Mechanism of reaction of hydrogen peroxide with horseradish peroxidase: identification of intermediates in the catalytic cycle

Reaction Discovery Using Spectroscopic Insights from an Enzymatic C-H Amination Intermediate

Engineered hemoproteins can selectively incorporate nitrogen from nitrene precursors like hydroxylamine, O-substituted hydroxylamines, and organic azides into organic molecules. Although iron-nitrenoids are often invoked as the reactive intermediates in these reactions, their innate reactivity and transient nature have made their characterization challenging. Here we characterize an iron-nitrosyl intermediate generated from NH2OH within a protoglobin active site that can undergo nitrogen-group transfer catalysis, using UV-vis, electron paramagnetic resonance (EPR) spectroscopy, and high-resolution electrospray ionization mass spectrometry (HR-ESI-MS) techniques. The mechanistic insights gained led to the discovery of aminating reagents─nitrite (NO2-), nitric oxide (NO), and nitroxyl (HNO)─that are new to both nature and synthetic chemistry. Based on the findings, we propose a catalytic cycle for C-H amination inspired by the nitrite reductase pathway. This study highlights the potential of engineered hemoproteins to access natural nitrogen sources for sustainable chemical synthesis and offers a new perspective on the use of biological nitrogen cycle intermediates in biocatalysis.

Capturing Enzyme-Loaded Diblock Copolymer Vesicles Using an Aldehyde-Functionalized Hydrophilic Polymer Brush

Compared to lipids, block copolymer vesicles are potentially robust nanocontainers for enzymes owing to their enhanced chemical stability, particularly in challenging environments. Herein we report that cis-diol-functional diblock copolymer vesicles can be chemically adsorbed onto a hydrophilic aldehyde-functional polymer brush via acetal bond formation under mild conditions (pH 5.5, 20 °C). Quartz crystal microbalance studies indicated an adsorbed amount, Γ, of 158 mg m-2 for vesicle adsorption onto such brushes, whereas negligible adsorption (Γ = 0.1 mg m-2) was observed for a control experiment conducted using a cis-diol-functionalized brush. Scanning electron microscopy and ellipsometry studies indicated a mean surface coverage of around 30% at the brush surface, which suggests reasonably efficient chemical adsorption. Importantly, such vesicles can be conveniently loaded with a model enzyme (horseradish peroxidase, HRP) using an aqueous polymerization-induced self-assembly formulation. Moreover, the immobilized vesicles remained permeable toward small molecules while retaining their enzyme payload. The enzymatic activity of such HRP-loaded vesicles was demonstrated using a well-established colorimetric assay. In principle, this efficient vesicle-on-brush strategy can be applied to a wide range of enzymes and functional proteins for the design of next-generation immobilized nanoreactors for enzyme-mediated catalysis.

Immobilized enzymatic membrane surfaces for biocatalytic organics removal and fouling resistance

This research reported on the immobilization of environmentally friendly enzymes, such as horseradish peroxidase (HRP) and laccase (L), along with the hydrophilic zwitterionic compound l-DOPA on nano-filtration (NF) membranes. This approach introduced biocatalytic membranes, leveraging combined effects between membranes and enzymes. The aim was to systematically assess the efficacy of the enzymatic modified membrane (HRP-NF) in degrading colors in the wastewater, as well as enhancing the membrane resistance toward organic fouling. The enzymatic immobilized membrane demonstrated 96.3 ± 1.8% to 96.6 ± 1.9% removal of colors, and 65.2 ± 1.3% to 67.2 ± 1.3% removal of TOC. This result was underpinned by the insights obtained from the radical scavenger coumarin, which was employed to trap and confirm the formation of PRs through the reaction of enzymes and H2O2. Furthermore, membranes modified with enzymes exhibited significantly improved antifouling properties. The HRP-NF membrane experienced an 8% decline in flux, while the co-immobilized HRP-L-NF membrane demonstrated as low as 6% flux decline, contributed by the synergistic effect of increased hydrophilicity and biocatalytic effects. These findings confirmed that the immobilized enzymatic surface has added function of degrading contaminants in addition to separation function of nanofiltration membrane. These l-DOPA-immobilized enzymatic membranes offered a promising hybrid biocatalytic membrane to eliminate dyes and mitigate membrane fouling, which can be applied in many industrial and domestic water and wastewater treatment.

Nanolevel of detection of ascorbic acid using horse-radish peroxidase inhibition assay

Ascorbic acid plays a significant role in regulation of various bodily functions with high concentrations in immune cells and being involved in connective tissue maintenance. Commonly it is detected through various colorimetric methods. In this study, we propose a one-step simple method based on the inhibitory activity of ascorbic acid on horseradish peroxidase and hydrogen peroxide. The detection is observed by colorimetric changes to TMB (3,3',5,5' tetramethylbenzidine). The enzyme inhibition unit was optimized with a high level of linearity (r2 = 0.9999) and the level of detection and level of quantification were found to be 1.35 nM and 4.08 nM, respectively with higher sensitive compared to the HPLC method (11 μM). Both intra and inter-assays showed high correlations at different AA concentrations. (r2 > 0.9999). Similar results were also observed for vitamin C tablets, ascorbate salts, fruits, and market products (r2 = 0.999). There was negligible effect of interference by citric acid, lactic acid, tartaric acids, and glucose with high recoveries (>98%) at 1 mg/mL to 0.0078 mg/mL concentration ranges. The recovery error (RE%) was found to be less than 10%. Our detection method is distinguished by its simplicity, nano-level of detection, reproducibility, and potential application and adaptability as a point-of-use test.

Unveiling the role of inorganic nanoparticles in Earth's biochemical evolution through electron transfer dynamics

This article explores the intricate interplay between inorganic nanoparticles and Earth's biochemical history, with a focus on their electron transfer properties. It reveals how iron oxide and sulfide nanoparticles, as examples of inorganic nanoparticles, exhibit oxidoreductase activity similar to proteins. Termed "life fossil oxidoreductases," these inorganic enzymes influence redox reactions, detoxification processes, and nutrient cycling in early Earth environments. By emphasizing the structural configuration of nanoparticles and their electron conformation, including oxygen defects and metal vacancies, especially electron hopping, the article provides a foundation for understanding inorganic enzyme mechanisms. This approach, rooted in physics, underscores that life's origin and evolution are governed by electron transfer principles within the framework of chemical equilibrium. Today, these nanoparticles serve as vital biocatalysts in natural ecosystems, participating in critical reactions for ecosystem health. The research highlights their enduring impact on Earth's history, shaping ecosystems and interacting with protein metal centers through shared electron transfer dynamics, offering insights into early life processes and adaptations.

