Understanding and improving NADPH-dependent reactions by nongrowing Escherichia coli cells

Strategies found not to be suitable for stabilizing high steroid hydroxylation activities of CYP450 BM3-based whole-cell biocatalysts

The implementation of biocatalytic steroid hydroxylation processes plays a crucial role in the pharmaceutical industry due to a plethora of medicative effects of hydroxylated steroid derivatives and their crucial role in drug approval processes. Cytochrome P450 monooxygenases (CYP450s) typically constitute the key enzymes catalyzing these reactions, but commonly entail drawbacks such as poor catalytic rates and the dependency on additional redox proteins for electron transfer from NAD(P)H to the active site. Recently, these bottlenecks were overcome by equipping Escherichia coli cells with highly active variants of the self-sufficient single-component CYP450 BM3 together with hydrophobic outer membrane proteins facilitating cellular steroid uptake. The combination of the BM3 variant KSA14m and the outer membrane pore AlkL enabled exceptionally high testosterone hydroxylation rates of up to 45 U gCDW-1 for resting (i.e., living but non-growing) cells. However, a rapid loss of specific activity heavily compromised final product titers and overall space-time yields. In this study, several stabilization strategies were evaluated on enzyme-, cell-, and reaction level. However, neither changes in biocatalyst configuration nor variation of cultivation media, expression systems, or inducer concentrations led to considerable improvement. This qualified the so-far used genetic construct pETM11-ksa14m-alkL, M9 medium, and the resting-cell state as the best options enabling comparatively efficient activity along with fast growth prior to biotransformation. In summary, we report several approaches not enabling a stabilization of the high testosterone hydroxylation rates, providing vital guidance for researchers tackling similar CYP450 stability issues. A comparison with more stable natively steroid-hydroxylating CYP106A2 and CYP154C5 in equivalent setups further highlighted the high potential of the investigated CYP450 BM3-based whole-cell biocatalysts. The immense and continuously developing repertoire of enzyme engineering strategies provides promising options to stabilize the highly active biocatalysts.

Synthetic gene circuit evolution: Insights and opportunities at the mid-scale

Directed evolution focuses on optimizing single genetic components for predefined engineering goals by artificial mutagenesis and selection. In contrast, experimental evolution studies the adaptation of entire genomes in serially propagated cell populations, to provide an experimental basis for evolutionary theory. There is a relatively unexplored gap at the middle ground between these two techniques, to evolve in vivo entire synthetic gene circuits with nontrivial dynamic function instead of single parts or whole genomes. We discuss the requirements for such mid-scale evolution, with hypothetical examples for evolving synthetic gene circuits by appropriate selection and targeted shuffling of a seed set of genetic components in vivo. Implementing similar methods should aid the rapid generation, functionalization, and optimization of synthetic gene circuits in various organisms and environments, accelerating both the development of biomedical and technological applications and the understanding of principles guiding regulatory network evolution.

Efficiency aspects of regioselective testosterone hydroxylation with highly active CYP450-based whole-cell biocatalysts

Steroid hydroxylations belong to the industrially most relevant reactions catalysed by cytochrome P450 monooxygenases (CYP450s) due to the pharmacological relevance of hydroxylated derivatives. The implementation of respective bioprocesses at an industrial scale still suffers from several limitations commonly found in CYP450 catalysis, that is low turnover rates, enzyme instability, inhibition and toxicity related to the substrate(s) and/or product(s). Recently, we achieved a new level of steroid hydroxylation rates by introducing highly active testosterone-hydroxylating CYP450 BM3 variants together with the hydrophobic outer membrane protein AlkL into Escherichia coli-based whole-cell biocatalysts. However, the activity tended to decrease, which possibly impedes overall productivities and final product titres. In this study, a considerable instability was confirmed and subject to a systematic investigation regarding possible causes. In-depth evaluation of whole-cell biocatalyst kinetics and stability revealed a limitation in substrate availability due to poor testosterone solubility as well as inhibition by the main product 15β-hydroxytestosterone. Instability of CYP450 BM3 variants was disclosed as another critical factor, which is of general significance for CYP450-based biocatalysis. Presented results reveal biocatalyst, reaction and process engineering strategies auguring well for industrial implementation of the developed steroid hydroxylation platform.

Identification and characterization of 3-ketosphinganine reductase activity encoded at the BT_0972 locus in Bacteroides thetaiotaomicron

Bacterial sphingolipid synthesis is important for the fitness of gut commensal bacteria with an implied potential for regulating mammalian host physiology. Multiple steps in bacterial sphingolipid synthesis pathways have been characterized previously, with the first step of de novo sphingolipid synthesis being well conserved between bacteria and eukaryotes. In mammals, the subsequent step of de novo sphingolipid synthesis is catalyzed by 3-ketosphinganine reductase, but the protein responsible for this activity in bacteria has remained elusive. In this study, we analyzed the 3-ketosphinganine reductase activity of several candidate proteins in Bacteroides thetaiotaomicron chosen based on sequence similarity to the yeast 3-ketosphinganine reductase gene. We further developed a metabolomics-based 3-ketosphinganine reductase activity assay, which revealed that a gene at the locus BT_0972 encodes a protein capable of converting 3-ketosphinganine to sphinganine. Taken together, these results provide greater insight into pathways for bacterial sphingolipid synthesis that can aid in future efforts to understand how microbial sphingolipid synthesis modulates host-microbe interactions.

Properties and Mechanisms of Flavin-Dependent Monooxygenases and Their Applications in Natural Product Synthesis

Natural products are usually highly complicated organic molecules with special scaffolds, and they are an important resource in medicine. Natural products with complicated structures are produced by enzymes, and this is still a challenging research field, its mechanisms requiring detailed methods for elucidation. Flavin adenine dinucleotide (FAD)-dependent monooxygenases (FMOs) catalyze many oxidation reactions with chemo-, regio-, and stereo-selectivity, and they are involved in the synthesis of many natural products. In this review, we introduce the mechanisms for different FMOs, with the classical FAD (C4a)-hydroperoxide as the major oxidant. We also summarize the difference between FMOs and cytochrome P450 (CYP450) monooxygenases emphasizing the advantages of FMOs and their specificity for substrates. Finally, we present examples of FMO-catalyzed synthesis of natural products. Based on these explanations, this review will expand our knowledge of FMOs as powerful enzymes, as well as implementation of the FMOs as effective tools for biosynthesis.

