Total Turnover Number Research Articles

Mapping the Initial Stages of a Protective Pathway that Enhances Catalytic Turnover by a Lytic Polysaccharide Monooxygenase.

Oxygenase and peroxygenase enzymes generate intermediates at their active sites which bring about the controlled functionalization of inert C-H bonds in substrates, such as in the enzymatic conversion of methane to methanol. To be viable catalysts, however, these enzymes must also prevent oxidative damage to essential active site residues, which can occur during both coupled and uncoupled turnover. Herein, we use a combination of stopped-flow spectroscopy, targeted mutagenesis, TD-DFT calculations, high-energy resolution fluorescence detection X-ray absorption spectroscopy, and electron paramagnetic resonance spectroscopy to study two transient intermediates that together form a protective pathway built into the active sites of copper-dependent lytic polysaccharide monooxygenases (LPMOs). First, a transient high-valent species is generated at the copper histidine brace active site following treatment of the LPMO with either hydrogen peroxide or peroxyacids in the absence of substrate. This intermediate, which we propose to be a CuII-(histidyl radical), then reacts with a nearby tyrosine residue in an intersystem-crossing reaction to give a ferromagnetically coupled (S = 1) CuII-tyrosyl radical pair, thereby restoring the histidine brace active site to its resting state and allowing it to re-enter the catalytic cycle through reduction. This process gives the enzyme the capacity to minimize damage to the active site histidine residues "on the fly" to increase the total turnover number prior to enzyme deactivation, highlighting how oxidative enzymes are evolved to protect themselves from deleterious side reactions during uncoupled turnover.

Engineering a Formate Dehydrogenase for NADPH Regeneration.

Nicotinamide adenine dinucleotide (NADH) and nicotinamide adenine dinucleotide phosphate (NADPH) constitute major hydrogen donors for oxidative/reductive bio-transformations. NAD(P)H regeneration systems coupled with formate dehydrogenases (FDHs) represent a dreamful method. However, most of the native FDHs are NAD+ -dependent and suffer from insufficient reactivity compared to other enzymatic tools, such as glucose dehydrogenase. An efficient and competitive NADP+ -utilizing FDH necessitates the availability and robustness of NADPH regeneration systems. Herein, we report the engineering of a new FDH from Candida dubliniensis (CdFDH), which showed no strict NAD+ preference by a structure-guided rational/semi-rational design. A combinatorial mutant CdFDH-M4 (D197Q/Y198R/Q199N/A372S/K371T/▵Q375/K167R/H16L/K159R) exhibited 75-fold intensification of catalytic efficiency (kcat /Km ). Moreover, CdFDH-M4 has been successfully employed in diverse asymmetric oxidative/reductive processes with cofactor total turnover numbers (TTNs) ranging from 135 to 986, making it potentially useful for NADPH-required biocatalytic transformations.

Ultra-small PdAuIr nanoparticles on amine-functionalized porous organic polymers for efficient dehydrogenation from FA at room temperature

Formic acid (FA) is a prospective hydrogen storage agent, which has attracted much attention for its low toxicity and stability and plays a significant role in the comprehensive implementation of the hydrogen economy. In this regard, it is very important to utilize additive free FA dehydrogenation, for which few heterogeneous catalysts are available. Herein, we report ultra-small PdAuIr nanoparticles (NPs) supported on amine-based amorphous porous organic polymers (POPs), which exhibit excellent FA dehydrogenation activity with an initial total turnover number (TOF) of 9635 h−1 without additives at room temperature and apparent activation energy (Eaapp) of 36.5 kJ/mol. The results show that the excellent performance can be attributed to the synergistic effect of trimetallic alloys and strong metal-support interaction effect (SMSI), as well as to the amine groups (-NH2) grafted on POPs which facilitates the O–H bond splitting on FA. Overall, the simple and efficient synthetic strategy provides a new method for the selective dehydrogenation of FA.

