A density functional investigation of hydrogen peroxide activation by high-valent heme centers: implications for the catalase catalytic cycle

2010 ◽  
Vol 14 (05) ◽  
pp. 371-374 ◽  
Author(s):  
Radu Silaghi-Dumitrescu
Keyword(s):  
Hydrogen Peroxide ◽  
Density Functional ◽  
Cytochromes P450 ◽  
Cation Radical ◽  
Hydrogen Atom Transfer ◽  
Catalytic Cycle ◽  
Compound I ◽  
Peroxide Activation

Catalases employ a tyrosinate-ligated ferric heme in order to catalyze the dismutation of hydrogen peroxide to O2 and water. In the first half of the catalytic cycle, H2O2 oxidizes Fe(III) to the formally Fe(V) state commonly referred to as Compound I. The second half of the cycle entails oxidation of a second hydrogen peroxide molecule by Compound I to dioxygen. The present study employs density functional (DFT) calculations to examine the nature of this second step of the catalatic reaction. In order to account for the unusual choice of tyrosinate as an axial ligand in catalases, oxidation of hydrogen peroxide by an imidazole-ligated Compound I is also examined, bearing in mind that imidazole-ligated hemoproteins such as myoglobin or horseradish peroxidase tend to display little, if any, catalatic activity. Furthermore, in order to gauge the importance of the cation radical of Compound I in peroxide activation, the performance of Compound II (the one-electron reduced version of Compound I, formally Fe(IV) ), is also examined. It is found that hydrogen peroxide oxidation occurs in a quasi-concerted manner, with two hydrogen-atom transfer reactions, and that the tyrosinate ligand is in no way required at this stage. We propose that the role of the tyrosinate is purely thermodynamic, in avoiding accumulation of the much less peroxide-reactive ferrous form in vivo – all in line with the predominantly thermodynamic role of the cysteinate ligands in enzymes such as cytochromes P450.

In vivo generation of hydrogen peroxide by rat kidney cortex and glomeruli

1989 ◽  
Vol 256 (1) ◽  
pp. F158-F164 ◽  
Author(s):  
B. R. Guidet ◽  
S. V. Shah
Keyword(s):  
Hydrogen Peroxide ◽  
Catalase Activity ◽  
Renal Cortex ◽  
Specific Activity ◽  
Rat Kidney ◽  
Kidney Cortex ◽  
Compound I ◽  
Rat Kidney Cortex ◽  
Hydrogen Peroxide Generation

The purpose of this study was to demonstrate in vivo generation of hydrogen peroxide by rat renal cortex and glomeruli. Aminotriazole irreversibly inactivates catalase only in the presence of hydrogen peroxide, and previous studies have shown that aminotriazole-mediated inhibition of catalase is a measure of in vivo changes in the hydrogen peroxide generation. Aminotriazole injected intraperitoneally caused a dose-dependent (0.1-1 g/kg) and a time-dependent (15, 30, 60, 90, 120 min) inhibition of the catalase activity in renal cortex. We confirmed that catalase inactivation by aminotriazole was due to formation of a catalase-hydrogen peroxide intermediate (compound I) because catalase inactivation was prevented by ethanol (2 g/kg), a competitive substrate for compound I. The specific activity of catalase in the glomeruli [0.27 +/- 0.026 k/mg protein (where k is the first-order reaction rate constant), n = 5] was significantly lower than the specific activity in the tubules (1.04 +/- 0.15 k/mg protein, n = 5) obtained from the same rats. The residual catalase activity (RCA) in the glomeruli (0.05 +/- 0.01 k/mg protein) was 19% of control values at 90 min after aminotriazole injection (1 g/kg). Taken together these data provide evidence for in vivo generation of hydrogen peroxide by rat renal cortex and glomeruli under normal conditions. Aminotriazole-mediated inhibition of catalase has been used in previous studies as a measure of in vivo changes in the hydrogen peroxide generation.(ABSTRACT TRUNCATED AT 250 WORDS)

Role of Cu,Zn-SOD in the synthesis of endogenous vasodilator hydrogen peroxide during reactive hyperemia in mouse mesenteric microcirculation in vivo

2008 ◽  
Vol 294 (1) ◽  
pp. H441-H448 ◽  
Author(s):  
Toyotaka Yada ◽  
Hiroaki Shimokawa ◽  
Keiko Morikawa ◽  
Aya Takaki ◽  
Yoshiro Shinozaki ◽  
...  
Keyword(s):  
Hydrogen Peroxide ◽  
Reactive Hyperemia ◽  
Fluorescent Microscopy ◽  
Artery Occlusion ◽  
Wild Type ◽  
Charge Coupled Device ◽  
In The Wild ◽  
Mesenteric Microcirculation