Combined active pocket and hinge region engineering to develop an NADPH-dependent phenylglycine dehydrogenase

NADPH-dependent amino acid dehydrogenases (AADHs) are favorable enzymes to construct artificial biosynthetic pathways in whole-cell for high-value noncanonical amino acids (NcAAs) production. Glutamate dehydrogenases (GluDHs) represent attractive candidates for the development of novel NADPH-dependent AADHs. Here, we report the development of a novel NADPH-dependent phenylglycine dehydrogenase by combining active pocket engineering and hinge region engineering of a GluDH from Pseudomonas putida (PpGluDH). The active pocket of PpGluDH was firstly tailored to optimize its binding mode with bulky substrate α-oxobenzeneacetic acid (α-OA), and then, the hinge region was further engineered to tune the protein conformational dynamics, which finally resulted in a mutant M3 (T196A/T121I/L123D) with a 103-fold increase of catalytic efficiency (k_cat/K_m) toward α-OA. The M3 mutant exhibited high catalytic performance in both in vitro biocatalysis preparation and in vivo biosynthesis of l-phenylglycine, indicating its promising practical applications. Our results demonstrated that co-engineering of the active pocket and hinge region is an effective strategy for developing novel NADPH-dependent AADHs from GluDHs for NcAAs production.

One-pot synthesis of 6-aminohexanoic acid from cyclohexane using mixed-species cultures

6-Aminohexanoic acid (6AHA) is a vital polymer building block for Nylon 6 production and an FDA-approved orphan drug. However, its production from cyclohexane is associated with several challenges, including low conversion and yield, and severe environmental issues. We aimed at overcoming these challenges by developing a bioprocess for 6AHA synthesis. A mixed-species approach turned out to be most promising. Thereby, Pseudomonas taiwanensis VLB120 strains harbouring an upstream cascade converting cyclohexane to either є-caprolactone (є-CL) or 6-hydroxyhexanoic acid (6HA) were combined with Escherichia coli JM101 strains containing the corresponding downstream cascade for the further conversion to 6AHA. ε-CL was found to be a better 'shuttle molecule' than 6HA enabling higher 6AHA formation rates and yields. Mixed-species reaction performance with 4 g l-1 biomass, 10 mM cyclohexane, and an air-to-aqueous phase ratio of 23 combined with a repetitive oxygen feeding strategy led to complete substrate conversion with 86% 6AHA yield and an initial specific 6AHA formation rate of 7.7 ± 0.1 U gCDW -1 . The same cascade enabled 49% 7-aminoheptanoic acid yield from cycloheptane. This combination of rationally engineered strains allowed direct 6AHA production from cyclohexane in one pot with high conversion and yield under environmentally benign conditions.

Dynamic control over feedback regulatory mechanisms improves NADPH flux and xylitol biosynthesis in engineered E. coli

We report improved NADPH flux and xylitol biosynthesis in engineered E. coli. Xylitol is produced from xylose via an NADPH dependent reductase. We utilize 2-stage dynamic metabolic control to compare two approaches to optimize xylitol biosynthesis, a stoichiometric approach, wherein competitive fluxes are decreased, and a regulatory approach wherein the levels of key regulatory metabolites are reduced. The stoichiometric and regulatory approaches lead to a 20-fold and 90-fold improvement in xylitol production, respectively. Strains with reduced levels of enoyl-ACP reductase and glucose-6-phosphate dehydrogenase, led to altered metabolite pools resulting in the activation of the membrane bound transhydrogenase and an NADPH generation pathway, consisting of pyruvate ferredoxin oxidoreductase coupled with NADPH dependent ferredoxin reductase, leading to increased NADPH fluxes, despite a reduction in NADPH pools. These strains produced titers of 200 g/L of xylitol from xylose at 86% of theoretical yield in instrumented bioreactors. We expect dynamic control over the regulation of the membrane bound transhydrogenase as well as NADPH production through pyruvate ferredoxin oxidoreductase to broadly enable improved NADPH dependent bioconversions or production via NADPH dependent metabolic pathways.

Assembly of Plant Enzymes in E. coli for the Production of the Valuable (-)-Podophyllotoxin Precursor (-)-Pluviatolide

Lignans are plant secondary metabolites with a wide range of reported health-promoting bioactivities. Traditional routes toward these natural products involve, among others, the extraction from plant sources and chemical synthesis. However, the availability of the sources and the complex chemical structures of lignans often limit the feasibility of these approaches. In this work, we introduce a newly assembled biosynthetic route in E. coli for the efficient conversion of the common higher-lignan precursor (+)-pinoresinol to the noncommercially available (-)-pluviatolide via three intermediates. (-)-Pluviatolide is considered a crossroad compound in lignan biosynthesis, because the methylenedioxy bridge in its structure, resulting from the oxidation of (-)-matairesinol, channels the biosynthetic pathway toward the microtubule depolymerizer (-)-podophyllotoxin. This oxidation reaction is catalyzed with high regio- and enantioselectivity by a cytochrome P450 monooxygenase from Sinopodophyllum hexandrum (CYP719A23), which was expressed and optimized regarding redox partners in E. coli. Pinoresinol-lariciresinol reductase from Forsythia intermedia (FiPLR), secoisolariciresinol dehydrogenase from Podophyllum pleianthum (PpSDH), and CYP719A23 were coexpressed together with a suitable NADPH-dependent reductase to ensure P450 activity, allowing for four sequential biotransformations without intermediate isolation. By using an E. coli strain coexpressing the enzymes originating from four plants, (+)-pinoresinol was efficiently converted, allowing the isolation of enantiopure (-)-pluviatolide at a concentration of 137 mg/L (ee ≥99% with 76% isolated yield).

Repurposing Inflatable Packaging Pillows as Bioreactors: a Convenient Synthesis of Glucosone by Whole-Cell Catalysis Under Oxygen

Biocatalysis using molecular oxygen as the electron acceptor has significant potential for selective oxidations at low cost. However, oxygen is poorly soluble in water, and its slow rate of mass transfer in the aqueous phase is a major obstacle, even for laboratory-scale syntheses. Oxygen transfer can be accelerated by vigorous mechanical methods, but these are often incompatible with biological catalysts. Gentler conditions can be achieved with shallow, high surface area bag reactors that are designed for single use and generally for specialized cell culture applications. As a less-expensive alternative to these high-end bioreactors, we describe repurposing inflatable shipping pillows with resealable valves to provide high surface area mixing under oxygen for preparative synthesis of glucosone (D-arabino-hexos-2-ulose) from D-glucose using non-growing Escherichia coli whole cells containing recombinant pyranose 2-oxidase (POX) as catalyst. Parallel reactions permitted systematic study of the effects of headspace composition (i.e., air vs 100% oxygen), cell density, exogenous catalase, and reaction volume in the oxidation of 10% glucose. Importantly, only a single charge of 100% oxygen is required for stoichiometric conversion on a multi-gram scale in 18 h with resting cells, and the conversion was successfully repeated with recycled cells.

