optimization of extraction system components for the safe and efficient recovery of a desired metabolite (e.g., 3-DMC) using artificial learning and evolutionary optimization techniques.

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.

Using small molecules as a new challenge to redirect metabolic pathway

The presence of acetate in the bacterial medium leads to a reduction in the growth rate of cells and recombinant protein production. In this study, three compounds including propionic acid, lithium chloride and butyric acid were added to the medium which decreased acetate levels and enhanced recombinant protein production (alpha-synuclein). In fact, propionic acid and lithium chloride are both known as acetate kinase inhibitors. The results obtained in the case of butyric acid were similar to those of the two other compounds indicating that butyric acid may act through a mechanism similar to propionic acid and lithium chloride. Consequently, it was shown that the presence of each of these supplements (5–200 μM) increased recombinant alpha-synuclein production and cell density by approximately 10–15 %. HPLC analysis showed that the levels of acetate in the media containing the supplements were considerably less than those of the control. Furthermore, pH values remained almost constant in the supplemented cultures. Growing the bacteria at lower temperatures (25 °C) indicated that the positive effects of these supplements were not as effective as at higher temperatures (37 °C), presumably due to the adequate balance between oxygen and carbon consumption. This study can confirm the viewpoint regarding the harmful effects of acetate on the recombinant protein production and cell density. Besides, such methods represent easy and complementary ways to increase target recombinant protein production without negatively affecting host cell density, and requiring complex genetic manipulation.

Use of a stress-minimisation paradigm in high cell density fed-batch Escherichia coli fermentations to optimise recombinant protein production

Production of recombinant proteins is an industrially important technique in the biopharmaceutical sector. Many recombinant proteins are problematic to generate in a soluble form in bacteria as they readily form insoluble inclusion bodies. Recombinant protein solubility can be enhanced by minimising stress imposed on bacteria through decreasing growth temperature and the rate of recombinant protein production. In this study, we determined whether these stress-minimisation techniques can be successfully applied to industrially relevant high cell density Escherichia coli fermentations generating a recombinant protein prone to forming inclusion bodies, CheY-GFP. Flow cytometry was used as a routine technique to rapidly determine bacterial productivity and physiology at the single cell level, enabling determination of culture heterogeneity. We show that stress minimisation can be applied to high cell density fermentations (up to a dry cell weight of >70 g L(-1)) using semi-defined media and glucose or glycerol as carbon sources, and using early or late induction of recombinant protein production, to produce high yields (up to 6 g L(-1)) of aggregation-prone recombinant protein in a soluble form. These results clearly demonstrate that stress minimisation is a viable option for the optimisation of high cell density industrial fermentations for the production of high yields of difficult-to-produce recombinant proteins, and present a workflow for the application of stress-minimisation techniques in a variety of fermentation protocols.

Whole-cell double oxidation of n-heptane

Biocascades allow one-pot synthesis of chemical building blocks omitting purification of reaction intermediates and expenses for downstream processing. Here we show the first whole cell double oxidation of n-heptane to produce chiral alcohols and heptanones. The concept of an artificial operon for co-expression of a monooxygenase from Bacillus megaterium (P450 BM3) and an alcohol dehydrogenase (RE-ADH) from Rhodococcus erythropolis is reported and compared to the widely used two-plasmid or Duet-vector expression systems. Both catalysts are co-expressed on a polycistronic constructs (single mRNA) that reduces recombinant DNA content and metabolic burden for the host cell, therefore increasing growth rate and expression level. Using the artificial operon system, the expression of P450 BM3 reached 81mgg(-1) cell dry weight. In addition, in situ cofactor regeneration through the P450 BM3/RE-ADH couple was enhanced by coupling to glucose oxidation by E. coli. Under optimized reaction conditions the artificial operon system displayed a product formation of 656mgL(-1) (5.7mM) of reaction products (heptanols+heptanones), which is 3-fold higher than the previously reported values for an in vitro oxidation cascade. In conjunction with the high product concentrations it was possible to obtain ee values of >99% for (S)-3-heptanol. Coexpression of a third alcohol dehydrogenase from Lactobacillus brevis (Lb-ADH) in the same host yielded complete oxidation of all heptanol isomers. Introduction of a second ADH enabled further to utilize both cofactors in the host cell (NADH and NADPH) which illustrates the simplicity and modular character of the whole cell oxidation concept employing an artificial operon system.