Impact of an Ionic Liquid Solution on Horseradish Peroxidase Activity

Horseradish peroxidase (HRP) is an enzyme that oxidizes pollutants from wastewater. A previous report indicated that peroxidases can have an enhancement in initial enzymatic activity in an aqueous solution of 0.26 M 1-ethyl-3-methylimidazolium ethyl sulfate ([EMIm][EtSO4]) at neutral pH. However, the atomistic details remain elusive. In the enzymatic landscape of HRP, compound II (Cpd II) plays a key role and involves a histidine (H42) residue. Cpd II exists as oxoferryl (2a) or hydroxoferryl (2b(FeIV)) forms, where 2a is the predominantly observed form in experimental studies. Intriguingly, the ferric 2b(FeIII) form seen in synthetic complexes has not been observed in HRP. Here, we have investigated the structure and dynamics of HRP in pure water and aqueous [EMIm][EtSO4] (0.26 M), as well as the reaction mechanism of 2a to 2b conversion using polarizable molecular dynamics (MD) simulations and quantum mechanics/molecular mechanics (QM/MM) calculations. When HRP is solvated in aq [EMIm][EtSO4], the catalytic water displaces, and H42 directly orients over the ferryl moiety, allowing a direct proton transfer (PT) with a significant energy barrier reduction. Conversely, in neat water, the reaction of 2a to 2b follows the previously reported mechanism. We further investigated the deprotonated form of H42. Analysis of the electric fields at the active site indicates that the aq [EMIm][EtSO4] medium facilitates the reaction by providing a more favorable environment compared with the system solvated in neat water. Overall, the atomic level supports the previous experimental observations and underscores the importance of favorable electric fields in the active site to promote catalysis.

Multifunctional 2D hemin-bridged MOF for the efficient removal and dual-mode detection of organophosphorus pesticides

The high-residual and bioaccumulation property of organophosphorus pesticides (OPs) creates enormous risks towards the ecological environment and human health, promoting the research for smart adsorbents and detection methods. Herein, 2D hemin-bridged MOF nanozyme (2D-ZHM) was fabricated and applied to the efficient removal and ultrasensitive dual-mode aptasensing of OPs. On the one hand, the prepared 2D-ZHM contained Zr-OH groups with high affinity for phosphate groups, endowing it with selective recognition and high adsorption capacity for OPs (285.7 mg g-1 for glyphosate). On the other hand, the enhanced peroxidase-mimicking biocatalytic property of 2D-ZHM allowed rapid H2O2-directed transformation of 3,3',5,5'-tetramethylbenzidine to oxidic product, producing detectable colorimetric or photothermal signals. Using aptamers of specific recognition capacity, the rapid quantification of two typical OPs, glyphosate and omethoate, was realized with remarkable sensitivity and selectivity. The limit of detections (LODs) of glyphosate were 0.004 nM and 0.02 nM for colorimetric and photothermal methods, respectively, and the LODs of omethoate were 0.005 nM and 0.04 nM for colorimetric and photothermal methods, respectively. The constructed dual-mode aptasensing platform exhibited outstanding performance for monitoring OPs in water and fruit samples. This work provides a novel pathway to develop MOF-based artificial peroxidase and integrated platform for pollutant removal and multi-mode aptasensing.

Inorganic Fe-O and Fe-S oxidoreductases: paradigms for prebiotic chemistry and the evolution of enzymatic activity in biology

Oxidoreductases play crucial roles in electron transfer during biological redox reactions. These reactions are not exclusive to protein-based biocatalysts; nano-size (<100 nm), fine-grained inorganic colloids, such as iron oxides and sulfides, also participate. These nanocolloids exhibit intrinsic redox activity and possess direct electron transfer capacities comparable to their biological counterparts. The unique metal ion architecture of these nanocolloids, including electron configurations, coordination environment, electron conductivity, and the ability to promote spontaneous electron hopping, contributes to their transfer capabilities. Nano-size inorganic colloids are believed to be among the earliest 'oxidoreductases' to have 'evolved' on early Earth, playing critical roles in biological systems. Representing a distinct type of biocatalysts alongside metalloproteins, these nanoparticles offer an early alternative to protein-based oxidoreductase activity. While the roles of inorganic nano-sized catalysts in current Earth ecosystems are intuitively significant, they remain poorly understood and underestimated. Their contribution to chemical reactions and biogeochemical cycles likely helped shape and maintain the balance of our planet's ecosystems. However, their potential applications in biomedical, agricultural, and environmental protection sectors have not been fully explored or exploited. This review examines the structure, properties, and mechanisms of such catalysts from a material's evolutionary standpoint, aiming to raise awareness of their potential to provide innovative solutions to some of Earth's sustainability challenges.

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.

Covalent Attachment of Horseradish Peroxidase to Single-Walled Carbon Nanotubes for Hydrogen Peroxide Detection

Single-walled carbon nanotubes (SWCNTs) are desirable nanoparticles for sensing biological analytes due to their photostability and intrinsic near-infrared fluorescence. Previous strategies for generating SWCNT nanosensors have leveraged nonspecific adsorption of sensing modalities to the hydrophobic SWCNT surface that often require engineering new molecular recognition elements. An attractive alternate strategy is to leverage pre-existing molecular recognition of proteins for analyte specificity, yet attaching proteins to SWCNT for nanosensor generation remains challenging. Towards this end, we introduce a generalizable platform to generate protein-SWCNT-based optical sensors and use this strategy to synthesize a hydrogen peroxide (H 2 O 2 ) nanosensor by covalently attaching horseradish peroxidase (HRP) to the SWCNT surface. We demonstrate a concentration-dependent response to H 2 O 2 , confirm the nanosensor can image H 2 O 2 in real-time, and assess the nanosensor's selectivity for H 2 O 2 against a panel of biologically relevant analytes. Taken together, these results demonstrate successful covalent attachment of enzymes to SWCNTs while preserving both intrinsic SWCNT fluorescence and enzyme function. We anticipate this platform can be adapted to covalently attach other proteins of interest including other enzymes for sensing or antibodies for targeted imaging and cargo delivery.

Bridging the Catalytic Turnover Gap Between Single-Atom Iron Nanozymes and Natural Enzymes by Engineering the First and Second Shell Coordination

Single-atom nanozymes (SAzymes) constitute a promising category of enzyme-mimicking materials with outstanding catalytic performance. The performance of SAzymes improves through modification of the coordination environments around the metal center. However, the catalytic turnover rates of SAzymes, which are key measures of the effectiveness of active site modifications, remain lower than those of natural enzymes, especially in peroxidase-reactions. Here, the first and second shell coordination tuning strategy that yields SAzymes with structures and activities analogous to those of natural enzymes is reported. The optimized SAzyme exhibits a turnover rate of 52.7 s-1 and a catalytic efficiency of 6.97 × 105 M-1 s-1 . A computational study reveals that axial S-ligands induce an alternative reaction mechanism, and SO2 - functional groups provide hydrogen bonds to reduce the activation energy. In addition, SAzyme shows superior anti-tumor ability in vitro and in vivo. These results demonstrate the validity of coordination engineering strategies and the carcinostatic potential of SAzymes.

