The Enigmatic P450 Decarboxylase OleT Is Capable of, but Evolved To Frustrate, Oxygen Rebound Chemistry

A chimeric membrane enzyme and an engineered whole-cell biocatalyst for efficient 1-alkene production

Bioproduction of 1-alkenes from naturally abundant free fatty acids offers a promising avenue toward the next generation of hydrocarbon-based biofuels and green commodity chemicals. UndB is the only known membrane-bound 1-alkene-producing enzyme, with great potential for 1-alkene bioproduction, but the enzyme exhibits limited turnovers, thus restricting its widespread usage. Here, we explore the molecular basis of the limitation of UndB activity and substantially improve its catalytic power. We establish that the enzyme undergoes peroxide-mediated rapid inactivation during catalysis. To counteract this inactivation, we engineered a chimeric membrane enzyme by conjugating UndB with catalase that protected UndB against peroxide and enhanced its number of turnovers tremendously. Notably, our chimeric enzyme is the only example of a membrane enzyme successfully engineered with catalase. We subsequently constructed a whole-cell biocatalytic system and achieved remarkable efficiencies (up to 95%) in the biotransformation of a wide range of fatty acids (both aliphatic and aromatic) into corresponding 1-alkenes with numerous biotechnological applications.

Discovery and Characterization of Pyridoxal 5'-Phosphate-Dependent Cycloleucine Synthases

Pyridoxal 5'-phosphate (PLP)-dependent enzymes are the most versatile biocatalysts for synthesizing nonproteinogenic amino acids. α,α-Disubstituted quaternary amino acids, such as 1-aminocyclopentane-1-carboxylic acid (cycloleucine), are useful building blocks for pharmaceuticals. In this study, starting with the biosynthesis of fusarilin A, we discovered a family of PLP-dependent enzymes that can facilitate tandem carbon-carbon forming steps to catalyze an overall [3 + 2]-annulation. In the first step, the cycloleucine synthases use SAM as the latent electrophile and an in situ-generated enamine as the nucleophile for γ-substitution. Whereas previously characterized γ-replacement enzymes protonate the resulting α-carbon and release the acyclic amino acid, cycloleucine synthases can catalyze an additional, intramolecular aldol or Mannich reaction with the nucleophilic α-carbon to form the substituted cyclopentane. Overall, the net [3 + 2]-annulation reaction can lead to 2-hydroxy or 2-aminocycloleucine products. These studies further expand the biocatalytic scope of PLP-dependent enzymes.

Functional Model of Compound II of Cytochrome P450: Spectroscopic Characterization and Reactivity Studies of a FeIV-OH Complex

Herein, we show that the reaction of a mononuclear FeIII(OH) complex (1) with N-tosyliminobenzyliodinane (PhINTs) resulted in the formation of a FeIV(OH) species (3). The obtained complex 3 was characterized by an array of spectroscopic techniques and represented a rare example of a synthetic FeIV(OH) complex. The reaction of 1 with the one-electron oxidizing agent was reported to form a ligand-oxidized FeIII(OH) complex (2). 3 revealed a one-electron reduction potential of -0.22 V vs Fc+/Fc at -15 °C, which was 150 mV anodically shifted than 2 (Ered = -0.37 V vs Fc+/Fc at -15 °C), inferring 3 to be more oxidizing than 2. 3 reacted spontaneously with (4-OMe-C6H4)3C• to form (4-OMe-C6H4)3C(OH) through rebound of the OH group and displayed significantly faster reactivity than 2. Further, activation of the hydrocarbon C-H and the phenolic O-H bond by 2 and 3 was compared and showed that 3 is a stronger oxidant than 2. A detailed kinetic study established the occurrence of a concerted proton-electron transfer/hydrogen atom transfer reaction of 3. Studying one-electron reduction of 2 and 3 using decamethylferrocene (Fc*) revealed a higher ket of 3 than 2. The study established that the primary coordination sphere around Fe and the redox state of the metal center is very crucial in controlling the reactivity of high-valent Fe-OH complexes. Further, a FeIII(OMe) complex (4) was synthesized and thoroughly characterized, including X-ray structure determination. The reaction of 4 with PhINTs resulted in the formation of a FeIV(OMe) species (5), revealing the presence of two FeIV species with isomer shifts of -0.11 mm/s and = 0.17 mm/s in the Mössbauer spectrum and showed FeIV/FeIII potential at -0.36 V vs Fc+/Fc couple in acetonitrile at -15 °C. The reactivity studies of 5 were investigated and compared with the FeIV(OH) complex (3).

Enhancing ferryl accumulation in H2O2-dependent cytochrome P450s

A facile strategy is presented to enhance the accumulation of ferryl (iron(IV)-oxo) species in H2O2 dependent cytochrome P450s (CYPs) of the CYP152 family. We report the characterization of a highly chemoselective CYP decarboxylase from Staphylococcus aureus (OleTSA) that is soluble at high concentrations. Examination of OleTSA Compound I (CpdI) accumulation with a variety of fatty acid substrates reveals a dependence on resting spin-state equilibrium. Alteration of this equilibrium through targeted mutagenesis of the proximal pocket favors the high-spin form, and as a result, enhances Cpd-I accumulation to nearly stoichiometric yields.