Application of the all-in-one electrode for in situ H2O2 generation in hydroxylation catalyzed by unspecific peroxygenase from Agrocybe aegerita

The versatile unspecific peroxygenase from the fungus Agrocybe aegerita (AaeUPO) is capable to perform hydroxylation of various substrates, even unactivated CH bonds. The technical application of AaeUPO is still limited due to an instability towards its co-substrate hydrogen peroxide (H2O2). Electrochemical synthesis of H2O2 is an attractive approach that provides controllable in situ generation of H2O2. The aims of this study are to promote catalyst efficiency and develop an individually tailored system for H2O2-dependent reactions. Herein, an All-in-One electrode (AiO) system with a carbon felt cathode is used to generate H2O2in situ and combined for the first time with an enzymatic hydroxylation. The AiO electrode combines the counter and working electrode in a single rod structure. This shape provides convenient integration into conventional bioreactors, thus, converting them into bioelectrochemical systems (BES). Hydroxylation of 4-ethylbenzoic acid (EBA) catalyzed by the recombinant AaeUPO (rAaeUPO) was chosen as the model reaction. Total turnover number (TTN) up to 450,000 mol mol−1 and turnover frequency (TOF) up to 7.7 s − 1 were achieved using the AiO electrode system. The H2O2 productivity was identified as the limiting factor in the hydroxylation of EBA. However, with numbering-up or surface enlargement, this process could have great potential as an optimizable platform for H2O2-dependent enzymatic reactions.

Catalytic Oxidation of Methane by Wild-Type Cytochrome P450BM3 with Chemically Evolved Decoy Molecules

Biological methane oxidation is a highly desirable method for the conversion of natural gas into a liquid to meet the increasing demand for fuel and chemical feedstock as well as reducing the potent greenhouse effects of methane emissions. Because natural hemoenzymes that can catalyze the conversion of methane to methanol have not been found, it has long been considered that hemoenzymes, including cytochrome P450s (P450s), cannot catalyze the oxidative conversion of methane. Herein, we report the catalytic oxidation of methane by wild-type P450BM3, without any mutagenesis, in the presence of chemically evolved dummy substrates (decoy molecules) under high-pressure methane at 10 MPa. Our studies showed that methane was catalytically converted into methanol at room temperature with a total turnover number of 4.

Synergy of ultra-low-loaded ruthenium with alumina stimulating the catalytic hydrogenation of levulinic acid into γ-valerolactone

Herein we report an efficient and recyclable catalyst combining extremely low-loaded Ru and abundant alumina supported on nitrogen-doped carbon (NC) for hydrogenation of cellulose-derived levulinic acid (LA) into valuable γ-valerolactone (GVL). The as-fabricated Ru0.18/Al2O3/NC shows extraordinarily catalytic efficiency in transforming LA to GVL under solvent-free conditions, achieving up to a 1.77 × 105 turnover number (TON) and a 26400 h−1 turnover frequency (TOF) in a single batch reaction, which is far better than other heterogeneous catalysts and comparable to the best optimized homogenous catalysts. This catalyst demonstrates a good stability and reusability, whose catalytic performance can be maintained in 22 times recycled reactions, thus reaching a 106-level value of total TONs. These results highlight the importance of the synergistic effects between infrequent Ru and abundant alumina, and provide an up-and-coming way to design more active and stable low Ru loading catalyst for such a transformation.

Catalytic Promiscuity of Horseradish Peroxidase: Aerobic Oxidative Regeneration of Oxidized Nicotinamide Cofactors

AbstractNAD(P)+ regeneration is of great importance for dehydrogenase (DH)‐catalyzed oxidations. In this work, we report horseradish peroxidase (HRP)‐catalyzed aerobic oxidation of the reduced nicotinamide cofactors, including natural and non‐natural ones, into the oxidized form at neutral and alkaline pH enabled by external amino acids (AAs). HRP showed promiscuous yet extremely low oxidase activity toward NADH at pH 7 in the absence of both AAs and mediators, as evidenced by the observation of compound III. The mechanistic study revealed that AAs as decoy molecules (likely act as acid‐base catalysts) and mediators significantly promoted the formation of compound III as well as its decay back to the ground state. The applicability of the aerobic cofactor regeneration method was demonstrated by the HRP total turnover number up to 88 000 as well as high yields (91–99 %) in bi‐enzymatic synthesis of carboxylic acids. The present work offers an alternative method to recycle NAD(P)+. Also, it may open up possibilities for HRP‐catalyzed aerobic oxidations.

