Selective oxidations catalyzed by peroxidases

Femtosecond near-infrared laser pulses elicit generation of reactive oxygen species in mammalian cells leading to apoptosis-like death

Two-photon excitation-based near-infrared (NIR) laser scanning microscopy is currently emerging as a new and versatile alternative to conventional confocal laser scanning microscopy, particularly for vital cell imaging in life sciences. Although this innovative microscopy has several advantages such as highly localized excitation, higher penetration depth, reduced photobleaching and photodamage, and improved signal to noise ratio, it has, however, recently been evidenced that high-power NIR laser irradiation can drastically inhibit cell division and induce cell death. In the present study we have investigated the cellular responses of unlabeled rat kangaroo kidney epithelium (PtK2) cells to NIR femtosecond laser irradiation. We demonstrate that NIR 170-fs laser pulses operating at 80-MHz pulse repetition frequency and at mean power of > or = 7 mW evoke generation of reactive oxygen species (ROS) such as H2O2 that can be visualized in situ by standard in vivo cytochemical analysis using Ni-3,3'-diaminobenzidine (Ni-DAB) as well as with a recently developed fluorescent probe Jenchrom px blue. The formation of the Ni-DAB reaction product as well as that of Jenchrom was relatively more pronounced when irradiated cells were incubated in alkaline solution (pH 8) than in those incubated in acidic solution (pH 6), suggesting peroxisomal localization of these reaction products. Two-photon time-lapse imaging of the internalization of the cell impermeate fluorescent dye propidium iodide revealed that the integrity of the plasma membrane of NIR irradiated cells is drastically compromised. Visualization of the nuclei with DNA-specific fluorescent probes such as 4',6-diamidino-2-phenylindole 24 h postirradiation further provided tangible evidence that the nuclei of these cells undergo several deformations and eventual fragmentation. That these NIR irradiated cells die by apoptosis has been established by in situ detection of DNA strand breaks using the terminal deoxynucleotidyl transferase-mediated dUTP nick-end labeling method. Because the reactive oxygen species such as H2O2 and OH* can cause noxious effects such as cell membrane injury by peroxidation of polyunsaturated lipids and proteins and oxidative phosphorylation, and alterations of ATP-dependent Ca2+ pumps, these ROS are likely to contribute to drastic cytological alterations observed in this study following NIR irradiation. Taken together, we have established that NIR laser irradiations at mean power > or = 7 mW delivered at pulse duration time of 170 fs generally used in two- and multiphoton microscopes cause oxidative stress (1) evoking production of ROS, (2) resulting in membrane barrier dysfunction, (3) inducing structural deformations and fragmentation of the nuclei as well as DNA strand breaks, (4) leading to cell death by apoptosis.

Improving the catalytic performance of peroxidases in organic synthesis

Peroxidases are ubiquitous enzymes that catalyze a variety of enantioselective oxygen-transfer reactions with hydrogen peroxide (H2O2). Although they have enormous potential, their industrial application is hampered by their high price and low operational stability. Recent developments, such as the controlled addition and in situ formation of the oxidant, protein engineering and the rational design of semi-synthetic peroxidases, aim to improve the operational stability of peroxidases.

Enzymes for chemical synthesis

New catalytic synthetic methods in organic chemistry that satisfy increasingly stringent environmental constraints are in great demand by the pharmaceutical and chemical industries. In addition, novel catalytic procedures are necessary to produce the emerging classes of organic compounds that are becoming the targets of molecular and biomedical research. Enzyme-catalysed chemical transformations are now widely recognized as practical alternatives to traditional (non-biological) organic synthesis, and as convenient solutions to certain intractable synthetic problems.

Engineering the active site of ascorbate peroxidase

The oxidation of a number of thioethers, namely methyl phenyl sulphide (1), ethyl phenyl sulphide (2), isopropyl phenyl sulphide (3), n-propyl phenyl sulphide (4), p-chlorophenyl methyl sulphide (5), p-nitrophenyl methyl sulphide (6) and methyl naphthalene sulphide (7), by recombinant pea cytosolic ascorbate peroxidase (rAPX) and a site-directed variant of rAPX in which the distal tryptophan 41 residue has been replaced with an alanine (W41A) has been examined. The electronic spectrum (pH 7.0, mu = 0.10 M, 25.0 degrees C) for the ferric derivative of W41A (lambda(max)/nm = 411, 534, 560, 632) is indicative of an increased quantity of 6-coordinate, high-spin and/or 6-coordinate, low-spin haem compared to rAPX. Steady state oxidation of sulphides 1-4 and 7, gave values for kcat that are approximately 10-fold and 100-fold, respectively, higher for W41A than for rAPX. For rAPX, essentially racemic mixtures of R- and S-sulphoxides were obtained for all sulphides. With the exception of sulphide 7, the W41A variant shows substantial enhancements in enantioselectivity, with R : S ratios varying between R : S = 63 : 37 (sulphides 1 and 4) and R : S = 85 : 15 (sulphide 6). Incubation of sulphide 2 with rAPX or W41A and [(18)O] H(2)O(2) shows 95% (rAPX) and 96% (W41A) transfer of labelled oxygen to the substrate. Structure-based modelling techniques have provided a fully quantitative rationalization of all the experimentally determined R : S ratios and have indicated that reorientation of the sidechain of Arg38, such that access to the haem is much less restricted, is influential in controlling the stereoselectivity for both rAPX and W41A.