Structure and Function of Alkane Monooxygenase (AlkB)

Every year, perhaps as much as 800 million tons of hydrocarbons enters the environment; alkanes make up a large percentage of it. Most are transformed by organisms that utilize these molecules as sources of energy and carbon. Both aerobic and anaerobic alkane transformation chemistries exist, capitalizing on the presence of alkanes in both oxic and anoxic environments. Over the past 40 years, tremendous progress has been made in understanding the structure and mechanism of enzymes that catalyze the transformation of methane. By contrast, progress involving enzymes that transform liquid alkanes has been slower with the first structures of AlkB, the predominant aerobic alkane hydroxylase in the environment, appearing in 2023. Because of the fundamental importance of C-H bond activation chemistries, interest in understanding how biology activates and transforms alkanes is high.In this Account, we focus on steps we have taken to understand the mechanism and structure of alkane monooxygenase (AlkB), the metalloenzyme that dominates the transformation of liquid alkanes in the environment (not to be confused with another AlkB that is an α-ketogluturate-dependent enzyme involved in DNA repair). First, we briefly describe what is known about the prevalence of AlkB in the environment and its role in the carbon cycle. Then we review the key findings from our recent high-resolution cryoEM structure of AlkB and highlight important similarities and differences in the structures of members of class III diiron enzymes. Functional studies, which we summarize, from a number of single residue variants enable us to say a great deal about how the structure of AlkB facilitates its function. Next, we overview work from our laboratories using mechanistically diagnostic radical clock substrates to characterize the mechanism of AlkB and contextualize the results we have obtained on AlkB with results we have obtained on other alkane-oxidizing enzymes and explain these results in light of the enzyme's structure. Finally, we integrate recent work in our laboratories with information from prior studies of AlkB, and relevant model systems, to create a holistic picture of the enzyme. We end by pointing to critical questions that still need to be answered, questions about the electronic structure of the active site of the enzyme throughout the reaction cycle and about whether and to what extent the enzyme plays functional roles in biology beyond simply initiating the degradation of alkanes.

Enzymatic polyethylene biorecycling: Confronting challenges and shaping the future

Polyethylene (PE) is a widely used plastic known for its resistance to biodegradation, posing a significant environmental challenge. Recent advances have shed light on microorganisms and insects capable of breaking down PE and identified potential PE-degrading enzymes (PEases), hinting at the possibility of PE biorecycling. Research on enzymatic PE degradation is still in its early stages, especially compared to the progress made with polyethylene terephthalate (PET). While PET hydrolases have been extensively studied and engineered for improved performance, even the products of PEases remain mostly undefined. This Perspective analyzes the current state of enzymatic PE degradation research, highlighting obstacles in the search for bona fide PEases and suggesting areas for future exploration. A critical challenge impeding progress in this field stems from the inert nature of the C-C and C-H bonds of PE. Furthermore, breaking down PE into small molecules using only one monofunctional enzyme is theoretically impossible. Overcoming these obstacles requires identifying enzymatic pathways, which can be facilitated using emerging technologies like omics, structure-based design, and computer-assisted engineering of enzymes. Understanding the mechanisms underlying PE enzymatic biodegradation is crucial for research progress and for identifying potential solutions to the global plastic pollution crisis.

P450 fatty acid decarboxylase

P450 fatty acid decarboxylases are able to utilize hydrogen peroxide as the sole cofactor to decarboxylate free fatty acids to produce α-olefins with abundant applications as drop-in biofuels and important chemical precursors. In this chapter, we review diverse approaches for discovery, characterization, engineering, and applications of P450 fatty acid decarboxylases. Information gained from structural data has been advancing our understandings of the unique mechanisms underlying alkene production, and providing important insights for exploring new activities. To build an efficient olefin-producing system, various engineering strategies have been proposed and applied to this unusual P450 catalytic system. Furthermore, we highlight a select number of applied examples of P450 fatty acid decarboxylases in enzyme cascades and metabolic engineering.

Dimer-assisted mechanism of (un)saturated fatty acid decarboxylation for alkene production

The enzymatic decarboxylation of fatty acids (FAs) represents an advance toward the development of biological routes to produce drop-in hydrocarbons. The current mechanism for the P450-catalyzed decarboxylation has been largely established from the bacterial cytochrome P450 OleTJE. Herein, we describe OleTPRN, a poly-unsaturated alkene-producing decarboxylase that outrivals the functional properties of the model enzyme and exploits a distinct molecular mechanism for substrate binding and chemoselectivity. In addition to the high conversion rates into alkenes from a broad range of saturated FAs without dependence on high salt concentrations, OleTPRN can also efficiently produce alkenes from unsaturated (oleic and linoleic) acids, the most abundant FAs found in nature. OleTPRN performs carbon-carbon cleavage by a catalytic itinerary that involves hydrogen-atom transfer by the heme-ferryl intermediate Compound I and features a hydrophobic cradle at the distal region of the substrate-binding pocket, not found in OleTJE, which is proposed to play a role in the productive binding of long-chain FAs and favors the rapid release of products from the metabolism of short-chain FAs. Moreover, it is shown that the dimeric configuration of OleTPRN is involved in the stabilization of the A-A' helical motif, a second-coordination sphere of the substrate, which contributes to the proper accommodation of the aliphatic tail in the distal and medial active-site pocket. These findings provide an alternative molecular mechanism for alkene production by P450 peroxygenases, creating new opportunities for biological production of renewable hydrocarbons.

Modeling the Ligand Effect on the Structure of CYP 450 Within the Density Functional Theory

An improved understanding of the P450 structure is relevant to the development of biomimetic catalysts and inhibitors for controlled CH-bond activation, an outstanding challenge of synthetic chemistry. Motivated by the experimental findings of an unusually short Fe-S bond of 2.18 Å for the wild-type (WT) OleT P450 decarboxylase relative to a cysteine pocket mutant form (A369P), a computational model that captures the effect of the thiolate axial ligand on the iron-sulfur distance is presented. With the computational efficiency and streamlined analysis in mind, this model combines a cluster representation of the enzyme─40-110 atoms, depending on the heme and ligand truncation level─with a density functional theory (DFT) description of the electronic structure (ES) and is calibrated against the experimental data. The optimized Fe-S distances show a difference of 0.25 Å between the low and high spin states, in agreement with the crystallographic structures of the OleT WT and mutant forms. We speculate that this difference is attributable to the packing of the ligand; the mutant is bulkier due to an alanine-to-proline replacement, meaning that it is excluded from the energetically favored low-spin minimum because of steric constraints. The presence of pure spin-state pairs and the intersection of the low/high spin states for the enzyme model is indicative of the limitations of single-reference ES methods in such systems and emphasizes the significance of using the proper state when modeling the hydrogen atom transfer (HAT) reaction catalyzed by OleT. At the same time, the correct characterization of both the short and long Fe-S bonds within a small DFT-based model of 42 atoms paves the way for quantum dynamics modeling of the HAT step, which initiates the OleT decarboxylation reaction.

