BackgroundEnzyme-aided valorization of lignocellulose represents a green and sustainable alternative to the traditional chemical industry. The recently discovered lytic polysaccharide monooxygenases (LPMOs) are important components of the state-of-the art enzyme cocktails for cellulose conversion. Yet, these monocopper enzymes are poorly characterized in terms of their kinetics, as exemplified by the growing evidence for that H2O2 may be a more efficient co-substrate for LPMOs than O2. LPMOs need external electron donors and one key question of relevance for bioprocess development is whether the required reducing power may be provided by the lignocellulosic substrate.ResultsHere, we show that the liquid fraction (LF) resulting from hydrothermal pretreatment of wheat straw supports LPMO activity on both chitin and cellulose. The initial, transient activity burst of the LPMO reaction was caused by the H2O2 present in the LF before addition of LPMO, while the steady-state rate of LPMO reaction was limited by the LPMO-independent production of H2O2 in the LF. H2O2 is an intermediate of LF oxidation as evidenced by a slow H2O2 accumulation in LF, despite high H2O2 production rates. This H2O2 scavenging ability of LF is important since high concentrations of H2O2 may lead to irreversible inactivation of LPMOs.ConclusionsOur results support the growing understanding that fine-tuned control over the rates of H2O2 production and consumption in different, enzymatic and non-enzymatic reactions is essential for harnessing the full catalytic potential of LPMOs in lignocellulose valorization.