Evidence that Ceriporiopsis subvermispora degrades nonphenolic lignin structures by a one-electron-oxidation mechanism

The white-rot fungus Ceriporiopsis subvermispora is able to degrade nonphenolic lignin structures but appears to lack lignin peroxidase (LiP), which is generally thought to be responsible for these reactions. It is well established that LiP-producing fungi such as Phanerochaete chrysosporium degrade nonphenolic lignin via one-electron oxidation of its aromatic moieties, but little is known about ligninolytic mechanisms in apparent nonproducers of LiP such as C. subvermispora. To address this question, C. subvermispora and P. chrysosporium were grown on cellulose blocks and given two high-molecular-weight, polyethylene glycol-linked model compounds that represent the major nonphenolic arylglycerol-beta-aryl ether structure of lignin. The model compounds were designed so that their cleavage via one-electron oxidation would leave diagnostic fragments attached to the polyethylene glycol. One model compound was labeled with C-13 at C-alpha of its propyl side chain and carried ring alkoxyl substituents that favor C-alpha-C-beta cleavage after one-electron oxidation. The other model compound was labeled with C-13 at C-beta of its propyl side chain and carried ring alkoxyl substituents that favor C-beta-O-aryl cleavage after one-electron oxidation. To assess fungal degradation of the models, the high-molecular-weight metabolites derived from them were recovered from the cultures and analyzed by C-13 nuclear magnetic resonance spectrometry. The results showed that both C. subvermispora and P. chrysosporium degraded the models by routes indicative of one-electron oxidation. Therefore, the ligninolytic mechanisms of these two fungi are similar. C. subvermispora might use a cryptic LiP to catalyze these C-alpha-C-beta and C-beta-O-aryl cleavage reactions, but the data are also consistent with the involvement of some other one-electron oxidant.