Sulfoxidation mechanism of vanadium bromoperoxidase from Ascophyllum nodosum. Evidence for direct oxygen transfer catalysis

We have previously shown that vanadium bromoperoxidase from Ascophyllum nodosum mediates production of the (R)-enantiomer of methyl phenyl sulfoxide with 91% enantiomeric excess. Investigation of the intrinsic selectivity of vanadium bromoperoxidase reveals that the enzyme catalyzes the sulfoxidation of methyl phenyl sulfide in a purely enantioselective manner. The K(m) of the enzyme for methyl phenyl sulfide was determined to be approximately 3.5 mM in the presence of 25% methanol or tert-butanol. The selectivity of the sulfoxidation of methyl phenyl sulfide is optimal in the temperature range 25-30 degrees C and can be further optimized by increasing the enzyme concentration, yielding selectivities with up to 96% enantiomeric excess. Furthermore, we established for the first time that vanadium bromoperoxidase is functional at temperatures up to 70 degrees C. A detailed investigation of the sulfoxidation activity of this enzyme using (18)O-labeled hydrogen peroxide shows that vanadium bromoperoxidase mediates the direct transfer of the peroxide oxygen to the sulfide. A schematic model of the vanadium haloperoxidase sulfoxidation mechanism is presented.

Design, synthesis and peroxidase-like activity of 3alpha-helix proteins covalently bound to heme

As a model of artificial peroxidase, de novo designed three-alpha-helix proteins, 3alpha-H9 and 3alpha-H12, covalently bound to Fe(III)-mesoporphyrin IX were synthesized and examined for a peroxidase-like activity. The activity was regulated according to the positions of His residues in the proteins, and the His residues played a role in an acid-base catalytic function.

Selective oxygen transfer catalysed by heme peroxidases: synthetic and mechanistic aspects

The synthetic and mechanistic aspects of the use of heme peroxidases as functional mimics of the cytochrome P450 monooxygenases in oxygen-transfer reactions have been described. The chloroperoxidase from Caldariomyces fumago (CPO) is the catalyst of choice in sulfoxidation, hydroxylation and epoxidation on account of its high activity and enantioselectivity. Other heme peroxidases were less active by orders of magnitude; protein engineering has resulted in impressive improvements but even the most active mutant was still at least an order of magnitude less active than CPO. The 'oxygen-rebound' mechanisms of oxygen transfer mediated by heme enzymes - as originally conceived - have proved to be untenable. Dual pathway mechanisms, via oxoferryl species that insert oxygen as well as iron hydroperoxide species that insert OH(+), have been proposed that accommodate all of the known experimental data.

Highly efficient immobilization of glycosylated enzymes into polyurethane foams

Glycosylated enzymes, including aminoacylase from Aspergillus melleus, chloroperoxidase from Caldariomyces fumago, and phytase from Aspergillus ficuum, were covalently immobilized into polyurethane foams with very high enzyme loadings of up to 0.2 g protein per gram dry foam. The immobilization efficiency (retained activity) ranged from 100% at a low loading to 60% at high loadings. In contrast to many other immobilization methods no leaching of the enzyme from the support took place under the reaction conditions. In short, a universal method for the immobilization of enzymes from fungal sources was developed, affording a highly active, stable, and reusable biocatalyst.

Peroxidase-catalyzed asymmetric sulfoxidation in organic solvents versus in water

Peroxidase-catalyzed asymmetric sulfoxidations, while synthetically attractive, suffer from relatively low reaction rates due to poor substrate solubilities in water and from appreciable spontaneous oxidation of substrates (especially aryl alkyl sulfides) with H(2)O(2). In this work, we found that both of these shortcomings could be alleviated by switching from aqueous solutions to certain nearly anhydrous (99.7%) organic solvents as sulfoxidation reaction media. The rates of spontaneous oxidation of the model prochiral substrate thioanisole in several organic solvents were observed to be some 100- to 1000-fold slower than in water. In addition, the rates of asymmetric sulfoxidation of thioanisole in isopropyl alcohol and in methanol catalyzed by horseradish peroxidase (HRP) were determined to be tens to hundreds of times faster than in water under otherwise identical conditions. This dramatic activation is due to a much higher substrate solubility in organic solvents than in water and occurs even though the intrinsic reactivity of HRP in isopropyl alcohol and in methanol is hundreds of times lower than in water. Sulfoxidation of thioanisole catalyzed by four other hemoproteins (soybean peroxidase, myoglobin, hemoglobin, and cytochrome c) is also much faster in isopropyl alcohol than in water.