In Vivo, High-Throughput Selection of Thermostable Cyclohexanone Monooxygenase (CHMO)

Cyclohexanone monooxygenase (CHMO) from Acinetobacter sp. NCIMB 9871 is characterized as having wide substrate versatility for the biooxidation of (cyclic) ketones into esters and lactones with high stereospecificity. Despite industrial potential, CHMO usage is restricted by poor thermostability. Limited high-throughput screening tools and challenges in rationally engineering thermostability have impeded CHMO engineering efforts. We demonstrate the application of an aerobic, high-throughput growth selection platform in Escherichia coli (strain MX203) for the discovery of thermostability enhancing mutations for CHMO. The selection employs growth for the easy readout of CHMO activity in vivo, by requiring nicotinamide adenine dinucleotide phosphate (NADPH)-consuming enzymes to restore cellular redox balance. In the presence of the native substrate cyclohexanone, variant CHMO GV (A245G-A288V) was discovered from a random mutagenesis library screened at 42 °C. This variant retained native activity, exhibited ~4.4-fold improvement in residual activity after 30 °C incubation, and demonstrated ~5-fold higher cyclohexanone conversion at 37 °C compared to the wild type. Molecular modeling indicates that CHMO GV experiences more favorable residue packing and supports additional backbone hydrogen bonding. Further rational design resulted in CHMO A245G-A288V-T415C with improved thermostability at 45 °C. Our platform for oxygenase evolution enabled the rapid engineering of protein stability critical for industrial scalability.

Cytochrome P450-mediated N-demethylation of noscapine by whole-cell biotransformation: process limitations and strategies for optimisation

Cytochrome P450 enzymes catalyse reactions of significant industrial interest but are underutilised in large-scale bioprocesses due to enzyme stability, cofactor requirements and the poor aqueous solubility and microbial toxicity of typical substrates and products. In this work, we investigate the potential for preparative-scale N-demethylation of the opium poppy alkaloid noscapine by a P450_BM3 (CYP102A1) mutant enzyme in a whole-cell biotransformation system. We identify and address several common limitations of whole-cell P450 biotransformations using this model N-demethylation process. Mass transfer into Escherichia coli cells was found to be a major limitation of biotransformation rate and an alternative Gram-positive expression host Bacillus megaterium provided a 25-fold improvement in specific initial rate. Two methods were investigated to address poor substrate solubility. First, a biphasic biotransformation system was developed by systematic selection of potentially biocompatible solvents and in silico solubility modelling using Hansen solubility parameters. The best-performing biphasic system gave a 2.3-fold improvement in final product titre compared to a single-phase system but had slower initial rates of biotransformation due to low substrate concentration in the aqueous phase. The second strategy aimed to improve aqueous substrate solubility using cyclodextrin and hydrophilic polymers. This approach provided a fivefold improvement in initial biotransformation rate and allowed a sixfold increase in final product concentration. Enzyme stability and cell viability were identified as the next parameters requiring optimisation to improve productivity. The approaches used are also applicable to the development of other pharmaceutical P450-mediated biotransformations.

Highly efficient asymmetric reduction of ketopantolactone to d-(−)-pantolactone by Escherichia coli cells expressing recombinant conjugated polyketone reductase and glucose dehydrogenase in a fed-batch biphasic reaction system

Enantiopure d-(−)-pantolactone was efficiently synthesized by Escherichia coli cells expressing recombinant CduCPR and BsuGDH in a fed-batch biphasic reaction system.

A multi-enzyme cascade reaction for the production of 6-hydroxyhexanoic acid

Multi-enzyme cascade reactions capture the essence of nature's efficiency by increasing the productivity of a process. Here we describe one such three-enzyme cascade for the synthesis of 6-hydroxyhexanoic acid. Whole cells of Escherichia coli co-expressing an alcohol dehydrogenase and a Baeyer-Villiger monooxygenase (CHMO) for internal cofactor regeneration were used without the supply of external NADPH or NADP+. The product inhibition caused by the ε-caprolactone formed by the CHMO was overcome by the use of lipase CAL-B for in situ conversion into 6-hydroxyhexanoic acid. A stirred tank reactor under fed-batch mode was chosen for efficient catalysis. By using this setup, a product titre of >20 g L-1 was achieved in a 500 mL scale with an isolated yield of 81% 6-hydroxyhexanoic acid.

The effect of CO2 in enhancing photosynthetic cofactor recycling for alcohol dehydrogenase mediated chiral synthesis in cyanobacteria

The light harvesting photosystem in cyanobacteria offers a potential pathway for the regeneration of the nicotinamide cofactor NADPH, thereby facilitating the application of cyanobacteria as excellent whole cell biocatalysts in oxidoreductase-mediated biotransformation. The use of cyanobacterial metabolism for cofactor recycling improves the atom economy of the process compared to the commonly employed enzyme-coupled cofactor recycling using enzymes such as glucose dehydrogenase. Here we report the asymmetric conversion of acetophenone to chiral 1-phenylethanol by recombinant Synechococcus elongatus PCC 7942 whole cell biocatalyst that expresses the NADPH dependent L. kefir alcohol dehydrogenase. Besides light, it was observed that carbon dioxide levels play a critical role in improving the bioconversion efficiency possibly due to the enhanced growth rate and improved cofactor availability at elevated CO₂ levels. Complete reduction of acetophenone to optically pure (R)-1-phenylethanol at 99% enantiomeric excess was achieved within 6 h with a relatively low cell density of 0.66 g/l by coupling optimum light and CO₂ levels and without the need for a co-substrate.