Z-Schematic Artificial Leaf Structure for Biosolar Oxyfunctionalization of Hydrocarbons

The natural Z-schematic photosynthesis is a promising catalytic model for solar-to-chemical conversion. Here, we construct a Z-schematic, wireless photoelectrocatalytic (PEC) system (i.e., artificial leaf) for biocatalytic oxyfunctionalization of hydrocarbons. The monolithic leaf structure consists of a tandem photoanode|photocathode configuration that uses sunlight as the sole energy source to drive redox reactions. Under solar light, the ferric oxyhydroxide-coated, molybdenum-doped bismuth vanadate (FeOOH|Mo:BVO) photoanode extracts electrons from H2O electron feedstock and transfers the electrons to the conjugated polyterthiophene (pTTh) photocathode. Meanwhile, the pTTh photocathode absorbs FeOOH|Mo:BVO-filtered light for O2 reduction to H2O2. The in situ generated H2O2 activates unspecific peroxygenases (UPOs) to drive enantioselective C–H oxyfunctionalization (e.g., hydroxylation and epoxidation). Furthermore, we solve HO•-mediated inactivation of UPOs using a cellulose membrane, which increases enzymatic productivity with a benchmark total turnover number of 193 000 among PEC and photocatalytic platforms that trigger UPO-mediated synthesis.

Total turnover number prediction of an aggregating biocatalyst: Amino ester hydrolase (AEH)

Amino ester hydrolase (AEH) from Xanthomonas campestris is a promising candidate for β-lactam synthesis but suffers from low thermostability and rapid deactivation. Wild-type AEH was found to deactivate with an apparent order that varies with temperature from n = 2 at 30 °C (close to the temperature of optimum activity Topt) to n = 1.5 at 25 °C. AEH features two transitional temperatures: (1) from native ‘N’ to partially unfolded, inactive intermediate ‘I’, and (2) from partially unfolded to fully unfolded entity ‘U’ at the melting temperature Tm. CD and light scattering data suggest aggregation near Topt. To determine the total turnover number TTN, AEH was deactivated by imposing a temperature gradient and recording instantaneous rates of cephalexin hydrolysis. The TTN increased ∼ 5-fold from the wild-type (WT) for the quadruple variant N186D/A275P/E143H/V622I (QVH) at 25 °C and 10 nM AEH but varied significantly with AEH concentration and temperature.

Effect of chromosomal integration on catalytic performance of a multi-component P450 system in Escherichia coli.

Cytochromes P450 are useful biocatalysts in synthetic chemistry and important bio-bricks in synthetic biology. Almost all bacterial P450s require separate redox partners for their activity, which are often expressed in recombinant Escherichia coli using multiple plasmids. However, the application of CRISPR/Cas recombineering facilitated chromosomal integration of heterologous genes which enables more stable and tunable expression of multi-component P450 systems for whole-cell biotransformations. Herein, we compared three E. coli strains W3110, JM109, and BL21(DE3) harboring three heterologous genes encoding a P450 and two redox partners either on plasmids or after chromosomal integration in two genomic loci. Both loci proved to be reliable and comparable for the model regio- and stereoselective two-step oxidation of (S)-ketamine. Furthermore, the CRISPR/Cas-assisted integration of the T7 RNA polymerase gene enabled an easy extension of T7 expression strains. Higher titers of soluble active P450 were achieved in E. coli harboring a single chromosomal copy of the P450 gene compared to E. coli carrying a medium copy pET plasmid. In addition, improved expression of both redox partners after chromosomal integration resulted in up to 80% higher (S)-ketamine conversion and more than fourfold increase in total turnover numbers.

Confinement of Cu2O by in-situ derived NH2-MIL-125@TiO2 for synergetic photothermal-driven hydrogen evolution from aqueous-phase methanol reforming