Oxidase activities in macrocyclic-resistant and -susceptible Haemonchus contortus

The role of oxidative metabolism in resistance to macrocyclic lactones in Haemonchus contortus was examined by measuring activities toward 2 model cytochrome P450 substrates, aldrin and ethoxycoumarin, in a susceptible and a resistant isolate of the parasite. Microsomal preparations from larvae and adults of the 2 isolates showed no differences in levels of NADPH- or cumene hydroperoxide-supported aldrin epoxidase or ethoxycoumarin O-deethylase activities. Intact adult nematodes showed an ability to catalyze the epoxidation of significant amounts of aldrin, although the nature of the enzyme group responsible was unknown. This epoxidase activity was greater in adults of the susceptible isolate. It is apparent that oxidase activities toward the 2 substrates are not enhanced in the resistant isolate, suggesting that the observed resistance to macrocyclic lactones may not involve enhanced oxidative metabolism.

Biocatalytic and biomimetic oxidations with vanadium

Approaches to the rational design of vanadium-based semi-synthetic enzymes and biomimetic models as catalysts for enantioselective oxidations are reviewed. Incorporation of vanadate ion into the active site of phytase (E.C. 3.1.3.8), which in vivo mediates the hydrolysis of phosphate esters, afforded a semi-synthetic peroxidase. It catalyzed the enantioselective oxidation of prochiral sulfides with H2O2 affording the S-sulfoxide, e.g. in 66% ee at quantitative conversion of thioanisole. Under the reaction conditions the semi-synthetic vanadium peroxidase was stable for more than 3 days with only a slight decrease in turnover frequency. Amongst the transition-metal oxoanions that are known to be potent inhibitors of phosphatases, only vanadate resulted in a semi-synthetic peroxidase when incorporated into phytase. In a biomimetic approach, vanadium complexes of chiral Schiff base complexes were encapsulated in the super cages of a hydrophobic zeolite Y. Unfortunately, these ship-in-a-bottle complexes afforded only racemic sulfoxide in the catalytic oxidation of thioanisole with H2O2.

Oxidation reactions catalyzed by vanadium chloroperoxidase from Curvularia inaequalis

Vanadium haloperoxidases have been reported to mediate the oxidation of halides to hypohalous acid and the sulfoxidation of organic sulfides to the corresponding sulfoxides in the presence of hydrogen peroxide. However, traditional heme peroxidase substrates were reported not to be oxidized by vanadium haloperoxidases. Surprisingly, we have now found that the recombinant vanadium chloroperoxidase from the fungus Curvularia inaequalis catalyzes the oxidation of 2,2'-azino-bis(3-ethylbenzthiazoline-6-sulfonic acid) (ABTS), a classical chromogenic heme peroxidase substrate. The enzyme mediates the oxidation of ABTS in the presence of hydrogen peroxide with a turnover frequency of 11 s(-1) at its pH optimum of 4.0. The Km of the recombinant enzyme for ABTS was observed to be approximately 35 microM at this pH value. In addition, the bleaching of an industrial sulfonated azo dye, Chicago Sky Blue 6B, catalyzed by the recombinant vanadium chloroperoxidase in the presence of hydrogen peroxide is reported.

Striking activation of oxidative enzymes suspended in nonaqueous media

The catalytic activity of four lyophilized oxidative enzymes-horseradish peroxidase, soybean peroxidase, Caldariomyces fumago chloroperoxidase, and mushroom polyphenol oxidase-is much lower when directly suspended in organic solvents containing little water than when they are introduced into the same largely nonaqueous media by first dissolving them in water and then diluting with anhydrous solvents. The lower the water content of the medium, the greater this discrepancy becomes. The mechanism of this phenomenon was found to arise from reversible denaturation of the oxidases on lyophilization: because of its conformational rigidity, the denatured enzyme exhibits very limited activity when directly suspended in largely nonaqueous media but renatures and thus yields much higher activity if first redissolved in water. Two independent means were discovered for dramatically minimizing the lyophilization-induced inactivation, both involving the addition of certain types of excipients to the aqueous enzyme solution before lyophilization. The first group of excipients consists of phenolic and aniline substrates as well as other hydrophobic compounds; these presumably bind to the hydrophobic pocket of the enzyme active site, thereby preventing its collapse during dehydration. The second group consists of general lyoprotectants such as polyols and polyethylen glycol that apparently preserve the overall enzyme structure during dehydration. The activation effects of such excipients can reach into the tens and hundreds of fold. Moreover, the activations afforded by the two excipient groups are additive, resulting in up to a complete protection against lyophilization-induced inactivation when representatives of the two are present together.