Escherichia coli Fails to Efficiently Maintain the Activity of an Important Flavin Monooxygenase in Recombinant Overexpression

This paper describes the measurement and analysis of in vivo activity and stability of cyclohexanone monooxygenase from Acinetobacter sp. NCIMB 9871 (CHMO), a model Baeyer–Villiger monooxygenase, in the recombinant host Escherichia coli. This enzyme was often described as poorly stable in vitro, and has recently been found to deactivate rapidly in the absence of its essential cofactors and antioxidants. Its stability in vivo was scarcely studied, so far. Under conditions common for the overexpression of CHMO we investigated the ability of the host to support these properties using metabolomics. Our results showed that E. coli failed to provide the intracellular levels of cofactors required to functionally stabilize the enzyme, although the biocatalyst was produced in high concentration, and was invariably detected after protein synthesis had stopped. We thus infer that biotechnological applications of CHMO with this host relied on a residual activity of approximately 5-10%. Other microorganisms might offer a more efficient solution for recombinant production of CHMO and related enzymes.

Library Growth and Protein Expression: Optimal and Reproducible Microtiter Plate Expression of Recombinant Enzymes in E. coli Using MTP Shakers

Escherichia coli (E. coli) as heterologous host enables the recombinant expression of the desired protein in high amounts. Nevertheless, the expression in such a host, especially by utilizing a strong induction system, can result in insoluble and/or inactive protein fractions (inclusion bodies). Furthermore, the expression of different enzyme variants often leads to a diverse growth behavior of the E. coli clones resulting in the identification of false-positives when screening a mutant library. Thus, we developed a protocol for an optimal and reproducible protein expression in microtiter plates showcased for the expression of the cyclohexanone monooxygenase (CHMO) from Acinetobacter sp. NCIMB 9871. By emerging this protocol, several parameters concerning the expression medium, the cultivation temperatures, shaking conditions as well as time and induction periods for CHMO were investigated. We employed a microtiter plate shaker with humidity and temperature control (Cytomat™) (integrated in a robotic platform) to obtain an even growth and expression over the plates. Our optimized protocol provides a comprehensive overview of the key factors influencing a reproducible protein expression and this should serve as basis for the adaptation to other enzyme classes.

ROS mediated selection for increased NADPH availability in Escherichia coli

The economical production of chemicals and fuels by microbial processes remains an intense area of interest in biotechnology. A key limitation in such efforts concerns the availability of key co-factors, in this case NADPH, required for target pathways. Many of the strategies pursued for increasing NADPH availability in Escherichia coli involve manipulations to the central metabolism, which can create redox imbalances and overall growth defects. In this study we used a reactive oxygen species based selection to search for novel methods of increasing NADPH availability. We report a loss of function mutation in the gene hdfR appears to increase NADPH availability in E. coli. Additionally, we show this excess NADPH can be used to improve the production of 3HP in E. coli.

In vivo plug-and-play: a modular multi-enzyme single-cell catalyst for the asymmetric amination of ketoacids and ketones

Background

Transaminases have become a key tool in biocatalysis to introduce the amine functionality into a range of molecules like prochiral α-ketoacids and ketones. However, due to the necessity of shifting the equilibrium towards the product side (depending on the amine donor) an efficient amination system may require three enzymes. So far, this well-established transformation has mainly been performed in vitro by assembling all biocatalysts individually, which comes along with elaborate and costly preparation steps. We present the design and characterization of a flexible approach enabling a quick set-up of single-cell biocatalysts producing the desired enzymes. By choosing an appropriate co-expression strategy, a modular system was obtained, allowing for flexible plug-and-play combination of enzymes chosen from the toolbox of available transaminases and/or recycling enzymes tailored for the desired application.

Results

By using a two-plasmid strategy for the recycling enzyme and the transaminase together with chromosomal integration of an amino acid dehydrogenase, two enzyme modules could individually be selected and combined with specifically tailored E. coli strains. Various plug-and-play combinations of the enzymes led to the construction of a series of single-cell catalysts suitable for the amination of various types of substrates. On the one hand the fermentative amination of α-ketoacids coupled both with metabolic and non-metabolic cofactor regeneration was studied, giving access to the corresponding α-amino acids in up to 96% conversion. On the other hand, biocatalysts were employed in a non-metabolic, “in vitro-type” asymmetric reductive amination of the prochiral ketone 4-phenyl-2-butanone, yielding the amine in good conversion (77%) and excellent stereoselectivity (ee = 98%).

Conclusions

The described modularized concept enables the construction of tailored single-cell catalysts which provide all required enzymes for asymmetric reductive amination in a flexible fashion, representing a more efficient approach for the production of chiral amines and amino acids.

Electronic supplementary material

The online version of this article (doi:10.1186/s12934-017-0750-5) contains supplementary material, which is available to authorized users.

Non-hazardous biocatalytic oxidation in Nylon-9 monomer synthesis on a 40 g scale with efficient downstream processing

This paper describes the development of a biocatalytic process on the multi-dozen gram scale for the synthesis of a precursor to Nylon-9, a specialty polyamide. Such materials are growing in demand, but their corresponding monomers are often difficult to synthesize, giving rise to biocatalytic approaches. Here, we implemented cyclopentadecanone monooxygenase as an Escherichia coli whole-cell biocatalyst in a defined medium, together with a substrate feeding-product removal concept, and an optimized downstream processing (DSP). A previously described hazardous peracid-mediated oxidation was thus replaced with a safe and scalable protocol, using aerial oxygen as oxidant, and water as reaction solvent. The engineered process converted 42 g (0.28 mol) starting material ketone to the corresponding lactone with an isolated yield of 70% (33 g), after highly efficient DSP with 95% recovery of the converted material, translating to a volumetric yield of 8 g pure product per liter. Biotechnol. Bioeng. 2017;114: 1670-1678. © 2017 Wiley Periodicals, Inc.

Enhanced xylitol production: Expression of xylitol dehydrogenase from Gluconobacter oxydans and mixed culture of resting cell

Xylitol has numerous applications in food and pharmaceutical industry, and it can be biosynthesized by microorganisms. In the present study, xdh gene, encoding xylitol dehydrogenase (XDH), was cloned from the genome of Gluconobacter oxydans CGMCC 1.49 and overexpressed in Escherichia coli BL21. Sequence analysis revealed that XDH has a TGXXGXXG NAD(H)-binding motif and a YXXXK active site motif, and belongs to the short-chain dehydrogenase/reductase family. And then, the enzymatic properties and kinetic parameter of purified recombinant XDH were investigated. Subsequently, transformations of xylitol from d-xylulose and d-arabitol, respectively, were studied through mixed culture of resting cells of G. oxydans wild-type strain and recombinant strain BL21-xdh. We obtained 28.80 g/L xylitol by mixed culture from 30 g/L d-xylulose in 28 h. The production was increased by more than three times as compared with that of wild-type strain. Furthermore, 25.10 g/L xylitol was produced by the mixed culture from 30 g/L d-arabitol in 30 h with a yield of 0.837 g/g, and the max volumetric productivity of 0.990 g/L h was obtained at 22 h. These contrast to the fact that wild-type strain G. oxydans only produced 8.10 g/L xylitol in 30 h with a yield of 0.270 g/g. To our knowledge, these values are the highest among the reported yields and productivity efficiencies of xylitol from d-arabitol with engineering strains.