Assembly and Healing: Capacitive and Conductive Plasmonic Interfacing via a Unified and Clean Wet Chemistry Route

Solution-based nanoparticle assembly represents a highly promising way to build functional metastructures based on a wealth of synthetic nanomaterial building blocks with well-controlled morphology and crystallinity. In particular, the involvement of DNA molecular programming in these bottom-up processes gradually helps the ambitious goal of customizable chemical nanofabrication. However, a fundamental challenge is to realize strong interunit coupling in an assembly toward emerging functions and applications. Herein, we present a unified and clean strategy to address this critical issue based on a H2O2-redox-driven "assembly and healing" process. This facile solution route is able to realize both capacitively coupled and conductively bridged colloidal boundaries, simply switchable by the reaction temperature, toward bottom-up nanoplasmonic engineering. In particular, such a "green" process does not cause surface contamination of nanoparticles by exogenous active metal ions or strongly passivating ligands, which, if it occurs, could obscure the intrinsic properties of as-formed structures. Accordingly, previously raised questions regarding the activities of strongly coupled plasmonic structures are clarified. The reported process is adaptable to DNA nanotechnology, offering molecular programmability of interparticle charge conductance. This work represents a new generation of methods to make strongly coupled nanoassemblies, offering great opportunities for functional colloidal technology and even metal self-healing.

Dopamine as a polymerizable reagent for enzyme-linked immunosorbent assay using horseradish peroxidase

We demonstrate that dopamine can be used as a reagent for colorimetric enzyme-linked immunosorbent assay (ELISA) using horseradish peroxidase (HRP). Dopamine was able to be polymerized in the presence of HRP and H2O2, and black polydopamine was obtained after the enzymatic reaction. Because of the black color, the absorbance was significantly changed in the whole range of the visible light region. Here, an indirect competitive ELISA based on the polymerization of dopamine was performed to detect a fluoroquinolone antibiotic, enrofloxacin. The antibiotic is commonly used in livestock farming. The anti-antibiotics antibody was produced from egg yolk from chicken hens. In the visible range, sufficient absorbance changes of ∼0.4∼0.5 and a low background level for the ELISA response were obtained, and the 50 % inhibitory concentration value at 450 nm was determined to be 26 ppb. The performance of the indirect competitive ELISA based on the polymerization of dopamine was compared to that based on the oxidation of catechol because dopamine has a catechol skeleton. By the complex of HRP and H2O2, catechol can be oxidized to o-benzoquinone having a maximum absorption wavelength of 420 nm. It was shown that the absorbance change in the case of polydopamine was about 2.5 times higher than that of catechol, where the background levels were similar. This confirms that the polymerization of dopamine significantly enhanced the photosignal.

Stabilized and Controlled Release of Radicals within Copper Formate-Based Nanozymes for Biosensing

Fenton-like radical processes are widely utilized to explain catalytic mechanisms of peroxidase-like nanozymes, which exhibit remarkable catalytic activity, cost-effectiveness, and stability. However, there is still a need for a comprehensive understanding of the formation, stabilization, and transformation of such radicals. Herein, a copper formate-based nanozyme (Cuf-TMB) was fabricated via a pre-catalytic strategy under ambient conditions. The as-prepared nanozyme shows comparable catalytic activity (Km, 1.02 × 10-5 mM-1; Kcat, 3.09 × 10-2 s-1) and kinetics to those of natural peroxidase toward H2O2 decomposition. This is attributed to the feasible oxidation by *OH species via the *O intermediate, as indicated by density functional theory calculations. The key ·OH radicals were detected to be stable for over 52 days and can be released in a controlled manner during the catalytic process via in situ electron spin-resonance spectroscopy measurements. Based on the understanding, an ultrasensitive biosensing platform was constructed for the sensitive monitoring of biochemical indicators in clinic settings.

Fluorescent Oligomeric Nanoparticle from Diaminopyridine Compound Via Enzyme-Catalyzed Oxidation

An oligomeric nanoparticle was obtained through an enzyme-catalyzed oxidation reaction using Horse Radish Peroxidase (HRP) with the 2,3-diaminopyridine (DAP) compound as the starting material. The structural characterizations of the synthesized oligomeric nanoparticles [[oligo(DAP)]Enz] were performed with 1H-NMR and FT-IR. The surface features were determined by scanning electron microscopy. The optical properties were determined by UV-Vis and fluorescence spectra. It was found that the obtained oligomeric nanoparticles had a different fluorescent character with 15.90% quantum yield from the monomer and emitted green color at 485 nm when excited with light at a wavelength of 420 nm. The electrochemical band gap of the oligomeric nanoparticles, whose electrochemical character was investigated by cyclic voltammetry, was calculated as 2.09 eV. The thermal stability of the oligomeric material was determined from the mass loss against increasing temperature. The monomer exhibited greater thermal stability in comparison to the polymer, both in terms of the temperature at which the maximum mass loss occurred and the residual amount after heating concluded.

A mutant R70V/E166A of short manganese peroxidase showing Mn2+-independent dye decolorization

Il-MnP1, a short-type manganese peroxidase from Irpex lacteus F17, can oxidize some substrates in the absence of Mn²⁺, but the catalysis was much lower than in the presence of Mn²⁺. Here, we report a mutant R70V/E166A of Il-MnP1 with some unique properties, which possessed clearly higher catalysis for the decolorization of anthraquinone and azo dyes in the absence of Mn²⁺ than that of Il-MnP1. Importantly, the optimum pH of R70V/E166A for decolorization of anthraquinone dyes (Reactive Blue 19, RB19) was 6.5, and the mutant achieved high decolorization activities in the range of pH 4.0-7.0, whereas Il-MnP1 only showed decolorization for RB19 at pH 3.5-4.0. In addition, the optimum H₂O₂ concentration of R70V/E166A for RB19 decolorization was eight times that of Il-MnP1 and the H₂O₂ stability has improved 1.4 times compared with Il-MnP1. Furthermore, Mn²⁺ competitively inhibited the oxidation of RB19 by R70V/E166A, explaining the higher catalytic activity of the mutant R70V/E166A in the absence of Mn²⁺. Molecular docking results suggested that RB19 binds to the distal side of the heme plane in mutant R70V/E166A, which extended from the heme δ-side to the heme γ-side, and close to the mutated residues of R70V and E166A, whereas RB19 could not access the heme pocket of Il-MnP1 due to the steric hindrance of the side-chain group of Arg 70. Thus, this study constructed a useful mutant R70V/E166A and analyzed its higher Mn²⁺-independent activity, which is very important for better understanding the Mn²⁺-independent catalytic mechanism for short manganese peroxidases. KEY POINTS: • The mutant R70V/E166A of atypical MnP1 of I. lacteus F17 shows unique catalytic properties. • At pH 6.5, the R70V/E166A had a strong ability to decolorize anthraquinone dyes in the absence of Mn²⁺. • The binding sites of Reactive Blue 19 in mutant R70V/E166A were elucidated.