We have recently demonstrated that endothelium-derived hydrogen peroxide (H2O2) is an endothelium-derived hyperpolarizing factor and that endothelial Cu/Zn-superoxide dismutase (SOD) plays an important role in the synthesis of endogenous H2O2 in both animals and humans. We examined whether SOD plays a role in the synthesis of endogenous H2O2 during in vivo reactive hyperemia (RH), an important regulatory mechanism. Mesenteric arterioles from wild-type and Cu,Zn-SOD−/− mice were continuously observed by a pencil-type charge-coupled device (CCD) intravital microscope during RH (reperfusion after 20 and 60 s of mesenteric artery occlusion) in the cyclooxygenase blockade under the following four conditions: control, catalase alone, NG-monomethyl-l-arginine (l-NMMA) alone, and l-NMMA + catalase. Vasodilatation during RH was significantly decreased by catalase or l-NMMA alone and was almost completely inhibited by l-NMMA + catalase in wild-type mice, whereas it was inhibited by l-NMMA and l-NMMA + catalase in the Cu,Zn-SOD−/− mice. RH-induced increase in blood flow after l-NMMA was significantly increased in the wild-type mice, whereas it was significantly reduced in the Cu,Zn-SOD−/− mice. In mesenteric arterioles of the Cu,Zn-SOD−/− mice, Tempol, an SOD mimetic, significantly increased the ACh-induced vasodilatation, and the enhancing effect of Tempol was decreased by catalase. Vascular H2O2 production by fluorescent microscopy in mesenteric arterioles after RH was significantly increased in response to ACh in wild-type mice but markedly impaired in Cu,Zn-SOD−/− mice. Endothelial Cu,Zn-SOD plays an important role in the synthesis of endogenous H2O2 that contributes to RH in mouse mesenteric smaller arterioles.

Methane Hydroxylation by Axially Ligated Iron (IV)-oxo Porphyrin Cation Radical Models

2016 ◽  
Vol 1 (2) ◽  
Author(s):  
Devendra Singh ◽  
Devesh Kumar ◽  
Sam P. de Visser
Keyword(s):  
Hydrogen Atom ◽  
Cytochromes P450 ◽  
Cation Radical ◽  
Axial Ligand ◽  
Hydrogen Atom Abstraction ◽  
Energy Splitting ◽  
Compound I ◽  
Model Complexes ◽  
Mononuclear Iron ◽  
Porphyrin Cation

Methane hydroxylation is a thermochemically difficult process due to the strength of the C-H bond that needs to be broken in the process. In Nature only the methane monoxygenases have a catalytic center that is active enough to perform this task. Other metalloenzymes, such as, mononuclear iron monoxygenases and dioxygenases, including the cytochromes P450, are not known to catalyze methane hydroxylation. The cytochromes P450 contain an iron heme group that in a catalytic cycle is converted into an iron(IV)-oxo heme cation radical (Compound I). To gain insight into the features that affect methane hydroxylation by Compound I and synthetic model complexes, we have done a detailed computational study. Thus, we investigated the chemical properties of iron(IV)-oxo porphyrins with varying axial ligands, including SH<sup>−</sup>, F<sup>−</sup>, OH<sup>−</sup>, CN<sup>−</sup>, CF<sub>3</sub>COO<sup>−</sup> and CH<sub>3</sub>COO<sup>−</sup>. In addition, we calculated the methane hydroxylation pathways for a selection of these oxidants and rationalize the obtained trends with thermochemical cycles and valence bond schemes. In general, the rate determining hydrogen atom abstraction barrier is dependent on the π<sub>xz</sub>/π*<sub>xz</sub> energy splitting along the Fe−O bond, the excitation energy from π<sub>xz</sub> to a<sub>2u</sub>, as well as the bond dissociation energies of the methane C−H bond and the newly formed O−H bond. Our studies predict that iron(IV)-oxo porphyrin cation radical models with hydroxide as axial ligand should be efficient oxidants of substrate hydroxylation reactions and able to activate methane at room temperature. However, changing the axial ligand to a weaker electron donating group decreases its activity and raises the hydrogen atom abstraction barriers dramatically. These studies show that subtle modifications to the oxidant can have a great impact on the catalytic ability of the active center.