A CYP21A2 based whole-cell system in Escherichia coli for the biotechnological production of premedrol

BACKGROUND

Synthetic glucocorticoids like methylprednisolone (medrol) are of high pharmaceutical interest and represent powerful drugs due to their anti-inflammatory and immunosuppressive effects. Since the chemical hydroxylation of carbon atom 21, a crucial step in the synthesis of the medrol precursor premedrol, exhibits a low overall yield because of a poor stereo- and regioselectivity, there is high interest in a more sustainable and efficient biocatalytic process. One promising candidate is the mammalian cytochrome P450 CYP21A2 which is involved in steroid hormone biosynthesis and performs a selective oxyfunctionalization of C21 to provide the precursors of aldosterone, the main mineralocorticoid, and cortisol, the most important glucocorticoid. In this work, we demonstrate the high potential of CYP21A2 for a biotechnological production of premedrol, an important precursor of medrol.

RESULTS

We successfully developed a CYP21A2-based whole-cell system in Escherichia coli by coexpressing the cDNAs of bovine CYP21A2 and its redox partner, the NADPH-dependent cytochrome P450 reductase (CPR), via a bicistronic vector. The synthetic substrate medrane was selectively 21-hydroxylated to premedrol with a max. yield of 90 mg L(-1) d(-1). To further improve the biocatalytic activity of the system by a more effective electron supply, we exchanged the CPR with constructs containing five alternative redox systems. A comparison of the constructs revealed that the redox system with the highest endpoint yield converted 70 % of the substrate within the first 2 h showing a doubled initial reaction rate compared with the other constructs. Using the best system we could increase the overall yield of premedrol to a maximum of 320 mg L(-1) d(-1) in shaking flasks. Optimization of the biotransformation in a bioreactor could further improve the premedrol gain to a maximum of 0.65 g L(-1) d(-1).

CONCLUSIONS

We successfully established a CYP21-based whole-cell system for the biotechnological production of premedrol, a pharmaceutically relevant glucocorticoid, in E. coli and could improve the system by optimizing the redox system concerning reaction velocity and endpoint yield. This is the first step for a sustainable replacement of a complicated chemical low-yield hydroxylation by a biocatalytic cytochrome P450-based whole-cell system.

Rational design of a synthetic Entner-Doudoroff pathway for improved and controllable NADPH regeneration

NADPH is an essential cofactor for the biosynthesis of several high-value chemicals, including isoprenoids, fatty acid-based fuels, and biopolymers. Tunable control over all potentially rate-limiting steps, including the NADPH regeneration rate, is crucial to maximizing production titers. We have rationally engineered a synthetic version of the Entner-Doudoroff pathway from Zymomonas mobilis that increased the NADPH regeneration rate in Escherichia coli MG1655 by 25-fold. To do this, we combined systematic design rules, biophysical models, and computational optimization to design synthetic bacterial operons expressing the 5-enzyme pathway, while eliminating undesired genetic elements for maximum expression control. NADPH regeneration rates from genome-integrated pathways were estimated using a NADPH-binding fluorescent reporter and by the productivity of a NADPH-dependent terpenoid biosynthesis pathway. We designed and constructed improved pathway variants by employing the RBS Library Calculator to efficiently search the 5-dimensional enzyme expression space and by performing 40 cycles of MAGE for site-directed genome mutagenesis. 624 pathway variants were screened using a NADPH-dependent blue fluorescent protein, and 22 were further characterized to determine the relationship between enzyme expression levels and NADPH regeneration rates. The best variant exhibited 25-fold higher normalized mBFP levels when compared to wild-type strain. Combining the synthetic Entner-Doudoroff pathway with an optimized terpenoid pathway further increased the terpenoid titer by 97%.

Construction of allitol synthesis pathway by multi-enzyme coexpression in Escherichia coli and its application in allitol production

An engineered strain for the conversion of D-fructose to allitol was developed by constructing a multi-enzyme coupling pathway and cofactor recycling system in Escherichia coli. D-Psicose-3-epimerase from Ruminococcus sp. and ribitol dehydrogenase from Klebsiella oxytoca were coexpressed to form the multi-enzyme coupling pathway for allitol production. The cofactor recycling system was constructed using the formate dehydrogenase gene from Candida methylica for continuous NADH supply. The recombinant strain produced 10.62 g/l allitol from 100 mM D-fructose. To increase the intracellular concentration of the substrate, the glucose/fructose facilitator gene from Zymomonas mobilis was incorporated into the engineered strain. The results showed that the allitol yield was enhanced significantly to 16.53 g/l with a conversion rate of 92 %. Through optimizing conversion conditions, allitol was produced effectively on a large scale by the whole-cell biotransformation system; the yield reached 48.62 g/l when 500 mM D-fructose was used as the substrate.

A recombinant CYP11B1 dependent Escherichia coli biocatalyst for selective cortisol production and optimization towards a preparative scale

BACKGROUND

Human mitochondrial CYP11B1 catalyzes a one-step regio- and stereoselective 11β-hydroxylation of 11-deoxycortisol yielding cortisol which constitutes not only the major human stress hormone but also represents a commercially relevant therapeutic drug due to its anti-inflammatory and immunosuppressive properties. Moreover, it is an important intermediate in the industrial production of synthetic pharmaceutical glucocorticoids. CYP11B1 thus offers a great potential for biotechnological application in large-scale synthesis of cortisol. Because of its nature as external monooxygenase, CYP11B1-dependent steroid hydroxylation requires reducing equivalents which are provided from NADPH via a redox chain, consisting of adrenodoxin reductase (AdR) and adrenodoxin (Adx).