Ferric heme b in aqueous micellar and vesicular systems: state-of-the-art and challenges

Ferric heme b (= ferric protoporphyrin IX = hemin) is an important prosthetic group of different types of enzymes, including the intensively investigated and widely applied horseradish peroxidase (HRP). In HRP, hemin is present in monomeric form in a hydrophobic pocket containing among other amino acid side chains the two imidazoyl groups of His170 and His42. Both amino acids are important for the peroxidase activity of HRP as an axial ligand of hemin (proximal His170) and as an acid/base catalyst (distal His42). A key feature of the peroxidase mechanism of HRP is the initial formation of compound I under heterolytic cleavage of added hydrogen peroxide as a terminal oxidant. Investigations of free hemin dispersed in aqueous solution showed that different types of hemin dimers can form, depending on the experimental conditions, possibly resulting in hemin crystallization. Although it has been recognized already in the 1970s that hemin aggregation can be prevented in aqueous solution by using micelle-forming amphiphiles, it remains a challenge to prepare hemin-containing micellar and vesicular systems with peroxidase-like activities. Such systems are of interest as cheap HRP-mimicking catalysts for analytical and synthetic applications. Some of the key concepts on which research in this fascinating and interdisciplinary field is based are summarized, along with major accomplishments and possible directions for further improvement. A systematic analysis of the physico-chemical properties of hemin in aqueous micellar solutions and vesicular dispersions must be combined with a reliable evaluation of its catalytic activity. Future studies should show how well the molecular complexity around hemin in HRP can be mimicked by using micelles or vesicles. Because of the importance of heme b in virtually all biological systems and the fact that porphyrins and hemes can be obtained under potentially prebiotic conditions, ideas exist about the possible role of heme-containing micellar and vesicular systems in prebiotic times.

Nanozyme-encoded luminescent detection for food safety analysis: An overview of mechanisms and recent applications

With the rapid growth in global food production, delivery, and consumption, reformative food analytical techniques are required to satisfy the monitoring requirements of speed and high sensitivity. Nanozyme-encoded luminescent detections (NLDs) integrating nanozyme-based rapid detections with luminescent output signals have emerged as powerful methods for food safety monitoring, not only because of their preeminent performance in analysis, such as rapid, facile, low background signal, and ultrasensitive, but also due to their strong attractiveness for future sensing research. However, the lack of a full understanding of the fundamentals of NLDs for food safety detection technologies limits their further application. In this review, a systematic overview of the mechanisms of NLDs and their applications in the food industry is summarized, which covers the nanozyme-mimicking types and their luminescent signal generation mechanisms, as well as their applications in monitoring common foodborne contaminants. As demonstrated by previous studies, NLDs are bridging the gap to practical-oriented food analytical technologies and various opportunities to improve their food analytical performance to be considered in the future are proposed.

Switchable Enzyme-mimicking catalysts Self-Assembled from de novo designed peptides and DNA G-quadruplex/hemin complex

Reconstruction of enzymatic active site in an artificial system is key to achieving high catalytic efficiency. Herein, we report the self-assembly of the lysine-containing peptides with guanine-rich DNA and hemin to form peroxidase-mimicking active sites and catalytic nanoparticles. The DNA strand self-folds into a G-quadruplex structure that provides a supramolecular scaffold and a potential axial ligand for hemin. The β-sheet forming capability of the lysine-containing peptides is found to affect the catalytic synergy between the G-quadruplex DNA and the peptide. It is hypothesized that the β-sheet formation of the peptides results in the enrichment of the lysine residues, which distribute on the distal side of hemin to promote the formation of Compound I, like distal arginine residue in natural heme pocket. Incorporation of the histidine residues into the lysine-containing peptides further enhanced the hemin activities, indicating the cooperation between the lysine and histidine. Furthermore, the peptide/DNA/hemin complexes can be switched between active and inactive state by reversible formation and deformation of the DNA G-quadruplex, which was attributed to the peptides-promoted conformational changes of the DNA components. This work opens an avenue to mimic the catalytic residues and their spatial distribution in the natural enzymes, and shed light on the design of the smart biocatalysts that can respond to the environmental stimuli.

Enzyme-Mimicking Materials from Designed Self-Assembly of Lysine-Rich Peptides and G-Quadruplex DNA/Hemin DNAzyme: Charge Effect of the Key Residues on the Catalytic Functions

In enzymatic active sites, the essential functional groups are spatially arranged as a result of the enzyme three-dimensional folding, which leads to remarkable catalytic properties. We are inspired to self-assemble the polylysine peptides with guanine-rich DNA and hemin as cofactor to fabricate the peroxidase-mimicking catalytic nanomaterials. The DNA can fold into G-quadruplex to provide a supramolecular scaffold and a nucleobase for supporting and coordinating hemin, and the polylysine provides amine as distal groups to promote the H₂O₂ adsorption to the iron of hemin. The polylysine and DNA components synergistically accelerated the hemin-catalyzed reactions, and the complex containing ε-polylysine exhibited higher activity than α-polylysine. This activity difference is attributed to the higher pK_a value and more susceptible protonation of amine of ε-polylysine than α-polylysine. The ε-polylysine/DNA/hemin had similar coordination states of hemin and conformations of the components to α-polylysine/DNA/hemin but accelerated the formation of the intermediate compound I faster than α-polylysine. Theoretical simulation reveals that the unprotonated NH₂ behaved like a base catalyst, similar to His-42 residue in the natural heme pocket, while the protonated NH₃⁺ acted as an acid, which indicated that the base catalyst on the distal side of the hemin pocket is more active than the acid. This work provides an avenue to control the distribution of the catalytic residues in an enzyme-like active site and to understand the roles of the key residues of native enzymes.

