Abstract
An extracellular lignin-degrading enzyme from the basidiomycete Phanerochaete chrysosporium Burdsall was purified to homogeneity by ion-exchange chromatography. The 42,000-dalton ligninase contains one protoheme IX per molecule. It catalyzes, nonstereospecifically, several oxidations in the alkyl side chains of lignin-related compounds: Cα—Cβ cleavage in lignin-related compounds of the type aryl—CαHOH—CβHR—CγH2OH (R = -aryl or -O-aryl), oxidation of benzyl alcohols to aldehydes or ketones, intradiol cleavage in phenylglycol structures, and hydroxylation of benzylic methylene groups. It also catalyzes oxidative coupling of phenols, perhaps explaining the long-recognized association between phenol oxidation and lignin degradation. All reactions require H2O2. The Cα—Cβ cleavage and methylene hydroxylation reactions involve substrate oxygenation; the oxygen atom is from O2 and not H2O2. Thus the enzyme is an oxygenase, unique in its requirement for H2O2.
Keywords: hemoprotein, stereospecificity, 18O2 incorporation, white-rot fungi, wood decay
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