Utilising haem enzymes in catalytic oxidation chemistry
2014-12-15T10:35:51Z (GMT) by
The catalytic oxidation of a range of substrates by recombinant pea cytosolic ascorbate peroxidase (rAPX), cytochrome P450 BM3 (P450 BM3) and poly-L-leucine has been examined and, for rAPX and P450 BM3, the results rationalised using computer-based molecular modelling. Transient and steady state kinetics, together with a range of chromatographic and spectroscopic techniques, have been used to establish the mechanism and the products of the H2O2-dependent oxidation of p-cresol by rAPX. The results are discussed in the more general context of APX-catalysed aromatic oxidations. The oxidation of a number of thioethers by rAPX and a site-directed variant of rAPX (W41A) has been examined. Steady state oxidation of the majority of sulphides studied, gave values for kcat that are approximately 10-fold to 100-fold higher for W41a than for rAPX. For rAPX, essentially racemic mixtures of R- and S-sulphoxides were obtained for all sulphides. The W14A variant shows substantial enhancements in enantioselectivity. Structure-based modelling techniques have provided a fully quantitative rationalisation of all the experimentally determined enantioselectivity and have indicated a role for Arg38 in the control of stereoselectivity. Cytochrome P450 BM3 is shown to be a versatile catalyst for the oxidation of a wide range of organic molecules. In this study we have shown that the enzyme is able to catalyse the regio- and stereo-selective oxidation of alkyl aryl sulphides to corresponding sulphoxides with varying degrees of enantioselectivity. The effects of both the nature of the alkyl group and the electronic properties of the aryl group on both rate an the stereoselectivity of the biotransformation have been explored. The results from this study have been compared to the theoretical prediction for the asymmetric induction expected for cytochrome P450 BM3 catalysed reactions. Poly-L-leucine proved to be a satisfactory catalyst for the oxidation of alkyl aryl sulphides, although this has been shown to occur with no significant asymmetric induction.