Development of fed-batch profiles for efficient biosynthesis of catechol-O-methyltransferase

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Constant feeds perform better than exponential feeds for hSCOMT production.

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A constant feed of 1 g/L/h yielded 40 OD₆₀₀ and a hSCOMT activity of 442 nmol/h/mg.

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A high percentage of viability was maintained in constant fed-batch fermentations.

Catechol-O-methyltransferase (COMT, EC 2.1.1.6) plays a crucial role in dopamine metabolism which has intimately linked this enzyme to some neurodegenerative diseases, such as Parkinson's disease. In recent years, in the attempt of developing new therapeutic strategies for Parkinson's disease, there has been a growing interest in the search for effective COMT inhibitors. In order to do so, large amounts of COMT in an active form are needed, and the best way to achieve this is by up-scaling its production through biotechnological processes. In this work, a fed-batch process for the biosynthesis of the soluble isoform of COMT in Escherichia coli is proposed. This final process was selected through the evaluation of the effect of different dissolved oxygen concentrations, carbon and nitrogen source concentrations and feeding profiles on enzymatic production and cell viability, while controlling various parameters (pH, temperature, starting time of the feeding and induction phases and carbon source concentration) during the process. After several batch and fed-batch experiments, a final specific COMT activity of 442.34 nmol/h/mg with approximately 80% of viable cells at the end of the fermentation were achieved. Overall, the results described herein provide a great improvement on hSCOMT production in recombinant bacteria and provide a new and viable option for the use of a fed-batch fermentation with a constant feeding profile to the large scale production of this enzyme.

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.

High-cell-density cultivation of recombinant Escherichia coli, purification and characterization of a self-sufficient biosynthetic octane ω-hydroxylase

We have recently described the biocatalytic characterization of a self-sufficent biosynthetic alkane hydroxylase based on CYP153A13a from Alcanivorax borkumensis SK2 (thereafter A13-Red). Despite remarkable regio- and chemo-selectivity, A13-Red suffers of a difficult-to-reproduce expression and moderate operational stability. In this study, we focused our efforts on the production of A13-Red using high-cell-density cultivation (HCDC) of recombinant Escherichia coli. We achieved 455 mg (5,000 nmol) of functional enzyme per liter of culture. Tight control of cultivation parameters rendered the whole process highly reproducible compared with flask cultivations. We optimized the purification of the biocatalyst that can be performed in either two or three steps depending on the application needed to afford A13-Red up to 95 % homogeneous. We investigated different reaction conditions and found that the total turnover numbers of A13-Red during the in vitro hydroxylation of n-octane could reach up to 3,250 to produce 1-octanol (1.6 mM) over a period of 78 h.

An efficient light-driven P450 BM3 biocatalyst

P450s are heme thiolate enzymes that catalyze the regio- and stereoselective functionalization of unactivated C-H bonds using molecular dioxygen and two electrons delivered by the reductase. We have developed hybrid P450 BM3 heme domains containing a covalently attached Ru(II) photosensitizer in order to circumvent the dependency on the reductase and perform P450 reactions upon visible light irradiation. A highly active hybrid enzyme with improved stability and a modified Ru(II) photosensitizer is able to catalyze the light-driven hydroxylation of lauric acid with total turnover numbers of 935 and initial reaction rate of 125 mol product/(mol enzyme/min).