Metalloenzymes for Fatty Acid-Derived Hydrocarbon Biosynthesis: Nature's Cryptic Catalysts

Waning resources, massive energy consumption, everdeepening global warming crisis, and climate change have raised grave concerns regarding continued dependence on fossil fuels as the predominant source of energy and generated tremendous interest for developing biofuels, which are renewable. Hydrocarbon-based 'drop-in' biofuels can be a proper substitute for fossil fuels such as gasoline or jet fuel. In Nature, hydrocarbons are produced by diverse organisms such as insects, plants, bacteria, and cyanobacteria. Metalloenzymes play a crucial role in hydrocarbons biosynthesis, and the past decade has witnessed discoveries of a number of metalloenzymes catalyzing hydrocarbon biosynthesis from fatty acids and their derivatives employing unprecedented mechanisms. These discoveries elucidated the enigma related to the divergent chemistries involved in the catalytic mechanisms of these metalloenzymes. There is substantial diversity in the structure, mode of action, cofactor requirement, and substrate scope among these metalloenzymes. Detailed structural analysis along with mutational studies of some of these enzymes have contributed significantly to identifying the key amino acid residues that dictate substrate specificity and catalytic intricacy. In this Review, we discuss the metalloenzymes that catalyze fatty acid-derived hydrocarbon biosynthesis in various organisms, emphasizing the active site architecture, catalytic mechanism, cofactor requirements, and substrate specificity of these enzymes. Understanding such details is essential for successfully implementing these enzymes in emergent biofuel research through protein engineering and synthetic biology approaches.

Exploring hitherto uninvestigated reactions of the fatty acid peroxygenase CYP152A1: catalase reaction and Compound I formation

CYP152A1 (cytochrome P450_BSβ) is a fatty acid peroxygenase, which specifically catalyses the oxidation of long-chain fatty acids using hydrogen peroxide as an oxidant. We have found that CYP152A1 possesses catalase activity, which competes with the hydroxylation of long-chain fatty acids, the oxidation of non-native substrates, and haem degradation. Using hydrogen peroxide, Compound I of CYP152A1 could not be observed, due to its swift decomposition via catalase activity, where Compound I reacts with another molecule of hydrogen peroxide to form O₂. In contrast, a clear spectral change indicative of Compound I formation was observed when mCPBA was employed as the oxidant. This work presents valuable insights into an important role for the catalase activity of CYP152A1 in avoiding enzyme deactivation when no substrate is available for oxidation.

Local Electric Fields Dictate Function: The Different Product Selectivities Observed for Fatty Acid Oxidation by Two Deceptively Very Similar P450-Peroxygenases OleT and BSβ

Cytochrome P450 peroxygenases use hydrogen peroxide to hydroxylate long-chain fatty acids by bypassing the use of O₂ and a redox partner. Among the peroxygenases, P450_OleT uniquely performs decarboxylation of fatty acids and production of terminal olefins. This route taken by P450_OleT is intriguing, and its importance is augmented by the practical importance of olefin production. As such, this mechanistic choice merits elucidation. To address this puzzle, we use hybrid QM/MM calculations and MD simulations for the OleT enzyme as well as for the structurally analogous enzyme, P450_BSβ. The study of P450_OleT reveals that the protonated His85 in the wild-type P450_OleT plays a crucial role in steering decarboxylation activity by stabilizing the corresponding hydroxoiron(IV) intermediate (Cpd II). In contrast, for P450_BSβ in which Q85 replaces H85, the respective Cpd II species is unstable and it reacts readily with the substrate radical by rebound, producing hydroxylation products. As shown, this single-site difference creates in P450_OleT a local electric field (LEF), which is significantly higher than that in P450_BSβ. In turn, these LEF differences are responsible for the different stabilities of the respective Cpd II/radical intermediates and hence for different functions of the two enzymes. P450_BSβ uses the common rebound mechanism and leads to hydroxylation, whereas P450_OleT proceeds via decarboxylation and generates terminal olefins. Olefin production projects the power of a single residue to alter the LEF and the enzyme's function.

Spectroscopic studies of the interaction between phosphorus heterocycles and cytochrome P450

P450 fatty acid decarboxylase OleT from Staphylococcus aureus (OleT_SA) is a novel cytochrome P450 enzyme that catalyzes the oxidative decarboxylation of fatty acids to yield primarily terminal alkenes and CO₂ or minor α- and β-hydroxylated fatty acids as side-products. In this work, the interactions between a series of cycloalkyl phosphorus heterocycles (CPHs) and OleT_SA were investigated in detail by fluorescence titration experiment, ultraviolet-visible (UV-vis) and ³¹P NMR spectroscopies. Fluorescence titration experiment results clearly showed that a dynamic quenching occurred when CPH-6, a representative CPHs, interacted with OleT_SA with a binding constant value of 15.2 × 10⁴ M^-1 at 293 K. The thermodynamic parameters (ΔH, ΔS and ΔG) showed that the hydrogen bond and van der Waals force played major roles in the interaction between OleT_SA and CPHs. The UV-vis and ³¹P NMR studies indicated the penetration of CPH-6 into the interior environment of OleT_SA, which greatly affects the enzymatic activity of OleT_SA. Therefore, our study revealed an effective way to use phosphorus heterocyclic compounds to modulate the activity of cytochrome P450 enzymes.

