Polyphenol Oxidase Products Are Priming Agents for LPMO Peroxygenase Activity

Polyphenol oxidases catalyze the hydroxylation of monophenols to diphenols, which are reducing agents for lytic polysaccharide monooxygenases (LPMOs) in their degradation of cellulose. In particular, the polyphenol oxidase MtPPO7 from Myceliophthora thermophila converts lignocellulose-derived monophenols, and under the new perspective of the peroxygenase reaction catalyzed by LPMOs, we aim to differentiate the role of the catalytic products of MtPPO7 in priming and fueling of LPMO activity. Exemplified by the activity of MtPPO7 towards guaiacol and by using the benchmark LPMO NcAA9C from Neurospora crassa we show that MtPPO7 catalytic products provide the initial electron for the reduction of Cu(II) to Cu(I) but cannot provide the required reducing power for continuous fueling of the LPMO. The priming reaction is shown to occur with catalytic amounts of MtPPO7 products and those compounds do not generate substantial amounts of H2O2 in situ to fuel the LPMO peroxygenase activity. Reducing agents with a low propensity to generate H2O2 can provide the means for controlling the LPMO catalysis through exogenous H2O2 and thereby minimize any enzyme inactivation.

Automated summary

Polyphenol oxidases (PPOs) catalyze the conversion of monophenols to diphenols, which act as reducing agents for lytic polysaccharide monooxygenases (LPMOs). The role of the catalytic products of the PPO MtPPO7 in priming and fueling the activity of the LPMO NcAA9C is investigated. It is found that the catalytic products of MtPPO7 provide the initial electron for the reduction of Cu(II) to Cu(I), but cannot provide sufficient reducing power for continuous fueling of the LPMO activity. This priming reaction occurs with low amounts of MtPPO7 products and does not generate significant levels of H2O2 to fuel the LPMO peroxygenase activity.