Synthesis of ω-hydroxy dodecanoic acid based on an engineered CYP153A fusion construct

A bacterial P450 monooxygenase-based whole cell biocatalyst using Escherichia coli has been applied in the production of ω-hydroxy dodecanoic acid from dodecanoic acid (C12-FA) or the corresponding methyl ester. We have constructed and purified a chimeric protein where the fusion of the monooxygenase CYP153A from Marinobacter aquaeloei to the reductase domain of P450 BM3 from Bacillus megaterium ensures optimal protein expression and efficient electron coupling. The chimera was demonstrated to be functional and three times more efficient than other sets of redox components evaluated. The established fusion protein (CYP153A_{M. aq.}-CPR) was used for the hydroxylation of C12-FA in in vivo studies. These experiments yielded 1.2 g l^–1 ω-hydroxy dodecanoic from 10 g l^–1 C12-FA with high regioselectivity (> 95%) for the terminal position. As a second strategy, we utilized C12-FA methyl ester as substrate in a two-phase system (5:1 aqueous/organic phase) configuration to overcome low substrate solubility and product toxicity by continuous extraction. The biocatalytic system was further improved with the coexpression of an additional outer membrane transport system (AlkL) to increase the substrate transfer into the cell, resulting in the production of 4 g l^–1 ω-hydroxy dodecanoic acid. We further summarized the most important aspects of the whole-cell process and thereupon discuss the limits of the applied oxygenation reactions referring to hydrogen peroxide, acetate and P450 concentrations that impact the efficiency of the production host negatively.

The influence of microbial physiology on biocatalyst activity and efficiency in the terminal hydroxylation of n-octane using Escherichia coli expressing the alkane hydroxylase, CYP153A6

BACKGROUND

Biocatalyst improvement through molecular and recombinant means should be complemented with efficient process design to facilitate process feasibility and improve process economics. This study focused on understanding the bioprocess limitations to identify factors that impact the expression of the terminal hydroxylase CYP153A6 and also influence the biocatalytic transformation of n-octane to 1-octanol using resting whole cells of recombinant E. coli expressing the CYP153A6 operon which includes the ferredoxin (Fdx) and the ferredoxin reductase (FdR).

RESULTS

Specific hydroxylation activity decreased with increasing protein expression showing that the concentration of active biocatalyst is not the sole determinant of optimum process efficiency. Process physiological conditions including the medium composition, temperature, glucose metabolism and product toxicity were investigated. A fed-batch system with intermittent glucose feeding was necessary to ease overflow metabolism and improve process efficiency while the introduction of a product sink (BEHP) was required to alleviate octanol toxicity. Resting cells cultivated on complex LB and glucose-based defined medium with similar CYP level (0.20 μmol gDCW-1) showed different biocatalyst activity and efficiency in the hydroxylation of octane over a period of 120 h. This was influenced by differing glucose uptake rate which is directly coupled to cofactor regeneration and cell energy in whole cell biocatalysis. The maximum activity and biocatalyst efficiency achieved presents a significant improvement in the use of CYP153A6 for alkane activation. This biocatalyst system shows potential to improve productivity if substrate transfer limitation across the cell membrane and enzyme stability can be addressed especially at higher temperature.

CONCLUSION

This study emphasises that the overall process efficiency is primarily dependent on the interaction between the whole cell biocatalyst and bioprocess conditions.

Optimization of the bacterial cytochrome P450 BM3 system for the production of human drug metabolites

Drug metabolism in human liver is a process involving many different enzymes. Among them, a number of cytochromes P450 isoforms catalyze the oxidation of most of the drugs commercially available. Each P450 isoform acts on more than one drug, and one drug may be oxidized by more than one enzyme. As a result, multiple products may be obtained from the same drug, and as the metabolites can be biologically active and may cause adverse drug reactions (ADRs), the metabolic profile of a new drug has to be known before this can be commercialized. Therefore, the metabolites of a certain drug must be identified, synthesized and tested for toxicity. Their synthesis must be in sufficient quantities to be used for metabolic tests. This review focuses on the progresses done in the field of the optimization of a bacterial self-sufficient and efficient cytochrome P450, P450 BM3 from Bacillus megaterium, used for the production of metabolites of human enzymes. The progress made in the improvement of its catalytic performance towards drugs, the substitution of the costly NADPH cofactor and its immobilization and scale-up of the process for industrial application are reported.