Photothermal synergism can effectively activate methanol at low operating temperature and significantly reduce the activation energy of the reaction, achieving more efficiently H2 release from methanol reforming. Here, a novel hierarchical heterojunction that integrating photo- and thermal-active Cu2O catalytic species with in-situ derived NH2-MIL-125@TiO2 framework was specifically designed for photothermal-driven aqueous phase reforming of methanol into H2. The afforded Cu2O/NH2-MIL-125@TiO2 realized an outstanding photothermal H2 production activity (apparent quantum efficiency of 22.3 % at 365 nm), ca. 13-times higher than that of thermocatalytic condition. Interestingly, the photothermal effect did confer the Cu2O/NH2-MIL-125@TiO2 with unexpected activity at low temperature subsided to 100 °C and accelerated the activation of MeOH/H2O with an obvious reduction of activation energy from 82.62 kJ·mol−1 to 52.40 kJ·mol−1.The improvement of catalyst stability and the promotion of charge separation also contributed to a long-term photothermal H2 evolution activity with average rate of 1.49x106 μmolgCu-1h−1 and a total turnover number (TON) up to 5971 in 63 h, almost 125-fold promotion compared with Cu2O.

Alternative Active Site Confinement in Squalene–Hopene Cyclase Enforces Substrate Preorganization for Cyclization

Confinement of an enzyme’s active site is critical to the efficiency of chemical reactions and has been recognized as an important tool for catalysis to accelerate reactions by proximity and via specific interactions with the substrate to control the course of the reaction. Despite these benefits, a strongly confined active site is inherently limited to compounds that resemble the native substrate, with only small deviations tolerated. To unlock the potential of modifying the terpene scaffold as building blocks of interest with branched isoprene/terpene motifs, the present study demonstrates a simple structure-guided strategy to create alternative confinement in the squalene–hopene cyclase from Alicyclobacillus acidocaldarius (AacSHC). This strategy aims for proximity and shape complementarity between the protein and substrate to enforce preorganization of geranyl acetone analogs with deviant isoprene patterns. Screening of AacSHC variants affecting the substrate tunnel allowed us to detect starting activity with the AacSHC G600M variant. Structural analysis led to the identification of reduced tunnel radii that resulted in steric interactions and proximity by a decrease in the average distances between the double bond of the substrate’s terminal isoprene unit and the catalytic site. Iterative saturation mutagenesis with SHC variants and screening with the geranyl acetone analog dihydropseudoirone were used to generate complementarity and demonstrated further evolvability. The final variant showed a 1174-fold increase in total turnover number and 111-fold increased catalytic efficiency compared to the wild-type. This approach to create alternative active site confinement demonstrates great potential to overcome limitations in the engineering of biocatalysts and generation of interesting building blocks.

Preparation of (3R)-quinuclidinol using heterogeneous biocatalytic hydrogenation in a dynamically-mixed continuous flow reactor

Introduction: This work was carried out to understand if a heterogeneous biocatalytic hydrogenation system could be implemented in a slurry mode continuous flow reactor, as a “slot-in” alternative to a metal/carbon catalyst with minimal process or chemical engineering development.Method: Biocatalytic hydrogenation was compared to metal (Pd/C) catalysed hydrogenation for ketone to chiral alcohol conversion in both a continuous flow reactor (Coflore ACR, AM Technology) and analogous scaled down batch reactions.Results and discussion: Initial results demonstrated that batch reactions can achieve high conversions in 30 min, with relatively low biocatalyst loadings, meeting critical criteria for operation as continuous slurry mode process. Further results demonstrated full conversion of quinuclidinone to (3R)-quinuclidinol in continuous flow under mild conditions (35°C, 2 bar H2). On intensification of the process to higher substrate loading (50 mM), conversion was similar to with Pd/C, however the biocatalytic system achieved far higher turnover frequency and total turnover number (65 min−1 and 20,000, respectively) than the metal system (0.16 min−1, 37). Comparison to an analogous batch reaction highlights that the biocatalytic system has promise for further optimisation and intensification in the scalable Coflore ACR. Overall, biocatalytic hydrogenation is shown to offer a decarbonised biocatalytic route and a “slot-in” replacement to metal catalysts for hydrogenation reactions in continuous flow reactors.

Improved 2α-Hydroxylation Efficiency of Steroids by CYP154C2 Using Structure-Guided Rational Design.