Construction of Pd Single Site Anchored on Nitrogen-Doped Porous Carbon and Its Application for Total Antioxidant Level Detection

Natural enzymes have excellent catalytic activity. However, due to their unstable nature and high cost, current research has turned to the synthesis and development of enzyme-like nanomaterials and single-atomic nanozymes. In this study, a single-atomic palladium-loaded nitrogen-doped porous carbon catalyst (SA-Pd/NPC) was prepared and used as a mimetic peroxidase to catalyze the substrates oxidation. The catalytic capability of the SA-Pd/NPC was tested by the TMB-H₂O₂ system, and it expressed a superior catalytic capability owing to the plentiful catalytic centers of the single-atom Pd, its high porosity, the large specific surface area, and the strong electron transfer capability of the NPC. For the color reaction of TMB, thiol antioxidants (e.g., glutathione, GSH) and non-thiol antioxidants (e.g., ascorbic acid, AA) are suitable for different inhibition mechanisms. GSH and AA are typical substances of these two main antioxidant types, respectively. Here, we demonstrate that this prepared catalyst could be used to simultaneously determine a variety of major known physiologically relevant thiol-containing and thiol-free antioxidants, accompanied by a blue color gradient change with UV-Vis spectra at 652 nm through the SA-Pd/NPC-catalyzed TMB-H₂O₂ system. Linear responses to GSH and AA could be obtained in the concentration ranges of 0.01-0.10 mM and 1-13 μM (both R² values were greater than 0.970), respectively, while the limits of detection were 3 μM and 0.3 μM, respectively. The ability of the nanozyme to detect overall antioxidant levels (TAL) was also confirmed in subsequent tests on artificial saliva and biological samples.

Insertion of Hemin into Metal-Organic Frameworks: Mimicking Natural Peroxidase Microenvironment for the Rapid Ultrasensitive Detection of Uranium

Constructing enzyme-like active sites in mimic enzyme systems is critical for achieving catalytic performances comparable to natural enzymes and can shed light on the natural development of enzymes. In this study, we described a specific hemin-based mimetic enzyme, which was facilely synthesized by the assembly of zeolitic imidazolate framework-l (ZIF-l) and hemin. The obtained hemin-based mimetic enzyme (denoted as ZIF-l-hemin) displayed enhanced peroxidase activity compared to free hemin in solution. Such excellent activity originated from the ZIF-l framework mimicking the active site cavity microenvironment of horseradish peroxidase in terms of axially coordinated histidine and distal histidine. Additionally, the constructed peroxidase mimetic was extremely resistant to a variety of severe circumstances that would normally denature natural enzymes. These characteristics made ZIF-l-hemin a potential platform for the colorimetric sensor of uranium (UO₂²⁺) with wide linear ranges (0.25-40 μM) and low limits of detection (0.079 μM). Moreover, the detection mechanism demonstrated that the coordination of uranyl ion with imidazole of ZIF-l-hemin reduced the catalytic efficiency of ZIF-l-hemin. The current work not only proposed a novel approach for fabricating artificial peroxidase but also offered facile colorimetric methods for selective radionuclide detection.

Structural Characterization of Two Short Unspecific Peroxygenases: Two Different Dimeric Arrangements

Unspecific peroxygenases (UPOs) are extracellular fungal enzymes of biotechnological interest as self-sufficient (and more stable) counterparts of cytochrome P450 monooxygenases, the latter being present in most living cells. Expression hosts and structural information are crucial for exploiting UPO diversity (over eight thousand UPO-type genes were identified in sequenced genomes) in target reactions of industrial interest. However, while many thousands of entries in the Protein Data Bank include molecular coordinates of P450 enzymes, only 19 entries correspond to UPO enzymes, and UPO structures from only two species (Agrocybe aegerita and Hypoxylon sp.) have been published to date. In the present study, two UPOs from the basidiomycete Marasmius rotula (rMroUPO) and the ascomycete Collariella virescens (rCviUPO) were crystallized after sequence optimization and Escherichia coli expression as active soluble enzymes. Crystals of rMroUPO and rCviUPO were obtained at sufficiently high resolution (1.45 and 1.95 Å, respectively) and the corresponding structures were solved by molecular replacement. The crystal structures of the two enzymes (and two mutated variants) showed dimeric proteins. Complementary biophysical and molecular biology studies unveiled the diverse structural bases of the dimeric nature of the two enzymes. Intermolecular disulfide bridge and parallel association between two α-helices, among other interactions, were identified at the dimer interfaces. Interestingly, one of the rCviUPO variants incorporated the ability to produce fatty acid diepoxides—reactive compounds with valuable cross-linking capabilities—due to removal of the enzyme C-terminal tail located near the entrance of the heme access channel. In conclusion, different dimeric arrangements could be described in (short) UPO crystal structures.

Surface Functionalization of Enzyme-Coronated Gold Nanoparticles with an Erythrocyte Membrane for Highly Selective Glucose Assays

Colorimetric glucose sensors using enzyme-coronated gold nanoparticles have been developed for high-throughput assays to monitor the blood glucose levels of diabetic patients. Although those sensors have shown sensitivity and wide linear detection ranges, they suffer from poor selectivity and stability in detecting blood glucose, which has limited their practical use. To address this limitation, herein, we functionalized glucose-oxidase-coronated gold nanoparticles with an erythrocyte membrane (EM-GOx-GNPs). Because the erythrocyte membrane (EM) selectively facilitates the permeation of glucose via glucose transporter-1 (GLUT1), the functionalization of GOx-GNPs with EM improved the stability, selectivity (3.3- to 15.8-fold higher), and limit of detection (LOD). Both membrane proteins, GLUT1 and aquaporin-1 (AQP1), on EM were shown to be key components for selective glucose detection by treatment with their inhibitors. Moreover, we demonstrated the stability of EM-GOx-GNPs in high-antioxidant-concentration conditions, under long-term storage (∼4 weeks) and a freeze-thaw cycle. Selectivity of the EM-GOx-GNPs against other saccharides was increased, which improved the LOD in phosphate-buffered saline and human serum. Our results indicated that the functionalization of colorimetric glucose sensors with EM is beneficial for improving selectivity and stability, which may make them candidates for use in a practical glucose sensor.

Electrochemically produced local pH changes stimulating (bio)molecule release from pH-switchable electrode-immobilized avidin-biotin systems

Immobilized avidin-biotin complexes were used to release biotinylated (bio)molecules upon producing local pH changes near an electrode surface by electrochemical reactions. The nitro-avidin complex with biotin was dissociated by increasing local pH with electrochemical O₂ reduction. The avidin complex with iminobiotin was split by decreasing local pH with electrochemical oxidation of ascorbate. Both studied systems were releasing molecule cargo species in response to small electrical potentials (-0.4 V or 0.2 V for the O₂ reduction or ascorbate oxidation, respectively) applied on the modified electrodes.

