Functional assembly of camphor converting two-component Baeyer-Villiger monooxygenases with a flavin reductase from E. coli

A Cold-Active Flavin-Dependent Monooxygenase from Janthinobacterium svalbardensis Unlocks Applications of Baeyer–Villiger Monooxygenases at Low Temperature

Cold-active enzymes maintain a large part of their optimal activity at low temperatures. Therefore, they can be used to avoid side reactions and preserve heat-sensitive compounds. Baeyer-Villiger monooxygenases (BVMO) utilize molecular oxygen as a co-substrate to catalyze reactions widely employed for steroid, agrochemical, antibiotic, and pheromone production. Oxygen has been described as the rate-limiting factor for some BVMO applications, thereby hindering their efficient utilization. Considering that oxygen solubility in water increases by 40% when the temperature is decreased from 30 to 10 °C, we set out to identify and characterize a cold-active BVMO. Using genome mining in the Antarctic organism Janthinobacterium svalbardensis, a cold-active type II flavin-dependent monooxygenase (FMO) was discovered. The enzyme shows promiscuity toward NADH and NADPH and high activity between 5 and 25 °C. The enzyme catalyzes the monooxygenation and sulfoxidation of a wide range of ketones and thioesters. The high enantioselectivity in the oxidation of norcamphor (eeS = 56%, eeP > 99%, E > 200) demonstrates that the generally higher flexibility observed in the active sites of cold-active enzymes, which compensates for the lower motion at cold temperatures, does not necessarily reduce the selectivity of these enzymes. To gain a better understanding of the unique mechanistic features of type II FMOs, we determined the structure of the dimeric enzyme at 2.5 Å resolution. While the unusual N-terminal domain has been related to the catalytic properties of type II FMOs, the structure shows a SnoaL-like N-terminal domain that is not interacting directly with the active site. The active site of the enzyme is accessible only through a tunnel, with Tyr-458, Asp-217, and His-216 as catalytic residues, a combination not observed before in FMOs and BVMOs.

Inter-Species Redox Coupling by Flavin Reductases and FMN-Dependent Two-Component Monooxygenases Undertaking Nucleophilic Baeyer-Villiger Biooxygenations

Using highly purified enzyme preparations throughout, initial kinetic studies demonstrated that the isoenzymic 2,5- and 3,6-diketocamphane mono-oxygenases from Pseudomonas putida ATCC 17453 and the LuxAB luciferase from Vibrio fischeri ATCC 7744 exhibit commonality in being FMN-dependent two-component monooxygenases that promote redox coupling by the transfer of flavin reductase-generated FMNH₂ by rapid free diffusion. Subsequent studies confirmed the comprehensive inter-species compatibility of both native and non-native flavin reductases with each of the tested monooxygenases. For all three monooxygenases, non-native flavin reductases from Escherichia coli ATCC 11105 and Aminobacter aminovorans ATCC 29600 were confirmed to be more efficient donators of FMNH₂ than the corresponding tested native flavin reductases. Some potential practical implications of these outcomes are considered for optimising FMNH₂-dependent biooxygenations of recognised practical and commercial value.

A Flavin-Dependent Monooxygenase Mediates Divergent Oxidation of Rifamycin

Rifamycins have been clinically utilized against mycobacterial infections for more than 50 years; however, their biosynthesis has not been fully elucidated. Here, on the basis of in vivo gene deletions, in vitro enzyme assays, isotope labeling, and site-directed mutations, we found that a flavin-dependent monooxygenase encoded by a rifamycin biosynthetic gene cluster, Rif-Orf17, not only converted the naphthoquinone chromophore of rifamycin S into benzo-γ-pyrone but also linearized rifamycin SV through phenolic hydroxylation. Both oxidation routes lead to inactivation of rifamycins.

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.

Regulation of Camphor Metabolism: Induction and Repression of Relevant Monooxygenases in Pseudomonas putida NCIMB 10007

For the first time, the differential rates of synthesis of all the key monooxygenases involved in the catabolism by Pseudomonas putida NCIMB 10007 of bicyclic (rac)-camphor to ∆2,5-3,4,4-trimethylpimelyl-CoA, the first aliphatic pathway intermediate, have been determined to help establish the relevant induction profile of each of the oxygen-dependent enzymes. The efficacy of both relevant substrates and pathway metabolites as inducers has been established. Further, inhibitors with characterised functionality have been used to indicate that the pertinent regulatory controls operate at the level of transcription of the corresponding genes.