RESULTS

We established an Escherichia coli based whole-cell system for selective cortisol production from 11-deoxycortisol by recombinant co-expression of the demanded 3 proteins. For the subsequent optimization of the whole-cell activity 3 different approaches were pursued: Firstly, CYP11B1 expression was enhanced 3.3-fold to 257 nmol∗L(-1) by site-directed mutagenesis of position 23 from glycine to arginine, which was accompanied by a 2.6-fold increase in cortisol yield. Secondly, the electron transfer chain was engineered in a quantitative manner by introducing additional copies of the Adx cDNA in order to enhance Adx expression on transcriptional level. In the presence of 2 and 3 copies the initial linear conversion rate was greatly accelerated and the final product concentration was improved 1.4-fold. Thirdly, we developed a screening system for directed evolution of CYP11B1 towards higher hydroxylation activity. A culture down-scale to microtiter plates was performed and a robot-assisted, fluorescence-based conversion assay was applied for the selection of more efficient mutants from a random library.

CONCLUSIONS

Under optimized conditions a maximum productivity of 0.84 g cortisol∗L(-1)∗d(-1) was achieved, which clearly shows the potential of the developed system for application in the pharmaceutical industry.

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.

Bioconversion of α-pinene by a novel cold-adapted fungus Chrysosporium pannorum

The psychrotrophic fungus Chrysosporium pannorum A-1 is reported for the first time as a novel biocatalyst for O₂-promoted oxidation of α-pinene. GC–MS analysis indicated that the main products of the reaction were compounds of a high commercial value, verbenol (1) and verbenone (2). Exponentially growing cells (days 2–3) were about twice as active as cells in the late stationary phase in terms of the total concentration of products. The highest yields of 1 and 2 were obtained using three-day and two-day-old mycelia and a medium containing 1.5 and 1 % (v/v) of the substrate, respectively. The optimal time for the bioconversion of α-pinene varied from 1 to 3 days, and depended on the kind of product desired. Most of 1 was produced at a relatively high concentration of 360 mg/L after the first six hours of α-pinene bioconversion [with an average yield of 69 mg/(g _{dry cell} L aqueous phase)]. The oxidative activity of C. pannorum was identified across a wide temperature range of 5–25 °C, 10 °C being the optimum for the production of 1 and 20 °C for the production of 2. Sequential addition of the substrate during 3 days of the biotransformation resulted in a significant increase in 1 and 2 up to 722 and 176 mg/L, respectively, and a 2-fold enhancement of product yield as compared to bioconversion with a single supply of α-pinene. The concentration of total conversion products in the culture medium reached 1.33 g/L [which corresponded product yield of 225 mg/(g _{dry cell} L)]. This represents probably the most promising result reported to date for oxidative biotransformation of α-pinene by a wild-type microorganism.

Influence of Cofactor Regeneration Strategies on Preparative-Scale, Asymmetric Carbonyl Reductions by Engineered Escherichia coli

This study was designed to determine whether whole cells or crude enzyme extracts are more effective for preparative-scale ketone reductions by dehydrogenases as well as learning which cofactor regeneration scheme is most effective. Based on results from three representative ketone substrates (an α-fluoro-β-keto ester, a bis-trifluoromethylated acetophenone, and a symmetrical β-diketone), our results demonstrate that several nicotinamide cofactor regeneration strategies can be applied to preparative-scale dehydrogenase-catalyzed reactions successfully.

Whole-cell biocatalysis for selective and productive C-O functional group introduction and modification

During the last decades, biocatalysis became of increasing importance for chemical and pharmaceutical industries. Regarding regio- and stereospecificity, enzymes have shown to be superior compared to traditional chemical synthesis approaches, especially in C-O functional group chemistry. Catalysts established on a process level are diverse and can be classified along a functional continuum starting with single-step biotransformations using isolated enzymes or microbial strains towards fermentative processes with recombinant microorganisms containing artificial synthetic pathways. The complex organization of respective enzymes combined with aspects such as cofactor dependency and low stability in isolated form often favors the use of whole cells over that of isolated enzymes. Based on an inventory of the large spectrum of biocatalytic C-O functional group chemistry, this review focuses on highlighting the potentials, limitations, and solutions offered by the application of self-regenerating microbial cells as biocatalysts. Different cellular functionalities are discussed in the light of their (possible) contribution to catalyst efficiency. The combined achievements in the areas of protein, genetic, metabolic, and reaction engineering enable the development of whole-cell biocatalysts as powerful tools in organic synthesis.

Improvement of NADPH bioavailability in Escherichia coli through the use of phosphofructokinase deficient strains

NADPH-dependent reactions play important roles in production of industrially valuable compounds. In this study, we used phosphofructokinase (PFK)-deficient strains to direct fructose-6-phosphate to be oxidized through the pentose phosphate pathway (PPP) to increase NADPH generation. pfkA or pfkB single deletion and double-deletion strains were tested for their ability to produce lycopene. Since lycopene biosynthesis requires many NADPH, levels of lycopene were compared in a set of isogenic strains, with the pfkA single deletion strain showing the highest lycopene yield. Using another NADPH-requiring process, a one-step reduction reaction of 2-chloroacrylate to 2-chloropropionic acid by 2-haloacrylate reductase, the pfkA pfkB double-deletion strain showed the highest yield of 2-chloropropionic acid product. The combined effect of glucose-6-phosphate dehydrogenase overexpression or lactate dehydrogenase deletion with PFK deficiency on NADPH bioavailability was also studied. The results indicated that the flux distribution of fructose-6-phosphate between glycolysis and the pentose phosphate pathway determines the amount of NAPDH available for reductive biosynthesis.

Efficient one-step production of (S)-1-phenyl-1,2-ethanediol from (R)-enantiomer plus NAD(+)-NADPH in-situ regeneration using engineered Escherichia coli

Background

Candida parapsilosis CCTCC M203011 catalyzes the stereoinversion of (R)-1-phenyl-1,2-ethanediol (PED) through oxidation and reduction. Its NAD⁺-linked (R)-carbonyl reductase (RCR) catalyzes the oxidization of (R)-PED to 2-hydroxyacetophenone (HAP), and its NADPH-dependent (S)-carbonyl reductase (SCR) catalyzes the reduction of HAP to (S)-PED. The reactions require NAD⁺ and NADPH as cofactors. However, even if NAD⁺ and NADPH are added, the biotransformation of (S)-PED from the (R)-enantiomer by an Escherichia coli strain co-expressing RCR and SCR is slow and gives low yields, probably as a result of insufficient or imbalanced redox cofactors. To prepare (S)-PED from the (R)-enantiomer in one-step efficiently, plus redox cofactor regeneration, we introduced pyridine nucleotide transhydrogenases (PNTs) from E. coli to the metabolic pathway of (S)-PED.