Halogen Transfer to Carbon Radicals by High-Valent Iron Chloride and Iron Fluoride Corroles

High-valent iron halide corroles were examined to determine their reactivity with carbon radicals and their ability to undergo radical rebound-like processes. Beginning with Fe(Cl)(ttppc) (1) (ttppc = 5,10,15-tris(2,4,6-triphenylphenyl)corrolato^3-), the new iron corroles Fe(OTf)(ttppc) (2), Fe(OTf)(ttppc)(AgOTf) (3), and Fe(F)(ttppc) (4) were synthesized. Complexes 3 and 4 are the first iron triflate and iron fluoride corroles to be structurally characterized by single crystal X-ray diffraction. The structure of 3 reveals an Ag^I-pyrrole (η²-π) interaction. The Fe(Cl)(ttppc) and Fe(F)(ttppc) complexes undergo halogen transfer to triarylmethyl radicals, and kinetic analysis of the reaction between (p-OMe-C₆H₄)₃C• and 1 gave k = 1.34(3) × 10³ M^-1 s^-1 at 23 °C and 2.2(2) M^-1 s^-1 at -60 °C, ΔH^⧧ = +9.8(3) kcal mol^-1, and ΔS^⧧ = -14(1) cal mol^-1 K^-1 through an Eyring analysis. Complex 4 is significantly more reactive, giving k = 1.16(6) × 10⁵ M^-1 s^-1 at 23 °C. The data point to a concerted mechanism and show the trend X = F^- > Cl^- > OH^- for Fe(X)(ttppc). This study provides mechanistic insights into halogen rebound for an iron porphyrinoid complex.

Unexpected Reactions of α,β-Unsaturated Fatty Acids Provide Insight into the Mechanisms of CYP152 Peroxygenases

CYP152 peroxygenases catalyze decarboxylation and hydroxylation of fatty acids using H₂ O₂ as cofactor. To understand the molecular basis for the chemo- and regioselectivity of these unique P450 enzymes, we analyze the activities of three CYP152 peroxygenases (OleT_JE , P450_SPα , P450_BSβ ) towards cis- and trans-dodecenoic acids as substrate probes. The unexpected 6S-hydroxylation of the trans-isomer and 4R-hydroxylation of the cis-isomer by OleT_JE , and molecular docking results suggest that the unprecedented selectivity is due to OleT_JE 's preference of C2-C3 cis-configuration. In addition to the common epoxide products, undecanal is the unexpected major product of P450_SPα and P450_BSβ regardless of the cis/trans-configuration of substrates. The combined H₂ ¹⁸ O₂ tracing experiments, MD simulations, and QM/MM calculations unravel an unusual mechanism for Compound I-mediated aldehyde formation in which the active site water derived from H₂ O₂ activation is involved in the generation of a four-membered ring lactone intermediate. These findings provide new insights into the unusual mechanisms of CYP152 peroxygenases.

Harnessing P450 Enzyme for Biotechnology and Synthetic Biology

Cytochrome P450 enzymes (P450s, CYPs) catalyze the oxidative transformation of a wide range of organic substrates. Their functions are crucial to xenobiotic metabolism and steroid transformation in humans and other organisms. The enzymes are promising for synthetic biology applications but limited by several drawbacks including low turnover rates, poor stability, the dependance of expensive cofactors and redox partners, and the narrow substrate scope. To conquer these obstacles, emerging strategies including substrate engineering, usage of decoy and decoy-based small molecules auxiliaries, designing of artificial enzyme cascades and the incorporation of materials have been explored based on the unique properties of P450s. These strategies can be applied to a wide range of P450s and can be combined with protein engineering to improve the enzymatic activities. This minireview will focus on some recent developments of these strategies which have been used to leverage P450 catalysis. Remaining challenges and future opportunities will also be discussed.

P450-driven plastic-degrading synthetic bacteria

Plastic contamination currently threatens a wide variety of ecosystems and presents damaging repercussions and negative consequences for many wildlife species. Sustainable plastic waste management is an important approach to environmental protection and a necessity in the current life cycle of plastics in nature. Plastic biodegradation by microorganisms is a notable possible solution. This opinion article includes a proposal to use hypothetical P450 enzymes with an engineered active site as potent trigger biocatalysts to biodegrade polyethylene (PE) via in-chain hydroxylation into smaller products of linear aliphatic alcohols and alkanoic acids based on cascade enzymatic reactions. Furthermore, we propose the adoption of P450 into plastic-eating synthetic bacteria for PE biodegradation. This strategy can be applicable to other dense plastics, such as polypropylene (PP) and polystyrene (PS).

Using Small Molecules to Enhance P450 OleT Enzyme Activity in Situ

Cytochrome P450 OleT is a fatty acid decarboxylase that catalyzes the production of olefins with biofuel and synthetic applications. However, the relatively sluggish catalytic efficiency of the enzyme limits its applications. Here, we report the application of a novel class of benzene containing small molecules to improve the OleT activity. The UV-Vis spectroscopy study and molecular docking results confirmed the high proximity of the small molecules to the heme group of OleT. Up to 6-fold increase of product yield has been achieved in the small molecule-modulated enzymatic reactions. Our work thus sheds the light to the application of small molecules to increase the OleT catalytic efficiency, which could be potentially used for future olefin productions.

A Photoclick-Based High-Throughput Screening for the Directed Evolution of Decarboxylase OleT

Enzymatic oxidative decarboxylation is an up-and-coming reaction yet lacking efficient screening methods for the directed evolution of decarboxylases. Here, we describe a simple photoclick assay for the detection of decarboxylation products and its application in a proof-of-principle directed evolution study on the decarboxylase OleT. The assay was compatible with two frequently used OleT operation modes (directly using hydrogen peroxide as the enzyme's co-substrate or using a reductase partner) and the screening of saturation mutagenesis libraries identified two enzyme variants shifting the enzyme's substrate preference from long chain fatty acids toward styrene derivatives. Overall, this photoclick assay holds promise to speed-up the directed evolution of OleT and other decarboxylases.

