Lignin and Mn peroxidase-catalyzed oxidation of phenolic lignin oligomers

The oxidation of phenolic oligomers by lignin and manganese peroxidases was studied by transient-state kinetic methods. The reactivity of peroxidase i ntermediates compound I and compound II was studied with the phenol guaiaco l along with a beta-O-4 phenolic dimer, trimer, and tetramer. Compound I of both peroxidases is much more reactive than compound II. The rate constant s for these substrates with Mn peroxidase compound I range from 1.0 x 10(5) M-1 s(-1) for guaiacol to 1.1 x 10(3) M-1 s(-1) for the tetramer. Reactivi ty is much higher with Lignin peroxidase compound I with rate constants ran ging from 1.2 x 10(6) M(-1)s(-1) for guaiacol to 3.6 x 10(5) M-1 s(-1) for the tetramer. Rate constants with compound II. are much lower with Mn perox idase exhibiting very little reactivity. The rate constants dramatically de creased with both peroxidases as the size of the substrate increased. The e xtent of the decrease was much more dramatic with Mn peroxidase, leading us to conclude that, despite its ability to oxidize phenols, Mn2+ is the only physiologically significant substrate. The rate decrease associated with i ncreasing substrate size was more gradual with lignin peroxidase. These dat a indicate that whereas Mn peroxidase cannot efficiently directly oxidize t he lignin polymer, Lignin peroxidase is well suited for direct oxidation of polymeric lignin.