Charge-Modulated Accessibility of Tyrosine Residues for Silk-Elastin Copolymer Cross-Linking

The modulation of reaction kinetics with horseradish peroxidase (HRP)-catalyzed cross-linking of proteins remains a useful strategy to modulate hydrogel formation. Here, we demonstrate that the presence of positively charged lysines in silk-elastin-like polymers impacts the thermal transition temperature of these proteins, while the location in the primary sequence modulates the reactivity of the tyrosines. The positively charged lysine side chains decreased π-π interactions among the tyrosines and reduced the rate of formation and number of HRP-mediated dityrosine bonds, dependent on the proximity of the charged group to the tyrosine. The results suggest that the location of repulsive charges can be used to tailor the reaction kinetics for enzymatic cross-linking, providing further control of gelation rates for in situ gel formation and the resulting protein-based gel characteristics.

Recombinant Arthromyces ramosus Peroxidase Has Similar Substrate Specificity Profiles as, but a Catalytic Efficiency up to 11-Fold Higher than, Horseradish Peroxidase

Fungal peroxidases are valuable enzymes. Arthromyces ramosus peroxidase (ARP) and horseradish peroxidase (HRP) share a conserved catalytic site. Both native ARP and recombinant ARP (rARP) were not commercially available. The substrate specificity and kinetic parameters of rARP and HRP were not well compared, particularly relevent to structure-activity relationship. In this work, rARP expressed by Komagataella phaffii had a production yield of 6.2 mg/L, up to 155-fold higher than ARP and other recombinant peroxidases, and a specific activity of 3240 units/mg toward 2,2'-azino-bis(3-ethylbenzothiazoline-6-sulfonic acid) (ABTS), up to 29-fold higher than HRP and other peroxidases. The Michaelis constant (K_m) and first-order rate constant (k_cat) of rARP showed 10-fold substrate affinity and consequently 6-fold catalytic efficiency of HRP toward ABTS. Under optimal conditions, rARP shared similar substrate specificity profiles as commercial HRP; the second-order rate constants (k_app) of rARP showed 2-11-fold catalytic efficiency of HRP toward well-known peroxidase substrates. rARP's higher catalytic efficiency was also in agreement with the shorter binding distance of H/N-His⁵⁶ in rARP/substrate in comparison to that of HRP/substrate, as illustrated by docking simulation. The rARP had similar substrate specificity profiles as, but higher specific activity and catalytic efficiency than, HRP, which merits its further structure-functional characterization and applications.

Oxidative dehalogenation of trichlorophenol catalyzed by a promiscuous artificial heme-enzyme

The miniaturized metalloenzyme Fe(iii)-mimochrome VI*a (Fe(iii)-MC6*a) acts as an excellent biocatalyst in the H₂O₂-mediated oxidative dehalogenation of the well-known pesticide and biocide 2,4,6-trichlorophenol (TCP). The artificial enzyme can oxidize TCP with a catalytic efficiency (k_cat/K^TCP_m = 150 000 mM⁻¹ s⁻¹) up to 1500-fold higher than the most active natural metalloenzyme horseradish peroxidase (HRP). UV-visible and EPR spectroscopies were used to provide indications of the catalytic mechanism. One equivalent of H₂O₂ fully converts Fe(iii)-MC6*a into the oxoferryl-porphyrin radical cation intermediate [(Fe(iv) Created by potrace 1.16, written by Peter Selinger 2001-2019 ]]> O)por˙⁺], similarly to peroxidase compound I (Cpd I). Addition of TCP to Cpd I rapidly leads to the formation of the corresponding quinone, while Cpd I decays back to the ferric resting state in the absence of substrate. EPR data suggest a catalytic mechanism involving two consecutive one-electron reactions. All results highlight the value of the miniaturization strategy for the development of chemically stable, highly efficient artificial metalloenzymes as powerful catalysts for the oxidative degradation of toxic pollutants.

The artificial metalloenzyme FeMC6*a is able to perform the H₂O₂-mediated dechlorination of 2,4,6-trichlorophenol with unrivalled catalytic efficiency, highlighting its potential application for the removal of toxic pollutants.

Removal of Persistent Sulfamethoxazole and Carbamazepine from Water by Horseradish Peroxidase Encapsulated into Poly(Vinyl Chloride) Electrospun Fibers

Enzymatic conversion of pharmaceutically active ingredients (API), using immobilized enzymes should be considered as a promising industrial tool due to improved reusability and stability of the biocatalysts at harsh process conditions. Therefore, in this study horseradish peroxidase was immobilized into sodium alginate capsules and then trapped into poly(vinyl chloride) electrospun fibers to provide additional enzyme stabilization and protection against the negative effect of harsh process conditions. Due to encapsulation immobilization, 100% of immobilization yield was achieved leading to loading of 25 μg of enzyme in 1 mg of the support. Immobilized in such a way, enzyme showed over 80% activity retention. Further, only slight changes in kinetic parameters of free (K_m = 1.54 mM) and immobilized horseradish peroxidase (K_m = 1.83 mM) were noticed, indicating retention of high catalytic properties and high substrate affinity by encapsulated biocatalyst. Encapsulated horseradish peroxidase was tested in biodegradation of two frequently occurring in wastewater API, sulfamethoxazole (antibiotic) and carbamazepine (anticonvulsant). Over 80% of both pharmaceutics was removed by immobilized enzyme after 24 h of the process from the solution at a concentration of 1 mg/L, under optimal conditions, which were found to be pH 7, temperature 25 °C and 2 mM of H₂O₂. However, even from 10 mg/L solutions, it was possible to remove over 40% of both pharmaceuticals. Finally, the reusability and storage stability study of immobilized horseradish peroxidase showed retention of over 60% of initial activity after 20 days of storage at 4 °C and after 10 repeated catalytic cycles, indicating great practical application potential. By contrast, the free enzyme showed less than 20% of its initial activity after 20 days of storage and exhibited no recycling potential.

