Epoxidations Monoxygenase Activity

The selective catalytic epoxidation of alkenes has become the most important reaction catalyzed by heme proteins in organic synthesis. As described above, the monoxygenase activity of a heme peroxidase is restricted to CPO due to the open substrate access of the ferryl subunit for this enzyme. HRP catalyzes epoxidations only after mutagenetic variations, as shown for the substrate trans-P-methylstyrene [234]. An exception of this rule is the regioselective epoxidation of (T.TJ-piperylpiperidide, which is successfully catalyzed by native HRP [265]. 

Excellent enantioselectivity is observed in the CP0/H202-catalyzed epoxidation of short-chain (Z)-alkenes with a chain length of nine of fewer carbon atoms, except for monosubstituted alkenes, which often function as reversible suicide inhibitors of the enzyme [266-271]. (E)-Alkenes are highly unreactive substrates and are converted to epoxides in yields below 5%. A number of functionalized (Z)-2-alkenes have been successfully epoxidized by CPO using tert-butyl hydroperoxide as the terminal oxidant [272]. This procedure appears to be more effective, especially in large-scale reactions, due to the fairly high sensitivity of CPO to hydrogen peroxide. 

Terminal alkenes can be suitable substrates if the chain is branched at the 2- or 3-position [249]. The oxidation of 3-hydroxy-l,4-pentadiene proceeds with 98% de and 65% ee, yielding predominantly (2S,3R)-l,2-epoxy-4-penten-3-ol, which is the 

The effect of chain length on the catalytic performance was investigated using a series of co-bromo-2-methylalkenes. In all cases the predominant enantiomer produced had the -configuration except for 3-bromo-2-methylpropene oxide, which was predominantly in the S-form due to the priority switch [274], The short propene and butene derivatives were converted quantitatively whereas the longer pentene, hexene and heptene substrates failed to convert completely. Many other functional groups such as carboxylic ester, methoxy, acetoxy and carbonic ester are accepted by the system. The epoxidation fails, however, for 4-hydroxy-2-methyl-l-butene as substrate [270]. 

Ortiz de Montellano, Cytochrome P450 - Structure, Mechanism and Biochemistry, 3rdedn., Kluwer Academic/ Plenum Publishers, New York, 2005. 

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Efforts to solubilize and isolate the P-450 monoxygenases from the microsomal membranes have been hampered by the facile conversion to a catalytically inactive form, cytochrome P-420. In 1968, however, a successful solubilization of catalytically active P-450 from hepatic microsomes was reported.30, 31 he solubilized system, which effected w-hydroxylation of fatty acids, required three coiq>onents for catalytic activity cytochrome P-450, NADPH-cytochrome c reductase, and a heat-stable, chloroform-soluble factor, the active component of which was identified as the microsomal lipid phosphatidylcholine.32 Further studies with this solubilized and reconstituted system have Indicated that the cytochrome P-450 and P-448 fractions have different catalytic activities 33-35 and that the terminal oxidase activity resides in the b-type cytochrome (P-450 or P-448) fraction rather than the cytochrome c reductase or lipid fractions.33, 36, 37 xhe cytochrome P-450 and P-448 were found to coiq>ete for reductase when present together.38 Cytochrome bs did not appear to be an obligatory component of the reconstituted systera.39 Cytochrome P-450 and P-448 fractions from rat liver were found to contain high levels of an epoxide hydrase, which can convert intermediate oxides to vicinal diols. Further purification has afforded fractions relatively free of cross-contamination of monoxygenase and hydrase enzymes. Recently, differential solubilization of monoxygenase activity towards a Type I substrate (naphthalene) and a Type II substrate (aniline) was... 

Andersson, T., M. Pesonen, and C. Johansson. 1985. Differential induction of cytochrome P-450-dependent monoxygenase, epoxide hydrolase, glutathione transferase and UDP glucuronyltransferase activities in the liver of the rainbow trout by 3-naphthoflavone or Clophen A50. Biochem. Pharmacol. 34 3309-3314. 

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