KINETIC CHARACTERIZATION OF A FUNGAL PEROXIDASE FROM COPRINUS-CINEREUS IN AQUEOUS AND ORGANIC MEDIA

Catalytic and structural characteristics of a fungal peroxidase from C oprinus cinereus (CiP) expressed by Aspergillus niger were investigate d in aqueous media and aqueous/dioxane solvents, and compared to the w ell-known peroxidase from horseradish (HRP). CiP is more thermostable than HRP; the half lives for the irreversible inactivation of CiP and HRP at 85 degrees C were 40 min and 21 min, respectively. The observed oxidation potential of CIP is similar to that of HRP. The kinetics fo r the oxidation of substituted phenols catalyzed by both peroxidases w ere measured to construct linear free energy relationships between the catalytic efficiency, V-max/K-m, of both enzymes and the electronic a nd hydrophobic properties of the phenolic substrates as represented by the Hammett constant, sigma and Hansch constant, pi, respectively. Ci P and HRP are similar in response to substrate hydrophobicity in diffe rent concentrations of dioxane; a finding not unexpected as substrate desolvation dominates hydrophobic interactions in organic media. The t wo peroxidases are quite dissimilar, however, in response to the elect ronic property of the substrate. In aqueous buffer, CiP is less sensit ive than HRP to the electron donating/withdrawing properties of the ph enolic substrates and shows a Hammett coefficient, rho, of -0.66. As t he dioxane content increased, however, CiP becomes more sensitive to t he electronic properties of the phenolic substrates with a rho value o f - 0.38 in 80% v/v dioxane. The dependence of rho on the dioxane cont ent suggests that the active site of CiP is exposed to the reaction me dium and this has a strong influence on the substrate specificity of t his fungal enzyme.