P450Jα : A New, Robust and α-Selective Fatty Acid Hydroxylase Displaying Unexpected 1-Alkene Formation

Oxyfunctionalization of fatty acids (FAs) is a key step in the design of novel synthetic pathways for biobased/biodegradable polymers, surfactants and fuels. Here, we show the isolation and characterization of a robust FA α-hydroxylase (P450_Jα ) which catalyses the selective conversion of a broad range of FAs (C6:0-C16:0) and oleic acid (C18:1) with H₂ O₂ as oxidant. Under optimized reaction conditions P450_Jα yields α-hydroxy acids all with >95 % regioselectivity, high specific activity (up to 15.2 U mg^-1 ) and efficient coupling of oxidant to product (up to 85 %). Lauric acid (C12:0) turned out to be an excellent substrate with respect to productivity (TON=394 min^-1 ). On preparative scale, conversion of C12:0 reached 83 % (0.9 g L^-1 ) when supplementing H₂ O₂ in fed-batch mode. Under similar conditions P450_Jα allowed further the first biocatalytic α-hydroxylation of oleic acid (88 % conversion on 100 mL scale) at high selectivity and in good yields (1.1 g L^-1 ; 79 % isolated yield). Unexpectedly, P450_Jα displayed also 1-alkene formation from shorter chain FAs (≤C10:0) showing that oxidative decarboxylation is more widely distributed across this enzyme family than reported previously.

Ru(II)-diimine complexes and cytochrome P450 working hand-in-hand

With a growing interest in utilizing visible light to drive biocatalytic processes, several light-harvesting units and approaches have been employed to harness the synthetic potential of heme monooxygenases and carry out selective oxyfunctionalization of a wide range of substrates. While the fields of cytochrome P450 and Ru(II) photochemistry have separately been prolific, it is not until the turn of the 21st century that they converged. Non-covalent and subsequently covalently attached Ru(II) complexes were used to promote rapid intramolecular electron transfer in bacterial P450 enzymes. Photocatalytic activity with Ru(II)-modified P450 enzymes was achieved under reductive conditions with a judicious choice of a sacrificial electron donor. The initial concept of Ru(II)-modified P450 enzymes was further improved using protein engineering, photosensitizer functionalization and was successfully applied to other P450 enzymes. In this review, we wish to present the recent contributions from our group and others in utilizing Ru(II) complexes coupled with P450 enzymes in the broad context of photobiocatalysis, protein assemblies and chemoenzymatic reactions. The merging of chemical catalysts with the synthetic potential of P450 enzymes has led to the development of several chemoenzymatic approaches. Moreover, strained Ru(II) compounds have been shown to selectively inhibit P450 enzymes by releasing aromatic heterocycle containing molecules upon visible light excitation taking advantage of the rapid ligand loss feature in those complexes.

Production of Propene from n-Butanol: A Three-Step Cascade Utilizing the Cytochrome P450 Fatty Acid Decarboxylase OleTJE

Propene is one of the most important starting materials in the chemical industry. Herein, we report an enzymatic cascade reaction for the biocatalytic production of propene starting from n-butanol, thus offering a biobased production from glucose. In order to create an efficient system, we faced the issue of an optimal cofactor supply for the fatty acid decarboxylase OleTJE , which is said to be driven by either NAD(P)H or H2 O2 . In the first system, we used an alcohol and aldehyde dehydrogenase coupled to OleTJE by the electron-transfer complex putidaredoxin reductase/putidaredoxin, allowing regeneration of the NAD+ cofactor. With the second system, we intended full oxidation of n-butanol to butyric acid, generating one equivalent of H2 O2 that can be used for the oxidative decarboxylation. As the optimal substrate is a long-chain fatty acid, we also tried to create an improved variant for the decarboxylation of butyric acid by using rational protein design. Within a mutational study with 57 designed mutants, we generated the mutant OleTV292I , which showed a 2.4-fold improvement in propene production in our H2 O2 -driven cascade system and reached total turnover numbers >1000.

Hydroxyl Transfer to Carbon Radicals by Mn(OH) vs Fe(OH) Corrole Complexes

The transfer of •OH from metal-hydroxo species to carbon radicals (R•) to give hydroxylated products (ROH) is a fundamental process in metal-mediated heme and nonheme C-H bond oxidations. This step, often referred to as the hydroxyl "rebound" step, is typically very fast, making direct study of this process challenging if not impossible. In this report, we describe the reactions of the synthetic models M(OH)(ttppc) (M = Fe (1), Mn (3); ttppc = 5,10,15-tris(2,4,6-triphenyl)phenyl corrolato^3-) with a series of triphenylmethyl carbon radical (R•) derivatives ((4-X-C₆H₄)₃C•; X = OMe, tBu, Ph, Cl, CN) to give the one-electron reduced M^III(ttppc) complexes and ROH products. Rate constants for 3 for the different radicals ranged from 11.4(1) to 58.4(2) M^-1 s^-1, as compared to those for 1, which fall between 0.74(2) and 357(4) M^-1 s^-1. Linear correlations for Hammett and Marcus plots for both Mn and Fe were observed, and the small magnitudes of the slopes for both correlations imply a concerted •OH transfer reaction for both metals. Eyring analyses of reactions for 1 and 3 with (4-X-C₆H₄)₃C• (X = tBu, CN) also give good linear correlations, and a comparison of the resulting activation parameters highlight the importance of entropy in these •OH transfer reactions. Density functional theory calculations of the reaction profiles show a concerted process with one transition state for all radicals investigated and help to explain the electronic features of the OH rebound process.