Cytochrome P450 enzymes are promising biocatalysts for industrial use because they catalyze site-selective C-H oxidation and have diverse catalytic reactions and a broad substrate range. In this study, the 2α-hydroxylation activity of CYP154C2 from Streptomyces avermitilis MA-4680T toward androstenedione (ASD) was identified by an in vitro conversion assay. The testosterone (TES)-bound structure of CYP154C2 was solved at 1.42 Å, and this structure was used to design eight mutants, including single, double, and triple mutants, to improve the conversion efficiency. Mutants L88F/M191F and M191F/V285L were found to enhance the conversion rates significantly (i.e., 8.9-fold and 7.4-fold for TES, 46.5-fold and 19.5-fold for ASD, respectively) compared with the wild-type (WT) enzyme while retaining high 2α-position selectivity. The substrate binding affinity of the L88F/M191F mutant toward TES and ASD was enhanced compared with that of WT CYP154C2, supporting the measured increase in the conversion efficiencies. Moreover, the total turnover number and kcat/Km of the L88F/M191F and M191F/V285L mutants increased significantly. Interestingly, all mutants containing L88F generated 16α-hydroxylation products, suggesting that L88 in CYP154C2 plays a vital role in substrate selectivity and that the amino acid corresponding to L88 in the 154C subfamily affects the orientation of steroid binding and substrate selectivity. IMPORTANCE Hydroxylated derivatives of steroids play essential roles in medicine. Cytochrome P450 enzymes selectively hydroxylate methyne groups on steroids, which can dramatically change their polarity, biological activity and toxicity. There is a paucity of reports on the 2α-hydroxylation of steroids, and documented 2α-hydroxylate P450s show extremely low conversion efficiency and/or low regio- and stereoselectivity. This study conducted crystal structure analysis and structure-guided rational engineering of CYP154C2 and efficiently enhanced the conversion efficiency of TES and ASD with high regio- and stereoselectivity. Our results provide an effective strategy and theoretical basis for the 2α-hydroxylation of steroids, and the structure-guided rational design of P450s should facilitate P450 applications in the biosynthesis of steroid drugs.

Engineering Cytochrome P450BM3 Enzymes for Direct Nitration of Unsaturated Hydrocarbons

AbstractApplications of the peroxidase activity of cytochrome P450 enzymes in synthetic chemistry remain largely unexplored. We present herein a protein engineering strategy to increase cytochrome P450BM3 peroxidase activity for the direct nitration of aromatic compounds and terminal aryl‐substituted olefins in the presence of a dual‐functional small molecule (DFSM). Site‐directed mutations of key active‐site residues allowed the efficient regulation of steric effects to limit substrate access and, thus, a significant decrease in monooxygenation activity and increase in peroxidase activity. Nitration of several phenol and aniline compounds also yielded ortho‐ and para‐nitration products with moderate‐to‐high total turnover numbers. Besides direct aromatic nitration by P450 variants using nitrite as a nitrating agent, we also demonstrated the use of the DFSM‐facilitated P450 peroxidase system for the nitration of the vinyl group of styrene and its derivatives.

Engineering Cytochrome P450BM3 Enzymes for Direct Nitration of Unsaturated Hydrocarbons.

Applications of the peroxidase activity of cytochrome P450 enzymes in synthetic chemistry remain largely unexplored. We present herein a protein engineering strategy to increase cytochrome P450BM3 peroxidase activity for the direct nitration of aromatic compounds and terminal aryl-substituted olefins in the presence of a dual-functional small molecule (DFSM). Site-directed mutations of key active-site residues allowed the efficient regulation of steric effects to limit substrate access and, thus, a significant decrease in monooxygenation activity and increase in peroxidase activity. Nitration of several phenol and aniline compounds also yielded ortho- and para-nitration products with moderate-to-high total turnover numbers. Besides direct aromatic nitration by P450 variants using nitrite as a nitrating agent, we also demonstrated the use of the DFSM-facilitated P450 peroxidase system for the nitration of the vinyl group of styrene and its derivatives.

Repertoire of Computationally Designed Peroxygenases for Enantiodivergent C-H Oxyfunctionalization Reactions.

The generation of enantiodivergent biocatalysts for C-H oxyfunctionalizations is ever more important in modern synthetic chemistry. Here, we have applied the FuncLib algorithm based on phylogenetic and Rosetta calculations to design a diverse repertoire of active, stable, and enantiodivergent fungal peroxygenases. 24 designs, each carrying 4-5 mutations in the catalytic core, were expressed functionally in yeast and benchmarked against characteristic model compounds. Several designs were active and stable in a range of temperature and pH, displaying unprecedented enantiodivergence, changing regioselectivity from alkyl to aromatic hydroxylation, and increasing catalytic efficiencies up to 10-fold, with 15-fold improvements in total turnover numbers over the parental enzyme. We find that this dramatic functional divergence stems from beneficial epistasis among the mutations and an extensive reorganization of the heme channel. Our work demonstrates that FuncLib can rapidly design highly functional libraries enriched in enantioselective peroxygenases not seen in nature for a range of biotechnological applications.