Results

The PNTs were successfully introduced into the E. coli strain RSAB. Most of the PNT activities occurred in the cell membrane of E. coli. The introduction of PNTs increased intracellular NAD⁺ and NADH concentrations and decreased the NADPH pool without affecting the total nucleotide concentration and cell growth properties. The presence of PNTs increased the NADH/NAD⁺ ratio slightly and reduced the NADPH/NADP⁺ ratio about two-fold; the ratio of NADPH/NADP⁺ to NADH/NAD⁺ was reduced from 36 to 17. So, the PNTs rebalanced the cofactor pathways: the rate of RCR was increased, while the rate of SCR was decreased. When the ratio of NAD⁺/NADPH was 3.0 or higher, the RSAB strain produced (S)-PED with the highest optical purity, 97.4%, and a yield of 95.2% at 6 h. The introduction of PNTs stimulated increases of 51.5% and 80.6%, respectively, in optical purity and yield, and simultaneously reduced the reaction time seven-fold.

Conclusions

In this work, PNTs were introduced into E. coli to rebalance the cofactor pools within the engineered (S)-PED pathways. The efficient one-step production of (S)-PED plus NAD⁺–NADPH in-situ regeneration was realized. This work provided new insights into cofactor rebalancing pathways, using metabolic engineering methods, for efficient chiral alcohol production.

Effects of NADH kinase on NADPH-dependent biotransformation processes in Escherichia coli

Sufficient supply of NADPH is one of the most important factors affecting the productivity of biotransformation processes. In this study, construction of an efficient NADPH-regenerating system was attempted using direct phosphorylation of NADH by NADH kinase (Pos5p) from Saccharomyces cerevisiae for producing guanosine diphosphate (GDP)-L-fucose and ε-caprolactone in recombinant Escherichia coli. Expression of Pos5p in a fed-batch culture of recombinant E. coli producing GDP-L-fucose resulted in a maximum GDP-L-fucose concentration of 291.5 mg/l, which corresponded to a 51 % enhancement compared with the control strain. In a fed-batch Baeyer-Villiger (BV) oxidation of cyclohexanone using recombinant E. coli expressing Pos5p, a maximum ε-caprolactone concentration of 21.6 g/l was obtained, which corresponded to a 96 % enhancement compared with the control strain. Such an increase might be due to the enhanced availability of NADPH in recombinant E. coli expressing Pos5p. These results suggested that efficient regeneration of NADPH was possible by functional expression of Pos5p in recombinant E. coli, which can be applied to other NADPH-dependent biotransformation processes in E. coli.

Production host selection for asymmetric styrene epoxidation: Escherichia coli vs. solvent-tolerant Pseudomonas

Selection of the ideal microbe is crucial for whole-cell biotransformations, especially if the target reaction intensively interacts with host cell functions. Asymmetric styrene epoxidation is an example of a reaction which is strongly dependent on the host cell owing to its requirement for efficient cofactor regeneration and stable expression of the styrene monooxygenase genes styAB. On the other hand, styrene epoxidation affects the whole-cell biocatalyst, because it involves toxic substrate and products besides the burden of additional (recombinant) enzyme synthesis. With the aim to compare two fundamentally different strain engineering strategies, asymmetric styrene epoxidation by StyAB was investigated using the engineered wild-type strain Pseudomonas sp. strain VLB120ΔC, a styrene oxide isomerase (StyC) knockout strain able to accumulate (S)-styrene oxide, and recombinant E. coli JM101 carrying styAB on the plasmid pSPZ10. Their performance was analyzed during fed-batch cultivation in two-liquid phase biotransformations with respect to specific activity, volumetric productivity, product titer, tolerance of toxic substrate and products, by-product formation, and product yield on glucose. Thereby, Pseudomonas sp. strain VLB120ΔC proved its great potential by tolerating high styrene oxide concentrations and by the absence of by-product formation. The E. coli-based catalyst, however, showed higher specific activities and better yields on glucose. The results not only show the importance but also the complexity of host cell selection and engineering. Finding the optimal strain engineering strategy requires profound understanding of bioprocess and biocatalyst operation. In this respect, a possible negative influence of solvent tolerance on yield and activity is discussed.

A stepwise approach for the reproducible optimization of PAMO expression in Escherichia coli for whole-cell biocatalysis

Background

Baeyer-Villiger monooxygenases (BVMOs) represent a group of enzymes of considerable biotechnological relevance as illustrated by their growing use as biocatalyst in a variety of synthetic applications. However, due to their increased use the reproducible expression of BVMOs and other biotechnologically relevant enzymes has become a pressing matter while knowledge about the factors governing their reproducible expression is scattered.

Results

Here, we have used phenylacetone monooxygenase (PAMO) from Thermobifida fusca, a prototype Type I BVMO, as a model enzyme to develop a stepwise strategy to optimize the biotransformation performance of recombinant E. coli expressing PAMO in 96-well microtiter plates in a reproducible fashion. Using this system, the best expression conditions of PAMO were investigated first, including different host strains, temperature as well as time and induction period for PAMO expression. This optimized system was used next to improve biotransformation conditions, the PAMO-catalyzed conversion of phenylacetone, by evaluating the best electron donor, substrate concentration, and the temperature and length of biotransformation. Combining all optimized parameters resulted in a more than four-fold enhancement of the biocatalytic performance and, importantly, this was highly reproducible as indicated by the relative standard deviation of 1% for non-washed cells and 3% for washed cells. Furthermore, the optimized procedure was successfully adapted for activity-based mutant screening.

Conclusions

Our optimized procedure, which provides a comprehensive overview of the key factors influencing the reproducible expression and performance of a biocatalyst, is expected to form a rational basis for the optimization of miniaturized biotransformations and for the design of novel activity-based screening procedures suitable for BVMOs and other NAD(P)H-dependent enzymes as well.

