Reductants fuel lytic polysaccharide monooxygenase activity in a pH-dependent manner

Polysaccharide-degrading mono-copper lytic polysaccharide monooxygenases (LPMOs) are efficient peroxygenases that require electron donors (reductants) to remain in the active Cu(I) form and to generate the H2O2 co-substrate from molecular oxygen. Here, we show how commonly used reductants affect LPMO catalysis in a pH-dependent manner. Between pH 6.0 and 8.0, reactions with ascorbic acid show little pH dependency, whereas reactions with gallic acid become much faster at increased pH. These dependencies correlate with the reductant ionization state, which affects its ability to react with molecular oxygen and generate H2O2. The correlation does not apply to l-cysteine because, as shown by stopped-flow kinetics, increased H2O2 production at higher pH is counteracted by increased binding of l-cysteine to the copper active site. The findings highlight the importance of the choice of reductant and pH in LPMO reactions.

Automated summary

The catalytic activity of polysaccharide-degrading mono-copper lytic polysaccharide monooxygenases (LPMOs) is affected by the choice of reductant and pH. The reaction rate with ascorbic acid is not pH-dependent between pH 6.0 and 8.0, while the reaction rate with gallic acid increases at higher pH. This correlation is attributed to the ionization state of the reductant, which affects its ability to generate H2O2 from molecular oxygen. However, l-cysteine shows a different behavior due to increased binding to the copper active site at higher pH, counteracting the increase in H2O2 production.