Enzyme catalytic nitration of aromatic compounds

Application of hydrophobic eutectic solvent in efficient biotransformation of total flavonoids of Herba Epimedii

Icaritin, a hydrolysate from total flavonoids of Epimedii (TFE), which has better anti-hepatoma activity than its glycosylated form. In this work, immobilized enzymes 4LP-Tpebgl3@Na-Y and DtRha@ES-107 were used to hydrolyze TFE to prepare icaritin. Five different hydrophobic deep eutectic solvents (HDES) were prepared and the most ideal HDES was successfully selected, which was composed of dodecyl alcohol and thymol with the molar ratio of 2:1. The relative enzyme activity of 4LP-Tpebgl3@Na-Y and DtRha@ES-107 was about 102.4 % and 112.5 %, respectively. In addition, the thermal and binding stability of 4LP-Tpebgl3@Na-Y and DtRha@ES-107 in HDES was not affected negatively. In the biphasic system composed of 50 % (v/v) HDES and Na2HPO4-citric acid buffer (50 mM, pH 5.5), 4LP-Tpebgl3@Na-Y (1.0 U/mL) and TFE (1 g/L) were reacted at 80 °C for 1 h, and then reacted with DtRha@ES-107 (20 U/mL) at 80 °C for 2 h. Finally, TFE was completely converted to 301.8 mg/L icaritin (0.82 mM). After 10 cycles, 4LP-Tpebgl3@Na-Y/DtRha@ES-107 still maintained 84.1 % original activity. In this study, we developed an efficient methodology for icaritin preparation through the integration of enzymatic catalysis and adsorption separation, presenting a viable approach for large-scale, cost-effective production of icaritin.

Emerging Strategies for Modifying Cytochrome P450 Monooxygenases into Peroxizymes

ConspectusCytochrome P450 monooxygenase is a versatile oxidizing enzyme with great potential in synthetic chemistry and biology. However, the dependence of its catalytic function on the nicotinamide cofactor NAD(P)H and redox partner proteins limits the practical catalytic application of P450 in vitro. An alternative to expensive cofactors is low-cost H2O2, which can be used directly to exploit the catalytic potential of P450s. However, the peroxide shunt pathway is generally inefficient at driving P450 catalysis compared to normal NAD(P)H-dependent activity. Over the last few decades, the scientific community has made continuous efforts to use directed evolution or site-directed mutagenesis to modify P450 monooxygenases into their peroxizyme modes─peroxygenase and peroxidase. Despite significant progress, obtaining efficient P450 peroxizymes remains a huge challenge. Here, we summarize our efforts to modulate peroxizyme activity in P450 monooxygenases and exploit their catalytic applications in challenging selective C-H oxidation, oxygenation, and oxyfunctionalization over the past seven years. We first developed a dual-functional small molecule (DFSM) strategy for transforming P450BM3 monooxygenase into peroxygenase. In this strategy, the typical DFSM, such as N-(ω-imidazolyl)-hexanoyl-l-phenylalanine (Im-C6-Phe), binds to the P450BM3 protein with an anchoring group at one end and plays a general acid-base catalytic role in the activation of H2O2 with an imidazolyl group at the other end. Compared with the O-O homolysis mechanism in the absence of DFSM, the addition of DFSM efficiently enables the heterolytic O-O cleavage of the adduct Fe-O-OH, thus being favored for the formation of active species compound I, which has been demonstrated by combining crystallographic and theoretical calculations. Furthermore, protein engineering showed the unique catalytic performance of DFSM-facilitated P450 peroxygenase for the highly difficult selective oxidation of C-H bonds. This catalytic performance was demonstrated during the chemoselective hydroxylation of gaseous alkanes, regioselective O-demethylation of aryl ethers, highly (R)-enantioselective epoxidation of styrene, and regio- and enantiomerically diverse hydroxylation of alkylbenzenes. Second, we demonstrated that DFSM-facilitated P450BM3 peroxygenase could be effectively switched to an efficient peroxidase mode through mechanism-guided protein engineering of redox-sensitive residues. Utilizing the peroxidase function of P450 enabled the direct nitration of unsaturated hydrocarbons including phenols, aromatic amines, and styrene derivatives, which was not only the P450-catalyzed direct nitration of phenols and aromatic amines for the first time but also the first example of the direct biological nitration of olefins. Finally, we report an H2O2 tunnel engineering strategy to enable peroxygenase activity in several different P450 monooxygenases for the first time, providing a general approach for accessing engineered P450 peroxygenases. In this Account, we highlight the emerging strategies we have developed for producing practical P450 peroxizyme biocatalysts. Although the DFSM strategy is primarily applied to P450BM3 to date, both strategies of redox-sensitive residue engineering and H2O2 tunnel engineering show great potential to extend to other P450s. These strategies have expanded the scope of applications of P450 chemistry and catalysis. Additionally, they provide a unique solution to the challenging selective oxidation of inert C-H bonds in synthetic chemistry.

