[Cp*Rh(bpy)(H2O)]2+ as a coenzyme substitute in enzymatic oxidations catalyzed by Baeyer–Villiger monooxygenases

Divorce in the two-component BVMO family: the single oxygenase for enantioselective chemo-enzymatic Baeyer-Villiger oxidations

Two-component flavoprotein monooxygenases consist of a reductase and an oxygenase enzyme. The proof of functionality of the latter without its counterpart as well as the mechanism of flavin transfer remains unanswered beyond doubt. To tackle this question, we utilized a reductase-free reaction system applying purified 2,5-diketocamphane-monooxygenase I (2,5-DKCMO), a FMN-dependent type II Baeyer-Villiger monooxygenase, and synthetic nicotinamide analogues (NCBs) as dihydropyridine derivatives for FMN reduction. This system demonstrated the stand-alone quality of the oxygenase, as well as the mechanism of FMNH2 transport by free diffusion. The efficiency of this reductase-free system strongly relies on the balance of FMN reduction and enzymatic (re)oxidation, since reduced FMN in solution causes undesired side reactions, such as hydrogen peroxide formation. Design of experiments allowed us to (i) investigate the effect of various reaction parameters, underlining the importance to balance the FMN/FMNH2 cycle, (ii) optimize the reaction system for the enzymatic Baeyer-Villiger oxidation of rac-bicyclo[3.2.0]hept-2-en-6-one, rac-camphor, and rac-norcamphor. Finally, this study not only demonstrates the reductase-independence of 2,5-DKCMO, but also revisits the terminology of two-component flavoprotein monooxygenases for this specific case.

Cooperative Catalysis of an Alcohol Dehydrogenase and Rhodium-Modified Periodic Mesoporous Organosilica

The combined use of a metal-complex catalyst and an enzyme is attractive, but typically results in mutual inactivation. A rhodium (Rh) complex immobilized in a bipyridine-based periodic mesoporous organosilica (BPy-PMO) shows high catalytic activity during transfer hydrogenation, even in the presence of bovine serum albumin (BSA), while a homogeneous Rh complex exhibits reduced activity due to direct interaction with BSA. The use of a smaller protein or an amino acid revealed a clear size-sieving effect of the BPy-PMO that protected the Rh catalyst from direct interactions. A combination of Rh-immobilized BPy-PMO and an enzyme (horse liver alcohol dehydrogenase; HLADH) promoted sequential reactions involving the transfer hydrogenation of NAD⁺ to give NADH followed by the asymmetric hydrogenation of 4-phenyl-2-butanone with high enantioselectivity. The use of BPy-PMO as a support for metal complexes could be applied to other systems consisting of a metal-complex catalyst and an enzyme.

Nonconventional regeneration of redox enzymes - a practical approach for organic synthesis?

Oxidoreductases have become useful tools in the hands of chemists to perform selective and mild oxidation and reduction reactions. Instead of mimicking native catalytic cycles, generally involving costly and unstable nicotinamide cofactors, more direct, NAD(P)-independent methodologies are being developed. The promise of these approaches not only lies with simpler and cheaper reaction schemes but also with higher selectivity as compared to whole cell approaches and their mimics.

Metabolic engineering of cofactor flavin adenine dinucleotide (FAD) synthesis and regeneration in Escherichia coli for production of α-keto acids

