Chloroperoxidase kinetics

An efficient method to prepare enantiomerically enriched hydroperoxides is the enzymatic kinetic resolution of racemic hydroperoxides using different kinds of enzymes (mainly lipases, chloroperoxidase, horseradish peroxidase). However, the scope of these reactions may be limit by the narrow substrate specificity of the enzyme. 

Hoft reported about the kinetic resolution of THPO (16b) by acylation catalyzed by different lipases (equation 12) °. Using lipases from Pseudomonas fluorescens, only low ee values were obtained even at high conversions of the hydroperoxide (best result after 96 hours with lipase PS conversion of 83% and ee of 37%). Better results were achieved by the same authors using pancreatin as a catalyst. With this lipase an ee of 96% could be obtained but only at high conversions (85%), so that the enantiomerically enriched (5 )-16b was isolated in poor yields (<20%). Unfortunately, this procedure was limited to secondary hydroperoxides. With tertiary 1-methyl-1-phenylpropyl hydroperoxide (17a) or 1-cyclohexyl-1-phenylethyl hydroperoxide (17b) no reaction was observed. The kinetic resolution of racemic hydroperoxides can also be achieved by chloroperoxidase (CPO) or Coprinus peroxidase (CiP) catalyzed enantioselective sulfoxidation of prochiral sulfides 22 with a racemic mixmre of chiral hydroperoxides. In 1992, Wong and coworkers and later Hoft and coworkers in 1995 ° investigated the CPO-catalyzed sulfoxidation with several chiral racemic hydroperoxides while the CiP-catalyzed kinetic resolution of phenylethyl hydroperoxide 16a was reported by Adam and coworkers (equation 13). The results are summarized in Table 4. 

As already reported in Section II.A.2, the enzymes chloroperoxidase (CPO) and Copri-nus peroxidase (CiP) catalyze the enantioselective oxidation of aryl alkyl sulfides. If a racemic mixture of a chiral secondary hydroperoxide is used as oxidant, kinetic resolution takes place and enantiomerically enriched hydroperoxides and the corresponding alcohols can be obtained together with the enantiomerically enriched sulfoxides. An overview of the results obtained in this reaction published by Wong and coworkers, Hoft and... 

Zhang R, Nagraj N, Lansakara-P DSP, Hager LP, Newcomb M (2006) Kinetics of Two-Electron Oxidations by the Compound I Derivative of Chloroperoxidase, A Model for Cytochrome P450 Oxidants. Org Lett 8 2731... 

Tanaka N, Hasan Z, Wever R (2003) Kinetic Characterization of Active Site Mutants Ser402Ala and Phe397His of Vanadium Chloroperoxidase from the Fungus Curvularia inaequalis. Inorg Chim Acta 356 288... 

Pasta P, Carrea G, Colonna S, Gaggero N (1994) Effects of Chloride on the Kinetics and Stereochemistry of Chloroperoxidase Catalyzed Oxidation of Sulfides. Biochim Biophys Acta 1209 203... 

Bhattacharjee369, who has brominated acetanilide suspended in aqueous solution of potassium bromide and hydrogen peroxide in presence of vanadium pent oxide catalyst. Chloroperoxidase has been mimicked using supported manganese phorphyrin catalyst in the oxychlorination of dimedone370 and by clay-supported iron(III) chloride in the oxy-chlorination of toluene and anisole371. The kinetics and mechanism of the haloper-oxidases have been studied extensively372,373. 

Steady-state kinetic studies showed that the kinetics of the enzyme resemble those of the vanadium bromoperoxidases. The chloroperoxidase exhibits a pH profile similar to vanadium bromoperoxidases although the optimal pH of 4.5-5.0 is at a lower value. At low pH the enzyme is inhibited by chloride in a competitive way whereas at higher pH values the activity displays normal Michaelis-Menten type of behavior (see Michaelis Constant). The log Km for chloride increases linearly with pH whereas that for hydrogen peroxide decreases with pH demonstrating that in the catalytic mechanism protons are involved. These observations have led to a simplified ping-pong type of mechanism for the chloroperoxidase similar to that shown in (Figure 1). 

Okazaki O, Guengerich FP. Evidence for specific base catalysis in M-dealkylation reactions catalyzed by cytochrome P450 and chloroperoxidase. Differences in rates of deprotonation of aminium radicals as an explanation for high kinetic hydrogen isotope effects observed with peroxidases. / Biol Chem 1993 268 1546-52. 

Since vanadium chloroperoxidase from Curvularia inaequalis is structurally closely related to acid phosphatases and transition metal oxoanions are potent inhibitors of the related phytases, (Figure 10.8) [35,36], Sheldon and coworkers investigated the peroxidase activity of phytase from Aspergillus ficuum in the presence of sodium orthovanadate Na3V04 [37-39]. Oxidation of thioanisole with H2O2 proceeded to produce the sulfoxide in quantitative yield in the presence of [VO4]-phytase. The reaction rate showed saturation kinetics with respect to the vanadate concentration, indicating a maximum rate of 120pmol/h (TOF = 11 min ) and a dissociation constant for the vanadate ion of 15.4 pM. 

In studies with chloroperoxidase (CPX), a peroxidative enzyme that is amazingly similar in certain properties to cytochrome P-450, we found the nitrosoarene metabolite to be the terminal product of arylamine oxidation (Fig. 6) (23, 31). CPX has been used to prepare nitrosoarene chemicals on a micro scale (17) because few chemical techniques are available for the direct conversion of arylamines to nitrosoarene compounds (28). It is probable that this enzymatic oxidation produces an intermediary hydroxylamine compound which, under the reaction conditions, is rapidly converted to the nitroso level. An apparent kinetic block in the oxidation of nitrosoarene to nitroaromatic compounds allows for the fairly selective production of the former by mild oxidants, particularly for those arylamines with electron-withdrawing substituents. 

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