MODE OF ACTION AND ACTIVE-SITE OF AN EXTRACELLULAR PEROXIDASE FROM PLEUROTUS-OSTREATUS

The properties of the haem environment of an extracellular peroxidase from Pleurotus ostreatus were studied by electronic absorption spectro scopy. A high-spin ferric form was predominant in the native enzyme an d a high-spin ferrous form in the reduced enzyme. Cyanide was readily bound to the haem iron in the native form, thereby changing the enzyme to a low-spin cyano adduct. The electronic absorption spectra of the enzyme were similar to those of lignin peroxidase from Phanerochaete c hrysosporium. Compound III of the enzyme was formed after the addition of an excess of H2O2 to the native enzyme, and thereafter spontaneous ly reverted to the native form. The enzyme oxidized phenyl)-2-(2-metho xy-phenoxy)-1,3-dihydroxypropane in the presence of H2O2 to produce 1- (3,5-dimethoxy-4-hydroxyphenyl)-2-(2-methoxy- phenoxy)-1-oxo-3-hydroxy propane, 2,6-dimethoxyhydroquinone, 2-(2-methoxyphenoxy)-3-hydroxyprop anal, 2-(2-methoxyphenoxy)-3-hydroxypropanoic acid, 2,6-dimethoxy-1,4- benzoquinone and guaiacol. A similar oxidation pattern was demonstrate d with a one-electron oxidant, ammonium cerium(IV) nitrate. Free radic als were detected as intermediates of the enzyme-mediated oxidation of -dimethoxy-4-hydroxyphenyl)-2-(2-methoxyphenoxy)-1 ,3-dihydroxypropan e and acetosyringone. These results can be explained by the mechanisms involving an initial one-electron oxidation of the lignin substructur e. This radical may undergo C-alpha-C-beta cleavage, C-alpha-oxidation and alkyl-phenyl cleavage.