Cofactor flavin adenine dinucleotide (FAD) plays a vital role in many FAD-dependent enzymatic reactions; therefore, how to efficiently accelerate FAD synthesis and regeneration is an important topic in biocatalysis and metabolic engineering. In this study, a system involving the synthesis pathway and regeneration of FAD was engineered in Escherichia coli to improve α-keto acid production-from the corresponding l-amino acids-catalyzed by FAD-dependent l-amino acid deaminase (l-AAD). First, key genes, ribH, ribC, and ribF, were overexpressed and fine-tuned for FAD synthesis. In the resulting E. coli strain PHCF7, strong overexpression of pma, ribC, and ribF and moderate overexpression of ribH yielded a 90% increase in phenylpyruvic acid (PPA) titer: 19.4 ± 1.1 g · L^-1 . Next, formate dehydrogenase (FDH) and NADH oxidase (NOX) were overexpressed to strengthen the regeneration rate of cofactors FADH₂ /FAD using FDH for FADH₂ /FAD regeneration and NOX for NAD⁺ /NADH regeneration. The resulting E. coli strain PHCF7-FDH-NOX yielded the highest PPA production: 31.4 ± 1.1 g · L^-1 . Finally, this whole-cell system was adapted to production of other α-keto acids including α-ketoglutaric acid, α-ketoisocaproate, and keto-γ-methylthiobutyric acid to demonstrate the broad utility of strengthening of FAD synthesis and FADH₂ /FAD regeneration for production of α-keto acids. Notably, the strategy reported herein may be generally applicable to other flavin-dependent biocatalysis reactions and metabolic pathway optimizations. Biotechnol. Bioeng. 2017;114: 1928-1936. © 2017 Wiley Periodicals, Inc.

An NAD(P)H-Dependent Artificial Transfer Hydrogenase for Multienzymatic Cascades

Enzymes typically depend on either NAD(P)H or FADH2 as hydride source for reduction purposes. In contrast, organometallic catalysts most often rely on isopropanol or formate to generate the reactive hydride moiety. Here we show that incorporation of a Cp*Ir cofactor possessing a biotin moiety and 4,7-dihydroxy-1,10-phenanthroline into streptavidin yields an NAD(P)H-dependent artificial transfer hydrogenase (ATHase). This ATHase (0.1 mol%) catalyzes imine reduction with 1 mM NADPH (2 mol%), which can be concurrently regenerated by a glucose dehydrogenase (GDH) using only 1.2 equiv of glucose. A four-enzyme cascade consisting of the ATHase, the GDH, a monoamine oxidase, and a catalase leads to the production of enantiopure amines.

Flavoprotein monooxygenases for oxidative biocatalysis: recombinant expression in microbial hosts and applications

External flavoprotein monooxygenases comprise a group of flavin-dependent oxidoreductases that catalyze the insertion of one atom of molecular oxygen into an organic substrate and the second atom is reduced to water. These enzymes are involved in a great number of metabolic pathways both in prokaryotes and eukaryotes. Flavoprotein monooxygenases have attracted the attention of researchers for several decades and the advent of recombinant DNA technology caused a great progress in the field. These enzymes are subjected to detailed biochemical and structural characterization and some of them are also regarded as appealing oxidative biocatalysts for the production of fine chemicals and valuable intermediates toward active pharmaceutical ingredients due to their high chemo-, stereo-, and regioselectivity. Here, we review the most representative reactions catalyzed both in vivo and in vitro by prototype flavoprotein monooxygenases, highlighting the strategies employed to produce them recombinantly, to enhance the yield of soluble proteins, and to improve cofactor regeneration in order to obtain versatile biocatalysts. Although we describe the most outstanding features of flavoprotein monooxygenases, we mainly focus on enzymes that were cloned, expressed and used for biocatalysis during the last years.

Towards recyclable NAD(P)H regeneration catalysts

Rh(III)-TsDPEN, an immobilized analog of the well-known [Cp*Rh(bpy)(H(2)O)](2+) was evaluated as a heterogeneous, recyclable regeneration catalyst for reduced oxidoreductase cofactors [NAD(P)H]. Repeated use of this catalyst was established and the catalytic properties were initially investigated. Apparently, Rh(III)-TsDPEN is prone to severe diffusion limitations, necessitating further developments. Overall, a promising concept for chemoenzymatic redox catalysis is proposed, which may overcome some of the current limitations such as catalyst cost and incompatibility of Rh with some biocatalysts.

