Oxidation capacity of laccases and peroxidases as reflected in experiments with methoxy-substituted benzyl alcohols

A set of methoxy-substituted benzyl alcohol (MBA) congeners were examined regarding susceptibility to oxidation by Trametes versicolor laccase, T. versicolor lignin peroxidase and horseradish peroxidase: 2,4,5-trimethoxybenzyl alcohol (TMBA), 3,4,5-TMBA, 2,3,4-TMBA, 2,5-dimethoxybenzyl alcohol (DMBA), 3,4-DMBA, and 2,3-DMBA. The corresponding methoxy-substituted benzaldehydes were strongly predominant as products on enzymic oxidation. This together with different reaction rates and redox potentials makes the MBAs suitable as substrates in the characterization of ligninolytic enzymes. For fungal laccase, the reaction rate order was: 2,4,5-TMBA >> 2,5-DMBA > 3,4-DMBA > 3,4,5-TMBA similar to 2,3,4-TMBA similar to 2,3-DMBA. Horseradish peroxidase displayed a similar reactivity order. Oxidation of some of the MBAs with laccase and horseradish peroxidase was only observed when the reactions were carried out at low pH and with relatively high-substrate concentration. 3,4-DMBA (veratryl alcohol) was the best substrate for lignin peroxidase and the reaction rate order was: 3,4-DMBA > 2,4,5-TMBA similar to 3,4,5-TMBA > 2,5-DMBA > 2,3,4-TMBA similar to 2,3-DMBA. The oxidation experiments with different MBAs elucidate the potential of the enzymes as oxidants in various applications.