Hydrogen Atom Abstraction by High-Valent Fe(OH) versus Mn(OH) Porphyrinoid Complexes: Mechanistic Insights from Experimental and Computational Studies

High-valent metal-hydroxide species have been implicated as key intermediates in hydroxylation chemistry catalyzed by heme monooxygenases such as the cytochrome P450s. However, in some classes of P450s, a bifurcation from the typical oxygen rebound pathway is observed, wherein the Fe^IV(OH)(porphyrin) species carries out a net hydrogen atom transfer reaction to form alkene metabolites. In this work, we examine the hydrogen atom transfer (HAT) reactivity of Fe^IV(OH)(ttppc) (1), ttppc = 5,10,15-tris(2,4,6-triphenyl)-phenyl corrole, toward substituted phenol derivatives. The iron hydroxide complex 1 reacts with a series of para-substituted 2,6-di-tert-butylphenol derivatives (4-X-2,6-DTBP; X = OMe, Me, Et, H, Ac), with second-order rate constants k₂ = 3.6(1)-1.21(3) × 10⁴ M^-1 s^-1 and yielding linear Hammett and Marcus plot correlations. It is concluded that the rate-determining step for O-H cleavage occurs through a concerted HAT mechanism, based on mechanistic analyses that include a KIE = 2.9(1) and DFT calculations. Comparison of the HAT reactivity of 1 to the analogous Mn complex, Mn^IV(OH)(ttppc), where only the central metal ion is different, indicates a faster HAT reaction and a steeper Hammett slope for 1. The O-H bond dissociation energy (BDE) of the M^III(HO-H) complexes were estimated from a kinetic analysis to be 85 and 89 kcal mol^-1 for Mn and Fe, respectively. These estimated BDEs are closely reproduced by DFT calculations and are discussed in the context of how they influence the overall H atom transfer reactivity.

Mechanistic Investigation of Oxygen Rebound in a Mononuclear Nonheme Iron Complex

An iron(III) methoxide complex reacts with para-substituted triarylmethyl radicals to give iron(II) and methoxyether products. Second-order rate constants for the radical derivatives were obtained. Hammett and Marcus plots suggest the radical transfer reactions proceed via a concerted process. Calculations support the concerted nature of these reactions involving a single transition state with no initial charge transfer. These findings have implications for the radical "rebound" step invoked in nonheme iron oxygenases, halogenases, and related synthetic catalysts.

Reduction Potentials of P450 Compounds I and II: Insight into the Thermodynamics of C-H Bond Activation

We present a mixed experimental/theoretical determination of the bond strengths and redox potentials that define the ground-state thermodynamics for C-H bond activation in cytochrome P450 catalysis. Using redox titrations with [Ir(IV)Cl₆]^2-, we have determined the compound II/ferric (or Fe(IV)OH/Fe(III)OH₂) couple and its associated D(O-H)_Ferric bond strength in CYP158. Knowledge of this potential as well as the compound II/ferric (or Fe(IV)O/Fe(III)OH) reduction potential in horseradish peroxidase and the two-electron compound I/ferric (or Fe(IV)O(Por^•)/Fe(III)OH₂(Por)) reduction potential in aromatic peroxidase has allowed us to gauge the accuracy of theoretically determined bond strengths. Using the restricted open shell (ROS) method as proposed by Wright and co-workers, we have obtained O-H bond strengths and associated redox potentials for charge-neutral H-atom reductions of these iron(IV)-hydroxo and -oxo porphyrin species that are within 1 kcal/mol of experimentally determined values, suggesting that the ROS method may provide accurate values for the P450-II O-H bond strength and P450-I reduction potential. The efforts detailed here indicate that the ground-state thermodynamics of C-H bond activation in P450 are best described as follows: E^0'_Comp-I = 1.22 V (at pH 7, vs NHE) with D(O-H)_Comp-II = 95 kcal/mol and E^0'_Comp-II = 0.99 V (at pH 7, vs NHE) with D(O-H)_Ferric = 90 kcal/mol.

Mechanistic Studies of Fatty Acid Activation by CYP152 Peroxygenases Reveal Unexpected Desaturase Activity

The majority of cytochrome P450 enzymes (CYPs) predominantly operate as monooxygenases, but recently a class of P450 enzymes was discovered, that can act as peroxygenases (CYP152). These enzymes convert fatty acids through oxidative decarboxylation, yielding terminal alkenes, and through α- and β-hydroxylation to yield hydroxy-fatty acids. Bioderived olefins may serve as biofuels, and hence understanding the mechanism and substrate scope of this class of enzymes is important. In this work, we report on the substrate scope and catalytic promiscuity of CYP OleT_JE and two of its orthologues from the CYP152 family, utilizing α-monosubstituted branched carboxylic acids. We identify α,β-desaturation as an unexpected dominant pathway for CYP OleT_JE with 2-methylbutyric acid. To rationalize product distributions arising from α/β-hydroxylation, oxidative decarboxylation, and desaturation depending on the substrate’s structure and binding pattern, a computational study was performed based on an active site complex of CYP OleT_JE containing the heme cofactor in the substrate binding pocket and 2-methylbutyric acid as substrate. It is shown that substrate positioning determines the accessibility of the oxidizing species (Compound I) to the substrate and hence the regio- and chemoselectivity of the reaction. Furthermore, the results show that, for 2-methylbutyric acid, α,β-desaturation is favorable because of a rate-determining α-hydrogen atom abstraction, which cannot proceed to decarboxylation. Moreover, substrate hydroxylation is energetically impeded due to the tight shape and size of the substrate binding pocket.

Biochemical characterization of three new α-olefin-producing P450 fatty acid decarboxylases with a halophilic property

BACKGROUND

The CYP152 family member OleT_JE from Jeotgalicoccus sp. ATCC 8456 has been well-known to catalyze the unusual one-step decarboxylation of free fatty acids towards the formation of terminal alkenes. Efforts to tune up its decarboxylation activity for better production of biological alkenes have been extensively explored via approaches such as site-directed mutagenesis and electron source engineering, but with limited success. To gain more insights into the decarboxylation mechanism and reaction bifurcation (decarboxylation versus hydroxylation), we turned to an alternative approach to explore the natural CYP152 resources for a better variety of enzyme candidates.

