Initial steps of ferulic acid polymerization by lignin peroxidase

The major products of the initial steps of ferulic acid polymerization by l ignin peroxidase included three dehydrodimers resulting from beta -5' and b eta-beta 'coupling and two trimers resulting from the addition of ferulic a cid moieties to decarboxylated derivatives of beta -O-4'- and beta -5'-coup led dehydrodimers. This is the first time that trimers have been identified from peroxidase-catalyzed oxidation of ferulic acid, and their formation a ppears to be favored by decarboxylation of dehydrodimer intermediates. Afte r initial oxidation, the coupling reactions appear to be determined by the chemistry of ferulic acid phenoxy radicals, regardless of the enzyme and of whether the reaction is performed in vitro or in vivo. This claim is suppo rted by our finding that horseradish peroxidase provides a similar product profile. Furthermore, two of the dehydrodimers were the two products obtain ed from laccase-catalyzed oxidation (Tatsumi, K. S., Freyer, A., Minard, R. D., and Bollag, J.-M.. (1994) Environ. Sci. Technol. 28, 210-215), and the most abundant dehydrodimer is the most prominent in grass cell walls (Ralp h, J,, Quideau, S,, Grabber, J. H., and Hatfield, R. D. (1994) J. Chem. Sec . Perkin Trans. 1, 3485-3498). Our results also indicate that the dehydrodi mers and trimers are further oxidized by lignin peroxidase, suggesting that they are only intermediates in the polymerization of ferulic acid. The ext ent of polymerization appears to be dependent on the ionization potential o f formed intermediates, H2O2 concentration, and, probably, enzyme stability .