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Fig. 6 | Biotechnology for Biofuels

Fig. 6

From: The liquid fraction from hydrothermal pretreatment of wheat straw provides lytic polysaccharide monooxygenases with both electrons and H2O2 co-substrate

Fig. 6

Possible mechanisms of liquid fraction (LF)-driven degradation of polysaccharides by LPMO. a According to this mechanism, LPMO uses O2 co-substrate and the LF (represented by a phenolic compound) drives the LPMO reaction by generating excited electrons that are used in a monooxygenase reaction. An analogous mechanism has been proposed by Cannela et al. to explain the LPMO activity-boosting effect of light in the presence of pigments [71]. The excited electrons generated in the LF are used for the initial reduction of the LPMO and subsequent catalysis via an LPMO/polysaccharide/O2 ternary complex and including delivery of a second electron. The generation of excited electrons in LF may be stimulated (e.g., by light as shown in the scheme by ) but stimulation is not a necessary assumption here. Note that many different mechanisms have been proposed for O2-driven catalysis but all these mechanisms assume the delivery of two electrons per one cleavage of glycosidic bond [27, 28]. b According to this mechanism the LPMO uses H2O2 as co-substrate and the LF drives LPMO reaction by generating H2O2. The slow, LPMO-independent, formation of H2O2 in the reaction between O2 and LF is rate-limiting for LPMO catalysis. This mechanism also assumes the delivery of electrons by LF but here the electrons are used only in the “priming reduction” of the LPMO (from Cu(II) to Cu(I) form). Primed LPMO can catalyze a number of oxidative cleavages of glycosidic bonds until the polysaccharide free LPMO happens to be re-oxidized, either by O2 or H2O2 [46, 48, 50]. Note that re-oxidation of the LPMO by H2O2 may lead to irreversible inactivation. Re-oxidation of a reduced LPMO by O2 may also generate H2O2 [73]. However, in our experiments, the rates of the routes involving LPMO re-oxidation must have been insignificant compared to the rate of formation of soluble LPMO products since product formation was independent of the LPMO concentration

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