RESULTS

We biochemically characterized three new P450 fatty acid decarboxylases including OleT_JH, OleT_SQ and OleT_SA, with respect to their substrate specificity, steady-state kinetics, and salt effects. These enzymes all act as an OleT_JE-like fatty acid decarboxylase being able to decarboxylate a range of straight-chain saturated fatty acids (C₈-C₂₀) to various degrees. Site-directed mutagenesis analysis to the lower activity P450 enzyme OleT_SA revealed a number of key amino acid residues within the substrate-binding pocket (T47F, I177L, V319A and L405I) that are important for delicate substrate positioning of different chain-length fatty acids and thus the decarboxylation versus hydroxylation chemoselectivity, in particular for the mid-chain fatty acids (C₈-C₁₂). In addition, the three new decarboxylases exhibited optimal catalytic activity and stability at a salt concentration of 0.5 M, and were thus classified as moderate halophilic enzymes.

CONCLUSION

The P450 fatty acid decarboxylases OleT_JE, OleT_JH, OleT_SQ and OleT_SA belong to a novel group of moderate halophilic P450 enzymes. OleT_JH from Jeotgalicoccus halophilus shows the decarboxylation activity, kinetic parameters, as well as salt tolerance and stability that are comparable to OleT_JE. Site-directed mutagenesis of several key amino acid residues near substrate-binding pocket provides important guidance for further engineering of these P450 fatty acid decarboxylases that hold promising application potential for production of α-olefin biohydrocarbons.

Structural and catalytic properties of the peroxygenase P450 enzyme CYP152K6 from Bacillus methanolicus

The CYP152 family of cytochrome P450 enzymes (P450s or CYPs) are bacterial peroxygenases that use hydrogen peroxide to drive hydroxylation and decarboxylation of fatty acid substrates. We have expressed and purified a novel CYP152 family member - CYP152K6 from the methylotroph Bacillus methanolicus MGA3. CYP152K6 was characterized using spectroscopic, analytical and structural methods. CYP152K6, like its peroxygenase counterpart P450SPα (CYP152B1) from Sphingomonas paucimobilis, does not undergo significant fatty acid-induced perturbation to the heme spectrum, with the exception of a minor Soret shift observed on binding dodecanoic acid. However, CYP152K6 purified from an E. coli expression system was crystallized and its structure was determined to 1.3 Å with tetradecanoic acid bound. No lipids were present in conditions used for crystallogenesis, and thus CYP152K6 must form a complex by incorporating the fatty acid from E. coli cells. Turnover studies with dodecanoic acid revealed several products, with 2-hydroxydodecanoic acid as the major product and much smaller quantities of 3-hydroxydodecanoic acid. Secondary turnover products were undec-1-en-1-ol, 2-hydroxydodec-2-enoic acid and 2,3-dihydroxydodecanoic acid. This is the first report of a 2,3-hydroxylated fatty acid product made by a peroxygenase P450, with the dihydroxylated product formed by CYP152K6-catalyzed 3-hydroxylation of 2-hydroxydodecanoic acid, but not by 2-hydroxylation of 3-hydroxydodecanoic acid.

Different Behaviors of a Substrate in P450 Decarboxylase and Hydroxylase Reveal Reactivity-Enabling Actors

Biological routes to the production of fuels from renewable feedstocks hold significant promise in our efforts towards a sustainable future. The fatty acid decarboxylase enzyme (OleT_JE) is a cytochrome P450 enzyme that converts long and medium chain fatty acids to terminal alkenes and shares significant similarities in terms of structure, substrate scope and mechanism with the hydroxylase cytochrome P450 (P450_BSβ). Recent reports have demonstrated that catalytic pathways in these enzymes bifurcate when the heme is in its iron-hydroxo (compound II) state. In spite of significant similarities, the fundamental underpinnings of their different characteristic wild-type reactivities remain ambiguous. Here, we develop point charges, modified parameters and report molecular simulations of this crucial intermediate step. Water occupancies and substrate mobility at the active site are observed to be vital differentiating aspects between the two enzymes in the compound II state and corroborate recent experimental hypotheses. Apart from increased substrate mobility in the hydroxylase, which could have implications for enabling the rebound mechanism for hydroxylation, OleT_JE is characterized by much stronger binding of the substrate carboxylate group to the active site arginine, implicating it as an important enabling actor for decarboxylation.

Characterization of Cytochrome P450 Enzymes and Their Applications in Synthetic Biology

The cytochrome P450 monooxygenase enzymes (P450s) catalyze a diverse array of chemical transformations, most originating from the insertion of an oxygen atom into a substrate that binds close to the P450 heme. The oxygen is delivered by a highly reactive heme iron-oxo species (compound I) and, according to the chemical nature of the substrate and its position in the active site, the P450 can catalyze a wide range of reactions including, e.g., hydroxylation, reduction, decarboxylation, sulfoxidation, N- and O-demethylation, epoxidation, deamination, CC bond formation and breakage, nitration, and dehalogenation. In this chapter, we describe the structural, biochemical, and catalytic properties of the P450s, along with spectroscopic and analytical methods used to characterize P450 enzymes and their redox partners. Important uses of P450 enzymes are highlighted, including how various P450s have been exploited for applications in synthetic biology.

Formation and Cleavage of C-C Bonds by Enzymatic Oxidation-Reduction Reactions

Many oxidation-reduction (redox) enzymes, particularly oxygenases, have roles in reactions that make and break C-C bonds. The list includes cytochrome P450 and other heme-based monooxygenases, heme-based dioxygenases, nonheme iron mono- and dioxygenases, flavoproteins, radical S-adenosylmethionine enzymes, copper enzymes, and peroxidases. Reactions involve steroids, intermediary metabolism, secondary natural products, drugs, and industrial and agricultural chemicals. Many C-C bonds are formed via either (i) coupling of diradicals or (ii) generation of unstable products that rearrange. C-C cleavage reactions involve several themes: (i) rearrangement of unstable oxidized products produced by the enzymes, (ii) oxidation and collapse of radicals or cations via rearrangement, (iii) oxygenation to yield products that are readily hydrolyzed by other enzymes, and (iv) activation of O₂ in systems in which the binding of a substrate facilitates O₂ activation. Many of the enzymes involve metals, but of these, iron is clearly predominant.