Isolation and initial characterization of a novel type of Baeyer-Villiger monooxygenase activity from a marine microorganism

A novel type of Baeyer–Villiger monooxygenase (BVMO) has been found in a marine strain of Stenotrophomonas maltophila strain PML168 that was isolated from a temperate intertidal zone. The enzyme is able to use NADH as the source of reducing power necessary to accept the atom of diatomic oxygen not incorporated into the oxyfunctionalized substrate. Growth studies have establish that the enzyme is inducible, appears to serve a catabolic role, and is specifically induced by one or more unidentified components of seawater as well as various anthropogenic xenobiotic compounds. A blast search of the primary sequence of the enzyme, recovered from the genomic sequence of the isolate, has placed this atypical BVMO in the context of the several hundred known members of the flavoprotein monooxygenase superfamily. A particular feature of this BVMO lies in its truncated C‐terminal domain, which results in a relatively small protein (357 amino acids; 38.4 kDa). In addition, metagenomic screening has been conducted on DNA recovered from an extensive range of marine environmental samples to gauge the relative abundance and distribution of similar enzymes within the global marine microbial community. Although low, abundance was detected in samples from many marine provinces, confirming the potential for biodiscovery in marine microorganisms.

Baeyer-Villiger monooxygenases in aroma compound synthesis

Baeyer-Villiger monooxygenases (BVMOs) are presented as highly selective and efficient biocatalysts for the synthesis of aroma lactones via kinetic resolution of 2-substituted cycloketones, exemplified with two δ-valerolactones, the jasmine lactones and their ε-caprolactone homologs. Analytical scale screens of our BVMO library ensued by preparative whole-cell biotransformations led to the identification of two enzymes (cyclohexanone monooxygenase from Arthrobacter BP2 and cyclododecanone monooxygenase from Rhodococcus SC1) perfectly suited for the task at hand: easily accessible racemic starting materials were bio-oxidized to almost enantiopure ketones and lactones in good yields (48-74%) and optical purities (ee 93% to >99%, E>100).

Chemically engineered papain as artificial formate dehydrogenase for NAD(P)H regeneration

Organometallic complexes of the general formula [(η(6)-arene)Ru(N⁁N)Cl](+) and [(η(5)-Cp*)Rh(N⁁N)Cl](+) where N⁁N is a 2,2'-dipyridylamine (DPA) derivative carrying a thiol-targeted maleimide group, 2,2'-bispyridyl (bpy), 1,10-phenanthroline (phen) or ethylenediamine (en) and arene is benzene, 2-chloro-N-[2-(phenyl)ethyl]acetamide or p-cymene were identified as catalysts for the stereoselective reduction of the enzyme cofactors NAD(P)(+) into NAD(P)H with formate as a hydride donor. A thorough comparison of their effectiveness towards NAD(+) (expressed as TOF) revealed that the Rh(III) complexes were much more potent catalysts than the Ru(II) complexes. Within the Ru(II) complex series, both the N⁁N and arene ligands forming the coordination sphere had a noticeable influence on the activity of the complexes. Covalent anchoring of the maleimide-functionalized Ru(II) and Rh(III) complexes to the cysteine endoproteinase papain yielded hybrid metalloproteins, some of them displaying formate dehydrogenase activity with potentially interesting kinetic parameters.

Combining bio- and chemo-catalysis: from enzymes to cells, from petroleum to biomass

In the future, biomass will continue to emerge as a viable source of chemicals. The development of new industries that utilize bio-renewables provides opportunities for innovation. For example, bio- and chemo-catalysts can be combined in 'one pot' to prepare chemicals of commercial value. This has been demonstrated using isolated enzymes and whole cells for a variety of chemical transformations. The one-pot approach has been successfully adopted to convert chemicals derived from biomass, and, in our opinion, it has an important role to play in the design of a more sustainable chemical industry. To implement new one-pot bio- and chemo-catalytic processes, issues of incompatibility must be overcome; the strategies for which are discussed in this opinion article.

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.

Investigating the coenzyme specificity of phenylacetone monooxygenase from Thermobifida fusca

