Abstract
The hemoprotein ligninase of Phanerochaete chrysosporium Burds. catalyzes the oxidative cleavage of lignin model dimers between C alpha and C beta of their propyl side chains. The model dimers hitherto used give multiple products and complex stoichiometries upon enzymatic oxidation. Here we present experiments with a new model dimer, 1-(3,4-dimethoxyphenyl)-2-phenylethanediol (dimethoxyhydrobenzoin, DMHB) which is quantitatively cleaved by ligninase in air to give benzaldehyde and veratraldehyde according to the stoichiometry: 2DMHB + O2----2PhCHO + 2Ph(OMe)2CHO. Catalytic amounts of H2O2 are required for this aerobic reaction. Under anaerobic conditions, ligninase uses H2O2 as the oxidant for cleavage: DMHB + H2O2----PhCHO + Ph(OMe)2CHO. Electron spin resonance experiments done in the presence of spin traps, 2-methyl-2-nitrosopropane or 5,5-dimethyl-1-pyrroline-N-oxide, show that C alpha-C beta cleavage yields alpha-hydroxybenzyl radicals as intermediate products. Under anaerobic conditions, these radicals react further to give the final aldehyde products. In air, O2 adds to the carbon-centered radicals, probably giving alpha-hydroxybenzylperoxyl radicals which fragment to yield superoxide, benzaldehyde, and veratraldehyde. These results lead us to propose a mechanism for C alpha-C beta cleavage in which attack by ligninase and H2O2 on the methoxylated ring of DMHB yields a cation radical, which then cleaves to give either benzaldehyde and an alpha-hydroxy(dimethoxybenzyl) radical or veratraldehyde and an alpha-hydroxybenzyl radical (cf. Kersten, P. J., Tien, M., Kalyanaraman, B., and Kirk, T.K. (1985) J. Biol. Chem. 260, 2609-2612; Snook, M. E., and Hamilton, G. A. (1974) J. Am. Chem. Soc. 96, 860-869). Similar mechanisms probably apply to the enzymatic C alpha-C beta cleavage of natural lignin.
Highlights
From the $Departmentof Bacteriology, University of Wisconsin, Madison, Wiconsin 53706,the $Forest Products Laboratory, United States Department of Agriculture, Forest Service, Madison, Wisconsin 53705,and the IINationul Biomedical ESR Center, Medical College of Wisconsin, Milwaukee, Wisconsin53226
Lignin model dimers between C, and CBof their propyl catalyzes this nonstereospecific reaction with lignin and with side chains
We present experiments with Both HzOzand 0, are consumed during the ligninolytic rea new model dimer, 1-(3,4-dimethoxypheny1)-2- actions catalyzed by this ferric hemoprotein (8, 10)
Summary
The situation is spin trap2s,-methyl-2-nitrosopropane or 5,5-di- further complicated by the fact that all of the model dimers methyl-1-pyrroline-N-oxide,show that C,-CB cleavage used until now undergo sequential, branching reactions with yields a-hydroxybenzyl radicalsas intermediate prod- ligninase, since some of the products of C,-Co cleavage are ucts. Under anaerobic conditions, these radicals react themselves substrates for the enzyme (6, 8).
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