Switch in Cofactor Specificity of a Baeyer-Villiger Monooxygenase

Baeyer-Villiger monooxygenases (BVMOs) catalyze the oxidation of ketones to esters or lactones by using molecular oxygen and a cofactor. Type I BVMOs display a strong preference for NADPH. However, for industrial purposes NADH is the preferred cofactor, as it is ten times cheaper and more stable. Thus, we created a variant of the cyclohexanone monooxygenase from Acinetobacter sp. NCIMB 9871 (CHMO_Acineto ); this used NADH 4200-fold better than NADPH. By combining structure analysis, sequence alignment, and literature data, 21 residues in proximity of the cofactor were identified and targeted for mutagenesis. Two combinatorial variants bearing three or four mutations showed higher conversions of cyclohexanone with NADH (79 %) compared to NADPH (58 %) as well as specificity. The structural reasons for this switch in cofactor specificity of a type I BVMO are especially a hydrogen-bond network coordinating the two hydroxy groups of NADH through direct interactions and bridging water molecules.

The oxygenating constituent of 3,6-diketocamphane monooxygenase from the CAM plasmid of Pseudomonas putida: the first crystal structure of a type II Baeyer-Villiger monooxygenase

The three-dimensional structures of the native enzyme and the FMN complex of the overexpressed form of the oxygenating component of the type II Baeyer-Villiger 3,6-diketocamphane monooxygenase have been determined to 1.9 Å resolution. The structure of this dimeric FMN-dependent enzyme, which is encoded on the large CAM plasmid of Pseudomonas putida, has been solved by a combination of multiple anomalous dispersion from a bromine crystal soak and molecular replacement using a bacterial luciferase model. The orientation of the isoalloxazine ring of the FMN cofactor in the active site of this TIM-barrel fold enzyme differs significantly from that previously observed in enzymes of the bacterial luciferase-like superfamily. The Ala77 residue is in a cis conformation and forms a β-bulge at the C-terminus of β-strand 3, which is a feature observed in many proteins of this superfamily.

A general protein purification and immobilization method on controlled porosity glass: biocatalytic applications

A general combined purification and immobilization method to facilitate biocatalytic process development is presented. The support material, EziG™, is based on controlled porosity glass (CPG) or polymer-coated versions thereof (HybCPG) and binds protein affinity tags. Biocatalytic reactions in aqueous and organic media with seven enzymes of biocatalytic interest are shown.

Multiple native flavin reductases in camphor-metabolizing Pseudomonas putida NCIMB 10007: functional interaction with two-component diketocamphane monooxygenase isoenzymes

Although they have been studied for nearly 50 years, the source of the FMNH2 needed for effective biooxidation by the 2,5- and 3,6-diketocamphane monooxygenase (DKCMO) isoenzymes induced by the growth of Pseudomonas putida NCIMB 10007 (ATCC 17453) on camphor remains incompletely characterized. Prior studies have focussed exclusively on enzymes present in cells harvested during late-exponential-phase growth despite considerable circumstantial evidence that the flavin reductase (FR) component of these multicomponent monooxygenases is subject to growth-phase-dependent variation. In this study, a number of alternative FMNH2-generating activities, including both conventional FRs and enzymes also able to serve as ferric reductases, were isolated from camphor-grown cells, and the relative level, and hence potential contribution, of these various proteins shown to vary considerably depending on the point of harvest of NCIMB 10007 within exponential-phase growth. While two constitutive monomeric ferric reductases (molecular masses 27.0 and 28.5 kDa) were found to be the major relevant sources of FMNH2 during the initial stages of growth on camphor-based media, a significant subsequent contribution throughout the mid- to late-exponential phases of growth was also made by the camphor-induced homodimeric 37.0 kDa FR Fred, recently reported to serve such a role exclusively. The possible involvement of camphor-induced putidaredoxin reductase (51.0 kDa) as a contributory activity was also investigated and considered. Studies with highly purified preparations of the isofunctional DKCMOs confirmed the potential of the various reductases to function effectively as sources of the requisite FMNH2 to both monooxygenases at different times throughout growth on camphor.

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.