Enzymatic Synthesis of Trimethyl-ε-caprolactone: Process Intensification and Demonstration on a 100 L Scale

Engineering of a Baeyer-Villiger monooxygenase to Improve Substrate Scope, Stereoselectivity and Regioselectivity

Baeyer-Villiger monooxygenases belong to a family of flavin-binding proteins that catalyze the Baeyer-Villiger (BV) oxidation of ketones to produce lactones or esters, which are important intermediates in pharmaceuticals or sustainable materials. Phenylacetone monooxygenase (PAMO) from Thermobifida fusca with moderate thermostability catalyzes the oxidation of aryl ketone substrates, but is limited by high specificity and narrow substrate scope. In the present study, we applied loop optimization by loop swapping followed by focused saturation mutagenesis in order to evolve PAMO mutants capable of catalyzing the regioselective BV oxidation of cyclohexanone and cyclobutanone derivatives with formation of either normal or abnormal esters or lactones. We further modulated PAMO to increase enantioselectivity. Crystal structure studies indicate that rotation occurs in the NADP-binding domain and that the high B-factor region is predominantly distributed in the catalytic pocket residues. Computational analyses further revealed dynamic character in the catalytic pocket and reshaped hydrogen bond interaction networks, which is more favorable for substrate binding. Our study provides useful insights for studying enzyme-substrate adaptations.

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.

Design of fusion enzymes for biocatalytic applications in aqueous and non-aqueous media

Biocatalytic cascades play a fundamental role in sustainable chemical synthesis. Fusion enzymes are one of the powerful toolboxes to enable the tailored combination of multiple enzymes for efficient cooperative cascades. Especially, this approach offers a substantial potential for the practical application of cofactor-dependent oxidoreductases by forming cofactor self-sufficient cascades. Adequate cofactor recycling while keeping the oxidized/reduced cofactor in a confined microenvironment benefits from the fusion fashion and makes the use of oxidoreductases in harsh non-aqueous media practical. In this mini-review, we have summarized the application of various fusion enzymes in aqueous and non-aqueous media with a focus on the discussion of linker design within oxidoreductases. The design and properties of the reported linkers have been reviewed in detail. Besides, the substrate loadings in these studies have been listed to showcase one of the key limitations (low solubility of hydrophobic substrates) of aqueous biocatalysis when it comes to efficiency and economic feasibility. Therefore, a straightforward strategy of applying non-aqueous media has been briefly discussed while the potential of using the fusion oxidoreductase of interest in organic media was highlighted.

Monitoring and control of the release of soluble O2 from H2 O2 inside porous enzyme carrier for O2 supply to an immobilized D-amino acid oxidase

While O₂ substrate for bio‐transformations in bulk liquid is routinely provided from entrained air or O₂ gas, tailored solutions of O₂ supply are required when the bio‐catalysis happens spatially confined to the microstructure of a solid support. Release of soluble O₂ from H₂O₂ by catalase is promising, but spatiotemporal control of the process is challenging to achieve. Here, we show monitoring and control by optical sensing within a porous carrier of the soluble O₂ formed by an immobilized catalase upon feeding of H₂O₂. The internally released O₂ is used to drive the reaction of d‐amino acid oxidase (oxidation of d‐methionine) that is co‐immobilized with the catalase in the same carrier. The H₂O₂ is supplied in portions at properly timed intervals, or continuously at controlled flow rate, to balance the O₂ production and consumption inside the carrier so as to maintain the internal O₂ concentration in the range of 100–500 µM. Thus, enzyme inactivation by excess H₂O₂ is prevented and gas formation from the released O₂ is avoided at the same time. The reaction rate of the co‐immobilized enzyme preparation is shown to depend linearly on the internal O₂ concentration up to the air‐saturated level. Conversions at a 200 ml scale using varied H₂O₂ feed rate (0.04–0.18 mmol/min) give the equivalent production rate from d‐methionine (200 mM) and achieve rate enhancement by ∼1.55‐fold compared to the same oxidase reaction under bubble aeration. Collectively, these results show an integrated strategy of biomolecular engineering for tightly controlled supply of O₂ substrate from H₂O₂ into carrier‐immobilized enzymes. By addressing limitations of O₂ supply via gas‐liquid transfer, especially at the microscale, this can be generally useful to develop specialized process strategies for O₂‐dependent biocatalytic reactions.

Flavoprotein monooxygenases: Versatile biocatalysts

Flavoprotein monooxygenases (FPMOs) are single- or two-component enzymes that catalyze a diverse set of chemo-, regio- and enantioselective oxyfunctionalization reactions. In this review, we describe how FPMOs have evolved from model enzymes in mechanistic flavoprotein research to biotechnologically relevant catalysts that can be applied for the sustainable production of valuable chemicals. After a historical account of the development of the FPMO field, we explain the FPMO classification system, which is primarily based on protein structural properties and electron donor specificities. We then summarize the most appealing reactions catalyzed by each group with a focus on the different types of oxygenation chemistries. Wherever relevant, we report engineering strategies that have been used to improve the robustness and applicability of FPMOs.

Process intensification for cytochrome P450 BM3-catalyzed oxy-functionalization of dodecanoic acid

Selective oxy‐functionalization of nonactivated C‐H bonds is a long‐standing “dream reaction” of organic synthesis for which chemical methodology is not well developed. Mono‐oxygenase enzymes are promising catalysts for such oxy‐functionalization to establish. Limitation on their applicability arises from low reaction output. Here, we showed an integrated approach of process engineering to the intensification of the cytochrome P450 BM3‐catalyzed hydroxylation of dodecanoic acid (C12:0). Using P450 BM3 together with glucose dehydrogenase for regeneration of nicotinamide adenine dinucleotide phosphate (NADPH), we compared soluble and co‐immobilized enzymes in O₂‐gassed and pH‐controlled conversions at high final substrate concentrations (≥40mM). We identified the main engineering parameters of process output (i.e., O₂ supply; mixing correlated with immobilized enzyme stability; foam control correlated with product isolation; substrate solubilization) and succeeded in disentangling their complex interrelationship for systematic process optimization. Running the reaction at O₂‐limited conditions at up to 500‐ml scale (10% dimethyl sulfoxide; silicone antifoam), we developed a substrate feeding strategy based on O₂ feedback control. Thus, we achieved high reaction rates of 1.86g·L⁻¹·hr⁻¹ and near complete conversion (≥90%) of 80mM (16g/L) C12:0 with good selectivity (≤5% overoxidation). We showed that “uncoupled reaction” of the P450 BM3 (~95% utilization of NADPH and O₂ not leading to hydroxylation) with the C12:0 hydroxylated product limited the process efficiency at high product concentration. Hydroxylated product (~7g; ≥92% purity) was recovered from 500ml reaction in 82% yield using ethyl‐acetate extraction. Collectively, these results demonstrate key engineering parameters for the biocatalytic oxy‐functionalization and show their integration into a coherent strategy for process intensification.