Laboratory evolution of peroxide-mediated cytochrome P450 hydroxylation

Enzyme-based chemical transformations typically proceed with high selectivity under mild conditions, and are becoming increasingly important in the pharmaceutical and chemical industries. Cytochrome P450 monooxygenases (P450s) constitute a large family of enzymes of particular interest in this regard. Their biological functions, such as detoxification of xenobiotics and steroidogenesis, are based on the ability to catalyse the insertion of oxygen into a wide variety of compounds. Such a catalytic transformation might find technological applications in areas ranging from gene therapy and environmental remediation to the selective synthesis of pharmaceuticals and chemicals. But relatively low turnover rates (particularly towards non-natural substrates), low stability and the need for electron-donating cofactors prohibit the practical use of P450s as isolated enzymes. Here we report the directed evolution of the P450 from Pseudomonas putida to create mutants that hydroxylate naphthalene in the absence of cofactors through the 'peroxide shunt' pathway with more than 20-fold higher activity than the native enzyme. We are able to screen efficiently for improved mutants by coexpressing them with horseradish peroxidase, which converts the products of the P450 reaction into fluorescent compounds amenable to digital imaging screening. This system should allow us to select and develop mono- and di-oxygenases into practically useful biocatalysts for the hydroxylation of a wide range of aromatic compounds.

Recent biotechnological developments in the use of peroxidases

Peroxidases are ubiquitous oxidoreductases that use hydrogen peroxide or alkyl peroxides as oxidants. Advances have recently been made in using them to prepare, under mild and controlled conditions, chiral organic molecules that are valuable for the chemoenzymatic synthesis of a wide range of useful compounds. Horseradish peroxidase can be converted into a peroxygenative enzyme by molecular engineering. Chloroperoxidase, the most versatile peroxidase, behaves like a 'true' monooxygenase in sulfoxidations with molecular oxygen and an external reductant, with substantial increases in enantioselectivity and enzyme stability.

Biotransformations with peroxidases

Enzymes are chiral catalysts and are able to produce optically active molecules from prochiral or racemic substrates by catalytic asymmetric induction. One of the major challenges in organic synthesis is the development of environmentally acceptable chemical processes for the preparation of enantiomerically pure compounds, which are of increasing importance as pharmaceuticals and agrochemicals. Enzymes meet this challenge! For example, a variety of peroxidases effectively catalyze numerous selective oxidations of electron-rich substrates, which include the hydroxylation of arenes, the oxyfunctionalizations of phenols and aromatic amines, the epoxidation and halogenation of olefins, the oxygenation of heteroatoms and the enantioselective reduction of racemic hydroperoxides. In this review, we summarize the important advances achieved in the last few years on peroxidase-catalyzed transformations, with major emphasis on preparative applications.

Chloroperoxidase-catalyzed enantioselective oxidation of methyl phenyl sulfide with dihydroxyfumaric acid/oxygen or ascorbic acid/oxygen as oxidants

The chloroperoxidase catalyzed oxidation of methyl phenyl sulfide to (R)-methyl phenyl sulfoxide was investigated, both in batch and membrane reactors, using as oxidant H2O2, or O2 in the presence of either dihydroxyfumaric acid or ascorbic acid. The effects of pH and nature and concentration of the oxidants on the selectivity, stability, and productivity of the enzyme were evaluated. The highest selectivity was displayed by ascorbic acid/O2, even though the activity of chloroperoxidase with this system was lower than that obtained with the others. When the reaction was carried out in a membrane reactor, it was possible to reuse the enzyme for several conversion cycles. The results obtained with ascorbic acid/O2 and dihydroxyfumaric acid/O2 as oxidants do not seem to be compatible with either a mechanism involving hydroxyl radicals as the active species or with the hypothesis that oxidation occurs through the initial formation of H2O2. Copyright 1999 John Wiley & Sons, Inc.

Unusual propargylic oxidations catalyzed by chloroperoxidase

This paper describes a new chloroperoxidase oxidative activity, namely, propargylic oxidations. Under appropriate conditions, chloroperoxidase catalyzes the oxidation of a variety of 2-alkynes to aldehydes via alcohol intermediates. Both hydrogen peroxide and t-butyl hydroperoxide can serve as terminal oxidants. The triple bond in the substrate is usually untouched. A free radical mechanism is proposed for the initial hydroxylation step in the overall reaction.