Catalytic ipso-Nitration of Organosilanes Enabled by Electrophilic N-Nitrosaccharin Reagent

Nitroaromatic compounds represent one of the essential classes of molecules that are widely used as feedstock for the synthesis of intermediates, the preparation of nitro-derived pharmaceuticals, agrochemicals, and materials on both laboratory and industrial scales. We herein disclose the efficient, mild, and catalytic ipso-nitration of organotrimethylsilanes, enabled by an electrophilic N-nitrosaccharin reagent and allows chemoselective nitration under mild reaction conditions, while exhibiting remarkable substrate generality and functional group compatibility. Additionally, the reaction conditions proved to be orthogonal to other common functionalities, allowing programming of molecular complexity via successive transformations or late-stage nitration. Detailed mechanistic investigation by experimental and computational approaches strongly supported a classical electrophilic aromatic substitution (SE Ar) mechanism, which was found to proceed through a highly ordered transition state.

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.

Characterization of a Nitro-Forming Enzyme Involved in Fosfazinomycin Biosynthesis

N-hydroxylating monooxygenases (NMOs) are a subclass of flavin-dependent enzymes that hydroxylate nitrogen atoms. Recently, unique NMOs that perform multiple reactions on one substrate molecule have been identified. Fosfazinomycin M (FzmM) is one such NMO, forming nitrosuccinate from aspartate (Asp) in the fosfazinomycin biosynthetic pathway in some Streptomyces sp. This work details the biochemical and kinetic analysis of FzmM. Steady-state kinetic investigation shows that FzmM performs a coupled reaction with Asp (k_cat, 3.0 ± 0.01 s^-1) forming nitrosuccinate, which can be converted to fumarate and nitrite by the action of FzmL. FzmM displays a 70-fold higher k_cat/K_M value for NADPH compared to NADH and has a narrow optimal pH range (7.5-8.0). Contrary to other NMOs where the k_red is rate-limiting, FzmM exhibits a very fast k_red (50 ± 0.01 s^-1 at 4 °C) with NADPH. NADPH binds at a K_D value of ∼400 μM, and hydride transfer occurs with pro-R stereochemistry. Oxidation of FzmM in the absence of Asp exhibits a spectrum with a shoulder at ∼370 nm, consistent with the formation of a C(4a)-hydroperoxyflavin intermediate, which decays into oxidized flavin and hydrogen peroxide at a rate 100-fold slower than the k_cat. This reaction is enhanced in the presence of Asp with a slightly faster k_ox than the k_cat, suggesting that flavin dehydration or Asp oxidation is partially rate limiting. Multiple sequence analyses of FzmM to NMOs identified conserved residues involved in flavin binding but not for NADPH. Additional sequence analysis to related monooxygenases suggests that FzmM shares sequence motifs absent in other NMOs.

Asymmetric Sulfoxidation of Thioether Catalyzed by Soybean Pod Shell Peroxidase to Form Enantiopure Sulfoxide in Water-in-Oil Microemulsions: A Kinetic Model

Esomeprazole with chiral sulfoxides structure is used to treat gastric ulcer disease. Soybean pod shell peroxidase (SPSP) is a peroxidase extracted from soybean pods shells which are one of the most abundant natural resources in the world. In the production of chiral sulfoxides catalyzed by SPSP, it is very important to establish the reaction kinetic model and explore the reaction mechanism for the development of the process, however, there is no report on the establishment of the model. Asymmetric sulfoxidation reactions catalyzed by SPSP in water-in-oil microemulsions were carried out, and the King-Altman approach was used to establish a kinetic model. A yield of 91% and e.e. value of 96% for esomeprazole were obtained at the activity of SPSP of 3200 U ml-1 and 50 °C for 5 h. The mechanism with a two-electron reduction of SPSP-I is accompanied with a single-electron transfer to SPSP-I and nonenzymatic reactions, indicating that three concomitant sub-mechanisms contribute to the asymmetric oxidation involving five enzymatic and two nonenzymatic reactions, which can represent the asymmetric sulfoxidation of organic sulfides to form enantiopure sulfoxides. With 5.44% of the average relative deviation, a kinetic model fitting experimental data was developed. The enzymatic reactions may follow ping-pong mechanism with substrate inhibition of H2 O2 and product inhibition of esomeprazole, while nonenzymatic reactions follow a power law. Those results indicate that SPSP with a lower cost and higher thermal stability may be used as an effective substitute for horseradish peroxidase.