Engineering yield and rate of reductive biotransformation in Escherichia coli by partial cyclization of the pentose phosphate pathway and PTS-independent glucose transport

Optimization of yields and productivities in reductive whole-cell biotransformations is an important issue for the industrial application of such processes. In a recent study with Escherichia coli, we analyzed the reduction of the prochiral β-ketoester methyl acetoacetate by an R-specific alcohol dehydrogenase (ADH) to the chiral hydroxy ester (R)-methyl 3-hydroxybutyrate (MHB) using glucose as substrate for the generation of NADPH. Deletion of the phosphofructokinase gene pfkA almost doubled the yield to 4.8 mol MHB per mole of glucose, and it was assumed that this effect was due to a partial cyclization of the pentose phosphate pathway (PPP). Here, this partial cyclization was confirmed by (13)C metabolic flux analysis, which revealed a negative net flux from glucose 6-phosphate to fructose 6-phosphate catalyzed by phosphoglucose isomerase. For further process optimization, the genes encoding the glucose facilitator (glf) and glucokinase (glk) of Zymomonas mobilis were overexpressed in recombinant E. coli strains carrying ADH and deletions of either pgi (phosphoglucose isomerase), or pfkA, or pfkA plus pfkB. In all cases, the glucose uptake rate was increased (30-47%), and for strains Δpgi and ΔpfkA also, the specific MHB production rate was increased by 15% and 20%, respectively. The yield of the latter two strains slightly dropped by 11% and 6%, but was still 73% and 132% higher compared to the reference strain with intact pgi and pfkA genes and expressing glf and glk. Thus, metabolic engineering strategies are presented for improving yield and rate of reductive redox biocatalysis by partial cyclization of the PPP and by increasing glucose uptake, respectively.

Biohydrogenation from biomass sugar mediated by in vitro synthetic enzymatic pathways

Different from NAD(P)H regeneration approaches mediated by a single enzyme or a whole-cell microorganism, we demonstrate high-yield generation of NAD(P)H from a renewable biomass sugar--cellobiose through in vitro synthetic enzymatic pathways consisting of 12 purified enzymes and coenzymes. When the NAD(P)H generation system was coupled with its consumption reaction mediated by xylose reductase, the NADPH yield was as high as 11.4 mol NADPH per cellobiose (i.e., 95% of theoretical yield--12 NADPH per glucose unit) in a batch reaction. Consolidation of endothermic reactions and exothermic reactions in one pot results in a very high energy-retaining efficiency of 99.6% from xylose and cellobiose to xylitol. The combination of this high-yield and projected low-cost biohydrogenation and aqueous phase reforming may be important for the production of sulfur-free liquid jet fuel in the future.

Increased NADPH availability in Escherichia coli: improvement of the product per glucose ratio in reductive whole-cell biotransformation

A basic requirement for the efficiency of reductive whole-cell biotransformations is the reducing capacity of the host. Here, the pentose phosphate pathway (PPP) was applied for NADPH regeneration with glucose as the electron-donating co-substrate using Escherichia coli as host. Reduction of the prochiral β-keto ester methyl acetoacetate to the chiral hydroxy ester (R)-methyl 3-hydroxybutyrate (MHB) served as a model reaction, catalyzed by an R-specific alcohol dehydrogenase. The main focus was maximization of the reduced product per glucose yield of this pathway-coupled cofactor regeneration with resting cells. With a strain lacking the phosphoglucose isomerase, the yield of the reference strain was increased from 2.44 to 3.78 mol MHB/mol glucose. Even higher yields were obtained with strains lacking either phosphofructokinase I (4.79 mol MHB/mol glucose) or phosphofructokinase I and II (5.46 mol MHB/mol glucose). These results persuasively demonstrate the potential of NADPH generation by the PPP in whole-cell biotransformations.

Improved NADPH supply for xylitol production by engineered Escherichia coli with glycolytic mutations

Escherichia coli engineered to uptake xylose while metabolizing glucose was previously shown to produce high levels of xylitol from a mixture of glucose and xylose when expressing NADPH-dependent xylose reductase from Candida boidinii (CbXR) (Cirino et al., Biotechnol Bioeng. 2006;95:1167-1176). We then described the effects of deletions of key metabolic pathways (e.g., Embden-Meyerhof-Parnas and pentose phosphate pathway) and reactions (e.g., transhydrogenase and NADH dehydrogenase) on resting-cell xylitol yield (Y RPG: moles of xylitol produced per mole of glucose consumed) (Chin et al., Biotechnol Bioeng. 2009;102:209-220). These prior results demonstrated the importance of direct NADPH supply by NADP+-utilizing enzymes in central metabolism for driving heterologous NADPH-dependent reactions. This study describes strain modifications that improve coupling between glucose catabolism (oxidation) and xylose reduction using two fundamentally different strategies. We first examined the effects of deleting the phosphofructokinase (pfk) gene(s) on growth-uncoupled xylitol production and found that deleting both pfkA and sthA (encoding the E. coli-soluble transhydrogenase) improved the xylitol Y RPG from 3.4 ± 0.6 to 5.4 ± 0.4. The second strategy focused on coupling aerobic growth on glucose to xylitol production by deleting pgi (encoding phosphoglucose isomerase) and sthA. Impaired growth due to imbalanced NADPH metabolism (Sauer et al., J Biol Chem. 2004;279:6613-6619) was alleviated upon expressing CbXR, resulting in xylitol production similar to that of the growth-uncoupled precursor strains but with much less acetate secretion and more efficient utilization of glucose. Intracellular nicotinamide cofactor levels were also quantified, and the magnitude of the change in the NADPH/NADP+ ratio measured from cells consuming glucose in the absence vs. presence of xylose showed a strong correlation to the resulting Y RPG.

Recent developments in the application of Baeyer-Villiger monooxygenases as biocatalysts

Baeyer-Villiger monooxygenases (BVMOs) represent a specific class of monooxygenases that are capable of catalyzing a variety of oxidation reactions, including Baeyer-Villiger oxidations. The recently elucidated BVMO crystal structures have provided a more detailed insight into the complex mechanism of these flavin-containing enzymes. Biocatalytic studies on a number of newly discovered BVMOs have shown that they are very potent oxidative biocatalysts. In addition to catalyzing the regio- and enantioselective Baeyer-Villiger oxidations of a wide range of carbonylic compounds, epoxidations, and enantioselective sulfoxidations have also been shown to be part of their catalytic repertoire. This review provides an overview on the recent developments in BVMO-mediated biocatalytic processes, identification of the catalytic role of these enzymes in metabolic routes and prodrug activation, as well as the efforts in developing effective biocatalytic methodologies to apply BVMOs for the synthesis of high added value compounds.