Density functional theory (DFT) and combined quantum mechanical/molecular mechanics (QM/MM) studies on the oxygen activation step in nitric oxide synthase enzymes

2009 ◽  
Vol 37 (2) ◽  
pp. 373-377 ◽  
Author(s):  
Sam P. de Visser
Keyword(s):  
Nitric Oxide ◽  
Density Functional Theory ◽  
Cytochrome P450 ◽  
Density Functional ◽  
Catalytic Cycle ◽  
Theoretical Studies ◽  
Cytochrome P450 Enzymes ◽  
Compound I ◽  
Functional Theory ◽  
Oxide Synthase

In this review paper, we will give an overview of recent theoretical studies on the catalytic cycle(s) of NOS (nitric oxide synthase) enzymes and in particular on the later stages of these cycles where experimental work is difficult due to the short lifetime of intermediates. NOS enzymes are vital for human health and are involved in the biosynthesis of toxic nitric oxide. Despite many experimental efforts in the field, the catalytic cycle of this important enzyme is still surrounded by many unknowns and controversies. Our theoretical studies were focused on the grey zones of the catalytic cycle, where intermediates are short-lived and experimental detection is impossible. Thus combined QM/MM (quantum mechanics/molecular mechanics) as well as DFT (density functional theory) studies on NOS enzymes and active site models have established a novel mechanism of oxygen activation and the conversion of L-arginine into Nω-hydroxo-arginine. Although NOS enzymes show many structural similarities to cytochrome P450 enzymes, it has long been anticipated that therefore they should have a similar catalytic cycle where molecular oxygen binds to a haem centre and is converted into an Fe(IV)-oxo haem(+•) active species (Compound I). Compound I, however, is elusive in the cytochrome P450s as well as in NOS enzymes, but indirect experimental evidence on cytochrome P450 systems combined with theoretical modelling have shown it to be the oxidant responsible for hydroxylation reactions in cytochrome P450 enzymes. By contrast, in the first catalytic cycle of NOS it has been shown that Compound I is first reduced to Compound II before the hydroxylation of arginine. Furthermore, substrate arginine in NOS enzymes appears to have a dual function, namely first as a proton donor in the catalytic cycle to convert the ferric-superoxo into a ferric-hydroperoxo complex and secondly as the substrate that is hydroxylated in the process leading to Nω-hydroxo-arginine.

scholarly journals Shoot Accumulation Kinetics and Effects on Herbivores of the Wound-Induced Antioxidant Indole Alkaloid Brachycerine of Psychotria brachyceras

2014 ◽  
Vol 9 (5) ◽  
pp. 1934578X1400900 ◽  
Author(s):  
Diogo D. Porto ◽  
Hélio N. Matsuura ◽  
Lúcia R. B. Vargas ◽  
Amélia T. Henriques ◽  
Arthur G. Fett-Neto
Keyword(s):  
Hydrogen Peroxide ◽  
Antioxidant Properties ◽  
Indole Alkaloid ◽  
Mechanical Damage ◽  
Chemical Defence ◽  
Generalist Herbivore ◽  
Phenolics Content ◽  
Accumulation Kinetics

A major shoot-specific monoterpene indole alkaloid produced by Psychotria brachyceras, brachycerine, is regulated by either wounding or jasmonate application. Highest concentrations of the alkaloid are found in inflorescences, suggesting a defence role. Brachycerine has antimutagenic and antioxidant properties, capable of quenching singlet oxygen, hydroxyl radical, and superoxide. This study aimed at characterizing the putative role of brachycerine in P. brachyceras responses to wounding and herbivory. Damage to leaves increased the content of brachycerine locally. Wounding did not affect phenolics content in P. brachyceras leaves, and no tannins were detected in the species. In generalist herbivore bioassays, neither brachycerine nor P. brachyceras extracts showed toxic effects. In vivo hydrogen peroxide staining assay showed less wound-generated peroxide accumulation in alkaloid treated tissues. This pattern was confirmed in quantitative assays measuring tissue hydrogen peroxide concentrations. Data indicate that brachycerine is not a herbivore deterrent, but rather an indirect chemical defence, modulating oxidative stress caused by mechanical damage.

One small step for cytochrome P450 in its catalytic cycle, one giant leap for enzymology

2019 ◽  
Vol 23 (04n05) ◽  
pp. 358-366 ◽  
Author(s):  
Huriye Erdogan
Keyword(s):  
Cytochrome P450 ◽  
Time Course ◽  
Catalytic Cycle ◽  
Small Step ◽  
Compound I ◽  
Unique Role ◽  
Natural Time ◽  
Giant Leap ◽  
High Valent

The intermediates operating in the cytochrome P450 catalytic cycle have been investigated for more than half a century, fascinating many enzymologists. Each intermediate has its unique role to carry out diverse oxidations. Natural time course of the catalytic cycle is quite fast, hence, not all of the reactive intermediates could be isolated during physiological catalysis. Different high-valent iron intermediates have been proposed as primary oxidants: the candidates are compound 0 (Cpd 0, [FeOOH][Formula: see text]P450) and compound I (Cpd I, Fe(IV)[Formula: see text]O por[Formula: see text]P450). Among them, the role of Cpd I in hydroxylation is fairly well understood due the discovery of the peroxide shunt. This review endeavors to put the outstanding research efforts conducted to isolate and characterize the intermediates together. In addition to spectral features of each intermediate in the catalytic cycle, the oxidizing powers of Cpd 0 and Cpd I will be discussed along with most recent scientific findings.

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