Recent advances in in situ oxygen-generating and oxygen-replenishing strategies for hypoxic-enhanced photodynamic therapy

Cancer is a leading cause of death worldwide, accounting for an estimated 10 million deaths by 2020. Over the decades, various strategies for tumor therapy have been developed and evaluated. Photodynamic therapy (PDT) has attracted increasing attention due to its unique characteristics, including low systemic toxicity and minimally invasive nature. Despite the excellent clinical promise of PDT, hypoxia is still the Achilles' heel associated with its oxygen-dependent nature related to increased tumor proliferation, angiogenesis, and distant metastases. Moreover, PDT-mediated oxygen consumption further exacerbates the hypoxia condition, which will eventually lead to the poor effect of drug treatment and resistance and irreversible tumor metastasis, even limiting its effective application in the treatment of hypoxic tumors. Hypoxia, with increased oxygen consumption, may occur in acute and chronic hypoxia conditions in developing tumors. Tumor cells farther away from the capillaries have much lower oxygen levels than cells in adjacent areas. However, it is difficult to change the tumor's deep hypoxia state through different ways to reduce the tumor tissue's oxygen consumption. Therefore, it will become more difficult to cure malignant tumors completely. In recent years, numerous investigations have focused on improving PDT therapy's efficacy by providing molecular oxygen directly or indirectly to tumor tissues. In this review, different molecular oxygen supplementation methods are summarized to alleviate tumor hypoxia from the innovative perspective of using supplemental oxygen. Besides, the existing problems, future prospects and potential challenges of this strategy are also discussed.

Fe-catalyzed sulfide oxidation in hydrothermal plumes is a source of reactive oxygen species to the ocean

Historically, the production of reactive oxygen species (ROS) in the ocean has been attributed to photochemical and biochemical reactions. However, hydrothermal vents emit globally significant inventories of reduced Fe and S species that should react rapidly with oxygen in bottom water and serve as a heretofore unmeasured source of ROS. Here, we show that the Fe-catalyzed oxidation of reduced sulfur species in hydrothermal vent plumes in the deep oceans supported the abiotic formation of ROS at concentrations 20 to 100 times higher than the average for photoproduced ROS in surface waters. ROS (measured as hydrogen peroxide) were determined in hydrothermal plumes and seeps during a series of Alvin dives at the North East Pacific Rise. Hydrogen peroxide inventories in emerging plumes were maintained at levels proportional to the oxygen introduced by mixing with bottom water. Fenton chemistry predicts the production of hydroxyl radical under plume conditions through the reaction of hydrogen peroxide with the abundant reduced Fe in hydrothermal plumes. A model of the hydroxyl radical fate under plume conditions supports the role of plume ROS in the alteration of refractory organic molecules in seawater. The ocean's volume circulates through hydrothermal plumes on timescales similar to the age of refractory dissolved organic carbon. Thus, plume-generated ROS can initiate reactions that may affect global ocean carbon inventories.

Biomimetic design of graphdiyne supported hemin for enhanced peroxidase-like activity

Effective electronic interactions between molecular catalysts and supports are critical for heterogeneous enzyme mimics, yet they are frequently neglected in most catalyst designs. Taking the enzyme mimics of hemin immobilized on graphdiyne (Hemin-GDY) as an example, we explicate for the first time the underlying role of GDY as a co-catalyst. Based on the robust conjugation between GDY and hemin, the delocalized π-electrons in GDY act as a ligand for Fe ions so that the orbital interactions including electron transport from GDY → Fe can induce the formation of an electron-rich Fe center and an electron-deficient π-electron conjugated system. This mechanism was validated by electron paramagnetic resonance (EPR), Raman spectroscopy, and DFT calculations. Moreover, both EPR spetra and Lineweaver-Burk plots revealed that Hemin-GDY could efficiently catalyze the decomposition of hydrogen peroxide (H₂O₂) to produce hydroxyl radical (•OH) and superoxide anion (O₂^•-) by a ping-pong type catalytic mechanism, and particularly, the catalytic activity was increased by 2.3-fold comparing to that of hemin immobilized on graphene (Hemin-GR). In addition, Hemin-GDY with the exceptional activity and stability was demonstrated for efficient catalytic degradation of organic pollutants under acidic conditions. Collectively, this work provides a theoretical basis for the design of GDY supported catalysts and renders great promises of the GDY based enzyme mimics.

Aspartate or arginine? Validated redox state X-ray structures elucidate mechanistic subtleties of FeIV = O formation in bacterial dye-decolorizing peroxidases

Structure determination of proteins and enzymes by X-ray crystallography remains the most widely used approach to complement functional and mechanistic studies. Capturing the structures of intact redox states in metalloenzymes is critical for assigning the chemistry carried out by the metal in the catalytic cycle. Unfortunately, X-rays interact with protein crystals to generate solvated photoelectrons that can reduce redox active metals and hence change the coordination geometry and the coupled protein structure. Approaches to mitigate such site-specific radiation damage continue to be developed, but nevertheless application of such approaches to metalloenzymes in combination with mechanistic studies are often overlooked. In this review, we summarize our recent structural and kinetic studies on a set of three heme peroxidases found in the bacterium Streptomyces lividans that each belong to the dye decolourizing peroxidase (DyP) superfamily. Kinetically, each of these DyPs has a distinct reactivity with hydrogen peroxide. Through a combination of low dose synchrotron X-ray crystallography and zero dose serial femtosecond X-ray crystallography using an X-ray free electron laser (XFEL), high-resolution structures with unambiguous redox state assignment of the ferric and ferryl (Fe^IV = O) heme species have been obtained. Experiments using stopped-flow kinetics, solvent-isotope exchange and site-directed mutagenesis with this set of redox state validated DyP structures have provided the first comprehensive kinetic and structural framework for how DyPs can modulate their distal heme pocket Asp/Arg dyad to use either the Asp or the Arg to facilitate proton transfer and rate enhancement of peroxide heterolysis.

A cationic copolymer as a cocatalyst for a peroxidase-mimicking heme-DNAzyme

Heme binds to a parallel-stranded G-quadruplex DNA to form a peroxidase-mimicking heme-DNAzyme. An interpolyelectrolyte complex between the heme-DNAzyme and a cationic copolymer possessing protonated amino groups was characterized and the peroxidase activity of the complex was evaluated to elucidate the effect of the polymer on the catalytic activity of the heme-DNAzyme. We found that the catalytic activity of the heme-DNAzyme is enhanced through the formation of the interpolyelectrolyte complex due to the general acid catalysis of protonated amino groups of the polymer, enhancing the formation of the iron(iv)oxo porphyrin π-cation radical intermediate known as Compound I. This finding indicates that the polymer with protonated amino groups can act as a cocatalyst for the heme-DNAzyme in the oxidation catalysis. We also found that the enhancement of the activity of the heme-DNAzyme by the polymer depends on the local heme environment such as the negative charge density in the proximity of the heme and substrate accessibility to the heme. These findings provide novel insights as to molecular design of the heme-DNAzyme for enhancing its catalytic activity.

