THE OXIDATION OF VERATRYL ALCOHOL, DIMERIC LIGNIN MODELS AND LIGNIN BY LIGNIN PEROXIDASE - THE REDOX CYCLE REVISITED

The mechanism of oxidation of veratryl alcohol and beta-O-4 dimeric lignin models is reviewed. Veratryl alcohol radicals are intermediates in both oxidation pathways. The possible role of the veratryl alcohol radical cation as a mediator is discussed. The lignin peroxidase (LiP) redox cycle is analyzed in terms of the Marcus theory of electron transfer. Reduction of both LiP-Compound I (LiP-I) and LiP-Compound II (LiP-II) by veratryl alcohol occurs in the endergonic region of the driving force. The reduction of LiP-II has a higher reorganization energy due to the change in spin state and the accompanying conformational change in the protein. It is suggested that a reversible nucleophilic addition of a carbohydrate residue located at the entrance of the active site channel plays a key role in the LiP redox cycle. Moreover, (polymeric) hydroxysubstituted benzyl radicals may reduce LiP-II via long-range electron transfer.