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 H 2 O 2 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 H 2 O 2 . The correlation does not apply to l-cysteine because, as shown by stopped-flow kinetics, increased H 2 O 2 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.