Effects of Heme Electronic Structure and Local Heme Environment on Catalytic Activity of a Peroxidase-Mimicking Heme-DNAzyme

The catalytic cycle of a peroxidase-mimicking heme-DNAzyme involves an iron(IV)oxo porphyrin π-cation radical intermediate known as compound I formed through heterolytic O-O bond cleavage of an Fe³⁺-bound hydroperoxo ligand (Fe-OOH) in compound 0, like that of a heme enzyme such as horseradish peroxidase (HRP). Peroxidase assaying of complexes composed of chemically modified hemes possessing various electron densities of the heme iron atom (ρ_Fe) and parallel-stranded tetrameric G-quadruplex DNAs of oligonucleotides d(TTAGGG), d(TTAGGGT), and d(TTAGGGA) was performed to elucidate the effects of the heme electronic structure and local heme environment on the catalytic activity of the heme-DNAzyme. The study revealed that the DNAzyme activity is enhanced through an increase in the ρ_Fe and general base catalysis of the adenine base adjacent to the heme, which are reminiscent of the "push" and "pull" mechanisms in the catalytic cycle of HRP, respectively, and that the activity of the heme-DNAzyme can be independently controlled through the heme electronic structure and local heme environment. These findings allow a deeper understanding of the structure-function relationship of the peroxidase-mimicking heme-DNAzyme.

Bound oxygen-atom transfer endows peroxidase-mimic M-N-C with high substrate selectivity

Advances in nanoscience have stimulated the wide exploration of nanozymes as alternatives to enzymes. Nonetheless, nanozymes often catalyze multiple reactions and are not specialized to a specific substrate, restricting their broad application. Here, we report that the substrate selectivity of the peroxidase-mimic M-N-C can be significantly altered via forming bound intermediates with variable interactions with substrates according to the type of metal. Taking two essential reactions in chemical sensing as an example, Fe-N-C and Co-N-C showed opposite catalytic selectivity for the oxidation of 3,3',5,5'-tetramethylbenzidine (TMB) and 3-aminophthalhydrazide (luminol), respectively, by factors of up to 200-fold. It was revealed that specific transition metal-N coordination was the origin of the selective activation of H2O2 forming critically bound oxygen intermediates (M[double bond, length as m-dash]O) for oxygen-atom transfer and the consequent oxidization of substrates. Notably, owing to the embedded ligands in the rigid graphitic framework, surprisingly, the selectivity of M-N-C was even superior to that of commonly used horseradish peroxidase (HRP).

Mechanism of action, sources, and application of peroxidases

Peroxidase is an enzyme in the group of oxidoreductases that is widely distributed in nature. It can catalyze the oxidation of various organic and inorganic substrates by reacting with hydrogen peroxide and similar molecules. Due to its wide catalytic activity, peroxidases can act in the removal of both phenolic compounds and peroxides, in chemical synthesis and, according to recent studies, in mycotoxin degradation. Therefore, this study aimed at introducing an overview of the mechanism of peroxidase action, extraction sources, mycotoxin degradation capacity and other potential applications in the food industry.

Immobilization of horseradish peroxidase on cationic microporous starch: Physico-bio-chemical characterization and removal of phenolic compounds

In the present study, two different modified starches; microporous starch (MPS) and cationic microporous starch (CMPS) were synthesized. The granules of MPS that distributed regularly were destroyed after the etherification reaction. The data depicted that the immobilization of horseradish peroxidase (HRP) on CMPS revealed highest immobilization efficiency (86%) at 100 mg of CMPS at pH = 6.0 and 100 units of enzyme. After 10 reuses of the CMPS-HRP, it retained 66% of initial activity. The soluble HRP showed broad pH optimum of 6.0-7.0, which changed to sharp pH = 6.0 for CMPS-HRP. Soluble-HRP and CMPS-HRP showed temperature optima at 30 °C and 40 °C, respectively. The CMPS-HRP showed high thermal stability up to 50 °C compared to the soluble HRP (40 °C). The Km values of soluble HRP and CMPS-HRP were 6.6 and 10.8 mM for H₂O₂ and 34 and 41.6 mM for guaiacol, respectively. CMPS-HRP showed higher affinity toward various substrates than the soluble-HRP. CMPS-HRP showed more resistance against heavy metals, urea, isopropanol, Triton X-100 and trypsin than soluble enzyme. The CMPS-HRP showed higher ability to remove phenol and p-chlorophenol compared to soluble-HRP.

Reactive intermediates for interactome mapping

The interactions of biomolecules underpin all cellular processes, and the understanding of their dynamic interplay can lead to significant advances in the treatment of disease through the identification of novel therapeutic strategies. Protein-protein interactions (PPIs) in particular play a vital role within this arena, providing the basis for the majority of cellular signalling pathways. Despite their great importance, the elucidation of weak or transient PPIs that cannot be identified by immunoprecipitation remains a significant challenge, particularly in a disease relevant cellular environment. Recent approaches towards this goal have utilized the in situ generation of high energy intermediates that cross-link with neighboring proteins, providing a snapshot of the biomolecular makeup of the local area or microenvironment, termed the interactome. In this tutorial review, we discuss these reactive intermediates, how they are generated, and the impact they have had on the discovery of new biology. Broadly, we believe this strategy has the potential to significantly accelerate our understanding of PPIs and how they affect cellular physiology.

Monitoring the heme iron state in horseradish peroxidase to detect ultratrace amounts of hydrogen peroxide in alcohols

Despite the importance of hydrogen peroxide (H2O2) in initiating oxidative damage and its connection to various diseases, the detection of low concentrations of H2O2 (<10 μM) is still limited using current methods, particularly in non-aqueous systems. One of the most common methods is based on examining the color change of a reducing substrate upon oxidation using UV/Vis spectrophotometry, fluorophotometry and/or paper test strips. In this study, we show that this method encounters low efficiency and sensitivity for detection of ultratrace amounts of H2O2 in non-aqueous media. Thus, we have developed a simple, fast, accurate and inexpensive method based on UV/Vis spectrophotometry to detect H2O2 in non-aqueous systems, such as alcohols. In this regard, we demonstrate that monitoring the Soret and Q-band regions of high-valent iron-oxo (ferryl heme) intermediates in horseradish peroxidase (HRP) is well suited to detect ultratrace amounts of H2O2 impurities in alcohols in the range of 0.001-1000 μM using UV/Vis spectrophotometry. We monitor the optical spectra of HRP solution for the red shift in the Soret and Q-band regions upon the addition of alcohols with H2O2 impurity. We also monitor the reversibility of this shift to the original wavelength over time to check the spontaneous decay of ferryl intermediates to the ferric state. Thus, we have found that the ferryl intermediates of HRP can be used for the detection of H2O2 in alcohols at μg L-1 levels through via UV/Vis spectrophotometric method.