Type I Baeyer–Villiger monooxygenases (BVMOs) strongly prefer NADPH over NADH as an electron donor. In order to elucidate the molecular basis for this coenzyme specificity, we have performed a site-directed mutagenesis study on phenylacetone monooxygenase (PAMO) from Thermobifida fusca. Using sequence alignments of type I BVMOs and crystal structures of PAMO and cyclohexanone monooxygenase in complex with NADP⁺, we identified four residues that could interact with the 2′-phosphate moiety of NADPH in PAMO. The mutagenesis study revealed that the conserved R217 is essential for binding the adenine moiety of the nicotinamide coenzyme while it also contributes to the recognition of the 2′-phosphate moiety of NADPH. The substitution of T218 did not have a strong effect on the coenzyme specificity. The H220N and H220Q mutants exhibited a ~3-fold improvement in the catalytic efficiency with NADH while the catalytic efficiency with NADPH was hardly affected. Mutating K336 did not increase the activity of PAMO with NADH, but it had a significant and beneficial effect on the enantioselectivity of Baeyer–Villiger oxidations and sulfoxidations. In conclusion, our results indicate that the function of NADPH in catalysis cannot be easily replaced by NADH. This finding is in line with the complex catalytic mechanism and the vital role of the coenzyme in BVMOs.

Nicotinamide-independent asymmetric bioreduction of CC-bonds via disproportionation of enones catalyzed by enoate reductases

The asymmetric bioreduction of activated CC-bonds catalyzed by a single flavoprotein was achieved via direct hydrogen transfer from a sacrificial 2-enone or 1,4-dione as hydrogen donor without requirement of a nicotinamide cofactor. Due to its simplicity, this system has clear advantages over conventional FAD-recycling systems.

Towards [Cp*Rh(bpy)(H2O]2+-promoted P450 catalysis: direct regeneration of CytC

The organometallic complex pentamethylcyclopentadienyl rhodium 2,2'-bipyridin ([Cp(*)Rh(bpy)(H(2)O)](2+)) was applied as regeneration catalyst for cytochrome C (CytC). Direct reduction of CytC-bound Fe(III) was achieved in this model system pointing towards a potential usefulness of this concept to promote cell-free P450 catalysis. In addition, controlled in situ provision with hydrogen peroxide was performed using [Cp(*)Rh(bpy)(H(2)O)](2+) resulting in improved CytC-catalyzed sulfoxidation of thioanisol This work represents the first step towards the direct-[Cp(*)Rh(bpy)(H(2)O)](2+) catalyzed regeneration of P450 monooxygenases and peroxidases.

Revealing the moonlighting role of NADP in the structure of a flavin-containing monooxygenase

Flavin-containing monooxygenases (FMOs) are, after cytochromes P450, the most important monooxygenase system in humans and are involved in xenobiotics metabolism and variability in drug response. The x-ray structure of a soluble prokaryotic FMO from Methylophaga sp. strain SK1 has been solved at 2.6-A resolution and is now the protein of known structure with the highest sequence similarity to human FMOs. The structure possesses a two-domain architecture, with both FAD and NADP(+) well defined by the electron density maps. Biochemical analysis shows that the prokaryotic enzyme shares many functional properties with mammalian FMOs, including substrate specificity and the ability to stabilize the hydroperoxyflavin intermediate that is crucial in substrate oxygenation. On the basis of their location in the structure, the nicotinamide ring and the adjacent ribose of NADP(+) turn out to be an integral part of the catalytic site being actively engaged in the stabilization of the oxygenating intermediate. This feature suggests that NADP(H) has a moonlighting role, in that it adopts two binding modes that allow it to function in both flavin reduction and oxygen reactivity modulation, respectively. We hypothesize that a relative domain rotation is needed to bring NADP(H) to these distinct positions inside the active site. Localization of mutations in human FMO3 that are known to cause trimethylaminuria (fish-odor syndrome) in the elucidated FMO structure provides a structural explanation for their biological effects.

Light-driven biocatalytic oxidation and reduction reactions: scope and limitations