Effect of hydrogen peroxide on Microcystic aeruginosa: Role of cytochromes P450

Cyanobacterial bloom has been rising as a worldwide issue owing to its adverse effects to water quality and ecological health. To solve this problem, hydrogen peroxide (H₂O₂) has been considered as a potential algaecide because no by-products are generated after treatment and because it kills cyanobacteria selectively. In addition, cytochromes P450 (CYPs) was reported to be related with H₂O₂, but the roles of CYPs in the regulation of H₂O₂ in cyanobacteria have yet to be investigated. In this study, the CYPs suicide inhibitor 1-aminobenzotriazole (ABT) was added to the representative cyanobacteria Microcystis aeruginosa (M. aeruginosa) exposed to H₂O₂. The results showed that CYPs mediates the effects of H₂O₂ on M. aeruginosa. To be exact, the addition of ABT induced greater inhibitory effects on the growth and higher reactive oxygen species levels in M. aeruginosa comparing to those treated with H₂O₂ alone. At the same time, photosynthetic parameters significantly decreased, and the content of extracellular microcystins (MCs) increased but the total MCs decreased due to the combined effect of H₂O₂ and ABT. ABT also intensified the aggregation of Fe, which might explain the effects on photosynthesis and synthesis of MCs. Furthermore, the transcriptional levels of MCs-synthesis genes (mcyA and mcyD) decreased but MCs-release gene (mcyH) increased, and photosynthetic genes (psaB, psbD1 and rbcL) decreased, which confirmed the effects on the MC production/release and electron transport of photosynthesis, respectively. In summary, this study illuminated the mediation role of CYPs in the adverse effects on M. aeruginosa induced by H₂O₂, thus providing new theoretical basis for the explanation of H₂O₂ as potential algaecide.

A Reactive Manganese(IV)-Hydroxide Complex: A Missing Intermediate in Hydrogen Atom Transfer by High-Valent Metal-Oxo Porphyrinoid Compounds

High-valent metal-hydroxide species are invoked as critical intermediates in both catalytic, metal-mediated O₂ activation (e.g., by Fe porphyrin in Cytochrome P450) and O₂ production (e.g., by the Mn cluster in Photosystem II). However, well-characterized mononuclear M^IV(OH) complexes remain a rarity. Herein we describe the synthesis of Mn^IV(OH)(ttppc) (3) (ttppc = tris(2,4,6-triphenylphenyl) corrole), which has been characterized by X-ray diffraction (XRD). The large steric encumbrance of the ttppc ligand allowed for isolation of 3. The complexes Mn^V(O)(ttppc) (4) and Mn^III(H₂O)(ttppc) (1·H₂O) were also synthesized and structurally characterized, providing a series of Mn complexes related only by the transfer of hydrogen atoms. Both 3 and 4 abstract an H atom from the O-H bond of 2,4-di- tert-butylphenol (2,4-DTBP) to give a radical coupling product in good yield (3 = 90(2)%, 4 = 91(5)%). Complex 3 reacts with 2,4-DTBP with a rate constant of k₂ = 2.73(12) × 10⁴ M^-1 s^-1, which is ∼3 orders of magnitude larger than 4 ( k₂ = 17.4(1) M^-1 s^-1). Reaction of 3 with a series of para-substituted 2,6-di- tert-butylphenol derivatives (4-X-2,6-DTBP; X = OMe, Me, tBu, H) gives rate constants in the range k₂ = 510(10)-36(1.4) M^-1 s^-1 and led to Hammett and Marcus plot correlations. Together with kinetic isotope effect measurements, it is concluded that O-H cleavage occurs by a concerted H atom transfer (HAT) mechanism and that the Mn^IV(OH) complex is a much more powerful H atom abstractor than the higher-valent Mn^V(O) complex, or even some Fe^IV(O) complexes.

Oxygen Activation and Radical Transformations in Heme Proteins and Metalloporphyrins

As a result of the adaptation of life to an aerobic environment, nature has evolved a panoply of metalloproteins for oxidative metabolism and protection against reactive oxygen species. Despite the diverse structures and functions of these proteins, they share common mechanistic grounds. An open-shell transition metal like iron or copper is employed to interact with O2 and its derived intermediates such as hydrogen peroxide to afford a variety of metal-oxygen intermediates. These reactive intermediates, including metal-superoxo, -(hydro)peroxo, and high-valent metal-oxo species, are the basis for the various biological functions of O2-utilizing metalloproteins. Collectively, these processes are called oxygen activation. Much of our understanding of the reactivity of these reactive intermediates has come from the study of heme-containing proteins and related metalloporphyrin compounds. These studies not only have deepened our understanding of various functions of heme proteins, such as O2 storage and transport, degradation of reactive oxygen species, redox signaling, and biological oxygenation, etc., but also have driven the development of bioinorganic chemistry and biomimetic catalysis. In this review, we survey the range of O2 activation processes mediated by heme proteins and model compounds with a focus on recent progress in the characterization and reactivity of important iron-oxygen intermediates. Representative reactions initiated by these reactive intermediates as well as some context from prior decades will also be presented. We will discuss the fundamental mechanistic features of these transformations and delineate the underlying structural and electronic factors that contribute to the spectrum of reactivities that has been observed in nature as well as those that have been invented using these paradigms. Given the recent developments in biocatalysis for non-natural chemistries and the renaissance of radical chemistry in organic synthesis, we envision that new enzymatic and synthetic transformations will emerge based on the radical processes mediated by metalloproteins and their synthetic analogs.

Transition-metal-catalyzed decarbonylation of carboxylic acids to olefins: exploiting acyl C–O activation for the production of high value products

In this article, we review the recent developments in the transition-metal-catalyzed decarbonylation of carboxylic acids to produce olefins by the formal acyl C–O activation mechanism and discuss future challenges in this field.