Clean enzymatic production of ursodeoxycholic acid enabled by a newly identified NADH-dependent 7β-hydroxysteroid dehydrogenase

7β-Hydroxysteroid dehydrogenases (7β-HSDHs) play an important role in the enzymatic synthesis of ursodeoxycholic acid (UDCA), which is a value-added compound with a range of pharmacological activities. Since the cofactor NADH is much cheaper than NADPH, the production of UDCA using an NADH-dependent 7β-HSDH has better prospects than that using an NADPH-dependent 7β-HSDH. However, a major bottleneck in UDCA biosynthesis is the poor catalytic activity of the current NADH-dependent 7β-HSDHs. In this work, a new and natively NADH-dependent Rs7β-HSDH from Roseococcus sp. was identified, which showed a high specific activity of 63.3 U mg−1protein toward 7-oxo-lithocholic acid, with a catalytic efficiency (kcat/KM) of 515 mM–1 s–1. In a preparative biotransformation (100-mL scale) using Rs7β-HSDH and an NAD+-dependent 7α-HSDH, with O2/NOX and HCOO−/FDH systems for the regeneration of cofactors (NAD+ and NADH), 25 mM chenodeoxycholic acid was completely converted to UDCA in one-pot two-step cascade, with an 80% isolated yield and a total turnover number (TTN) of 6586 for Rs7β-HSDH. The environmental factor (E-factor) of this process was 8.37 when water was excluded, much lower than those obtained from other processes reported so far, indicating a great potential of this Rs7β-HSDH for more sustainable and cleaner enzymatic production of UDCA.

Multienzyme Coimmobilization on Triheterofunctional Supports.

Immobilized multienzyme systems are gaining momentum in applied biocatalysis; however, the coimmobilization of several enzymes on one carrier is still challenging. In this work, we exploited a heterofunctional support activated with three different chemical functionalities to immobilize a wide variety of different enzymes. This support is based on agarose microbeads activated with aldehyde, amino, and cobalt chelate moieties that allow a fast and irreversible immobilization of enzymes, enhancing the thermostability of most of the heterogeneous biocatalysts (up to 21-fold higher than the soluble one). Furthermore, this trifunctional support serves to efficiently coimmobilize a multienzyme system composed of an alcohol dehydrogenase, a reduced nicotinamide adenine dinucleotide (NADH) oxidase, and a catalase. The confined multienzymatic system demonstrates higher performance than its free counterpart, achieving a total turnover number (TTN) of 1 × 105 during five batch consecutive cycles. We envision this solid material as a platform for coimmobilizing multienzyme systems with enhanced properties to catalyze stepwise biotransformations.

Solvent-free oxidation of toluene to benzaldehyde using electron-rich Au clusters confined in Silicalite-1

The selective oxidation of the primary CH bonds in hydrocarbons to corresponding high-valued oxygenates is highly desired but challenging to realize. The difficulty arises from the high homolytic bond dissociation enthalpy of the CH bond, the relatively high reactivity of oxygenated intermediates, and the diversity of resultant products. Herein, we reported the in situ confinement of Au clusters in Silicalite-1 for the selective solvent-free oxidation of toluene to benzaldehyde. The doping of an alkali metal (K or Na) resulted in the formation of electron-rich Au species, which readily facilitate the activation of oxygen, In turn, toluene is activated by the produced reactive oxygen species in a single step, as confirmed through density functional theory (DFT) calculations. At optimal alkali metal doping, the selectivity to benzaldehyde could reach > 90 %, with the toluene conversion being 19.8 %. Remarkably, the total turnover number (TTN) of Au@Silicalite-1–0.2 K was found to be 29,658, which is one or two orders of magnitudes higher than the previously reported values. These findings are expected to inspire the design of highly efficient catalysts for the activation of fundamental feedstock with CH bonds.