Environmental occurrence, toxicity concerns, and remediation of recalcitrant nitroaromatic compounds

Nitroaromatic compounds (NACs) are considered important groups of chemicals mainly produced by human and industrial activities. The large-scale application of these xenobiotics creates contamination of the water and soil environment. Despite applicability, NACs have been caused severe hazardous side effects in animals and human systems like different cancers, anemia, skin irritation, liver damage and mutagenic effects. The effective remediation of the NACs from the environment is a significant concern. Researchers have implemented physicochemical and biological methods for the remediation of NACs from the environment. Most of the applied methods are based on adsorption and degradation approaches. Among these methods, degradation is considered a versatile method for the subsequent removal of NACs due to its exceptional properties like simplicity, easy operation, cost-effectiveness, and availability. Most importantly, the degradation process does not generate hazardous side products and wastes compared to other methods. Hence, the importance of NACs, their remediation, and supreme attributes of the degradation method have encouraged us to review the recent progress and development for the removal of these perilous materials using degradation as a versatile method. Therefore, in this review, (i) NACs, physicochemical properties, and their hazardous side effects on humans and animals are discussed; (ii) Physicochemical methods, microbial, anaerobic bioremediation, mycoremediation, and aerobic degradation approaches for the degradation of NACs were thoroughly vetted; (iii) The possible mechanisms for degradation of NACs were investigated and discussed. (iv) The applied kinetic models for evaluation of the rate of degradation were also assessed and discussed. Finally, (vi) current challenges and future prospects of proposed methods for degradation and removal of NACs were also directed.

Efficacy of Gastric Electrical Stimulation for Gastroparesis: US/European Comparison

BACKGROUND

Gastric electrical stimulation (GES) is used in both the US and Europe, but little research has investigated the demographics of gastroparesis patients receiving GES by geographic location.

METHODS

We compared data from 380 patients, 296 female and 84 males, mean age 42 years, 246 idiopathic (ID), 107 diabetic (DM), and 27 post-surgical (PS). The statistical significance was calculated by Chi-square test and a P-value obtained for ID, DM, and PS. The statistical significance was calculated by Fischer exact test and a P-value obtained comparing male vs. female.

RESULTS

European centers had 61 GES patients compared to 319 from the US. In Europe, 100% of patients had gastric emptying test (GET) values available; in the US, it was 75% of patients. European centers had more DM patients (59%) than the US (22%), and a smaller proportion of ID patients (25%) than the US (72%). There was a statistical difference between the causes of gastroparesis in the patients receiving GES (P-value < 0.00001). There was also significant difference in the gender of the patients receiving GES, with a greater proportion of women in the US (P value = 0.0023).

CONCLUSIONS

Comparing GES in US vs. Europe demonstrated significant differences in gastroparesis demographics and percentage of patients with GET data. After analyzing the previously discussed results and reviewing recent updates in evidence-based medicine guidelines, the discrepancy and variance in patient populations in the US and Europe emphasizes the need for a database that allows better analysis and treatment of gastroparesis patients worldwide including stimulation therapies.

New chemistry from natural product biosynthesis

Catalysts are a vital part of synthetic chemistry. However, there are still many important reactions for which catalysts have not been developed. The use of enzymes as biocatalysts for synthetic chemistry is growing in importance due to the drive towards sustainable methods for producing both bulk chemicals and high value compounds such as pharmaceuticals, and due to the ability of enzymes to catalyse chemical reactions with excellent stereoselectivity and regioselectivity. Such challenging transformations are a common feature of natural product biosynthetic pathways. In this mini-review, we discuss the potential to use biosynthetic pathways as a starting point for biocatalyst discovery. We introduce the reader to natural product assembly and tailoring, then focus on four classes of enzyme that catalyse C─H bond activation reactions to functionalize biosynthetic precursors. Finally, we briefly discuss the challenges involved in novel enzyme discovery.

Copper–cobalt synergy in Cu1−xCoxFe2O4spinel ferrite as a highly efficient and regioselective nanocatalyst for the synthesis of 2,4-dinitrotoluene

Highly regioselective dinitration of toluene with nitric acid as nitrating agent in the presence of Cu_1−xCo_xFe₂O₄(0 ≤x≤ 1) as nanocatalysts is described.