The quest for practical regeneration concepts for nicotinamide-dependent oxidoreductases continues. Recently we proposed the use of visible light to promote the direct reductive regeneration of a flavin-dependent monooxygenase. With this enzyme (PAMO-P3) light-driven enantioselective Baeyer-Villiger oxidations were performed. In spite of the significant reduction in the complexity achieved, catalytic performance of the novel approach did not meet the requirements for an efficient biocatalytic oxygenation system. Driven by this ultimate goal, we further investigated the limiting factors of our particular system. We discovered that oxidative uncoupling of the flavin-regeneration reaction from enzymatic O2-activation accounts for the futile consumption of approximately 95% of the reducing equivalents provided by the sacrificial electron donor, EDTA. Furthermore, it was found that the apparent turnover frequency (TOF) for PAMO-P3 in the present setup is approximately two orders of magnitude lower than in conventional setups that use NADPH as reductant. This finding was traced to sluggish electron transfer kinetics that arose from an impeded interaction between PAMO-P3-bound FAD and the reducing catalyst. The limiting factors and potential approaches for their circumvention are discussed. Furthermore, we broadened the light-driven regeneration approach to the class of flavin-dependent reductases. By using the Old Yellow Enzyme homologue YqjM as a model system, a significantly higher catalytic turnover for the enzyme catalyst was achieved, which we assign to a higher accessibility of the prosthetic group as well as to the absence of oxidative uncoupling.

Kinetic mechanism of phenylacetone monooxygenase from Thermobifida fusca

Phenylacetone monooxygenase (PAMO) from Thermobifida fusca is a FAD-containing Baeyer-Villiger monooxygenase (BVMO). To elucidate the mechanism of conversion of phenylacetone by PAMO, we have performed a detailed steady-state and pre-steady-state kinetic analysis. In the catalytic cycle ( k cat = 3.1 s (-1)), rapid binding of NADPH ( K d = 0.7 microM) is followed by a transfer of the 4( R)-hydride from NADPH to the FAD cofactor ( k red = 12 s (-1)). The reduced PAMO is rapidly oxygenated by molecular oxygen ( k ox = 870 mM (-1) s (-1)), yielding a C4a-peroxyflavin. The peroxyflavin enzyme intermediate reacts with phenylacetone to form benzylacetate ( k 1 = 73 s (-1)). This latter kinetic event leads to an enzyme intermediate which we could not unequivocally assign and may represent a Criegee intermediate or a C4a-hydroxyflavin form. The relatively slow decay (4.1 s (-1)) of this intermediate yields fully reoxidized PAMO and limits the turnover rate. NADP (+) release is relatively fast and represents the final step of the catalytic cycle. This study shows that kinetic behavior of PAMO is significantly different when compared with that of sequence-related monooxygenases, e.g., cyclohexanone monooxygenase and liver microsomal flavin-containing monooxygenase. Inspection of the crystal structure of PAMO has revealed that residue R337, which is conserved in other BVMOs, is positioned close to the flavin cofactor. The analyzed R337A and R337K mutant enzymes were still able to form and stabilize the C4a-peroxyflavin intermediate. The mutants were unable to convert either phenylacetone or benzyl methyl sulfide. This demonstrates that R337 is crucially involved in assisting PAMO-mediated Baeyer-Villiger and sulfoxidation reactions.

Flavoprotein monooxygenases, a diverse class of oxidative biocatalysts

During the last decades a large number of flavin-dependent monooxygenases have been isolated and studied. This has revealed that flavoprotein monooxygenases are able to catalyze a remarkable wide variety of oxidative reactions such as regioselective hydroxylations and enantioselective sulfoxidations. These oxidation reactions are often difficult, if not impossible, to be achieved using chemical approaches. Analysis of the available genome sequences has indicated that many more flavoprotein monooxygenases exist and await biocatalytic exploration. Based on the known biochemical properties of a number of flavoprotein monooxygenases and sequence and structural analyses, flavoprotein monooxygenases can be classified into six distinct flavoprotein monooxygenase subclasses. This review provides an inventory of known flavoprotein monooxygenases belonging to these different enzyme subclasses. Furthermore, the biocatalytic potential of a selected number of flavoprotein monooxygenases is highlighted.

Non-enzymatic regeneration of nicotinamide and flavin cofactors for monooxygenase catalysis

Biocatalytic oxygenation chemistry is a rapidly evolving field in which monooxygenases are the tools of choice. Monooxygenases catalyze many industrially important synthetic transformations; however, their use in preparative applications is hampered by their intrinsic requirement for reducing equivalents. As a result, non-enzymatic strategies--where the reducing equivalents are introduced directly into the catalytic cycle--are being developed to supersede the well-established enzymatic NAD(P)H regeneration systems currently in use. In this review we summarize and evaluate recent achievements in this area.