P450(BM3) (CYP102A1): connecting the dots

P450(BM3) (CYP102A1), a fatty acid hydroxylase from Bacillus megaterium, has been extensively studied over a period of almost forty years. The enzyme has been redesigned to catalyse the oxidation of non-natural substrates as diverse as pharmaceuticals, terpenes and gaseous alkanes using a variety of engineering strategies. Crystal structures have provided a basis for several of the catalytic effects brought about by mutagenesis, while changes to reduction potentials, inter-domain electron transfer rates and catalytic parameters have yielded functional insights. Areas of active research interest include drug metabolite production, the development of process-scale techniques, unravelling general mechanistic aspects of P450 chemistry, methane oxidation, and improving selectivity control to allow the synthesis of fine chemicals. This review draws together the disparate research themes and places them in a historical context with the aim of creating a resource that can be used as a gateway to the field.

Improved expression of recombinant cytochrome P450 monooxygenase in Escherichia coli for asymmetric oxidation of sulfides

Escherichia coli BL21 as production strain for the production of cytochrome P450 monooxygenase (P450SMO) from Rhodococcus sp. in high yields was developed. The expression was first optimized with a series of flask experiments testing several key parameters for their influence on the expression level and enzyme activity. The optimal process parameters found in the flask experiments were verified in a cultivation process in a 5-L bioreactor. Glycerol proved to be superior over glucose as carbon source. Low dissolved oxygen (DO) concentration (<10%) during expression was found to be critical for active P450s production, resulting in expression level of 400 nM for P450SMO. Intact cells were used to establish an efficient bioconversion system for the production of sulfoxidation product. With p-chlorothioanisole as a representative substrate, the desired product (S-sulfoxide) was afforded with 99% ee and highest production of 130 mg/L within 12 h.

Recombinant hemoglobin II from Lucina pectinata: a large-scale method for hemeprotein expression in E. coli

Hemoglobin II from the clam L. pectinata is an O(2) reactive protein that remains oxygenated in the presence of other molecules. To determine the mechanism of ligand selection in this hemoglobin, rHbII was expressed in large quantities using an improved fermentation process. The highest protein yield was obtained by: transforming HbII into the BLi5 cells, inducing and supplementing the culture during the mid-log phase with 1 mM IPTG, 30 microg/mL hemin chloride and 1% glucose, and decreasing the temperature to 30 degrees C after induction. In addition, cell culture density was greatly enhanced by using glycerol, adding MgSO(4), supplementing the media with glucose after the glycerol was consumed and maintaining the dissolved oxygen at 35%. Under these conditions the maximum protein yield obtained was approximately 2,300 mg/L. The results indicate that rHbII is similar to the native protein. The protocol was validated with other hemoglobins, indicating that it can be extended to other hemeproteins.

Crystal structure of inhibitor-bound P450BM-3 reveals open conformation of substrate access channel

P450BM-3 is an extensively studied P450 cytochrome that is naturally fused to a cytochrome P450 reductase domain. Crystal structures of the heme domain of this enzyme have previously generated many insights into features of P450 structure, substrate binding specificity, and conformational changes that occur on substrate binding. Although many P450s are inhibited by imidazole, this compound does not effectively inhibit P450BM-3. Omega-imidazolyl fatty acids have previously been found to be weak inhibitors of the enzyme and show some unusual cooperativity with the substrate lauric acid. We set out to improve the properties of these inhibitors by attaching the omega-imidazolyl fatty acid to the nitrogen of an amino acid group, a tactic that we used previously to increase the potency of substrates. The resulting inhibitors were significantly more potent than their parent compounds lacking the amino acid group. A crystal structure of one of the new inhibitors bound to the heme domain of P450BM-3 reveals that the mode of interaction of the amino acid group with the enzyme is different from that previously observed for acyl amino acid substrates. Further, required movements of residues in the active site to accommodate the imidazole group provide an explanation for the low affinity of imidazole itself. Finally, the previously observed cooperativity with lauric acid is explained by a surprisingly open substrate-access channel lined with hydrophobic residues that could potentially accommodate lauric acid in addition to the inhibitor itself.