Effect of lignin fractions isolated from different biomass sources on cellulose oxidation by fungal lytic polysaccharide monooxygenases

Abstract

BackgroundLytic polysaccharide monooxygenases (LPMOs) are copper-dependent enzymes that oxidatively cleave recalcitrant lignocellulose in the presence of oxygen or hydrogen peroxide as co-substrate and a reducing agent as electron donor. One of the possible systems that provide electrons to the LPMOs active site and promote the polysaccharide degradation involves the mediation of phenolic agents, such as lignin, low-molecular-weight lignin-derived compounds and other plant phenols. In the present work, the interaction of the bulk insoluble lignin fraction extracted from pretreated biomass with LPMOs and the ability to provide electrons to the active site of the enzymes is studied.ResultsThe catalytic efficiency of three LPMOs, namely MtLPMO9 with C1/C4 regioselectivity, PcLPMO9D which is a C1 active LPMO and NcLPMO9C which is a C4 LPMO, was evaluated in the presence of different lignins. It was correlated with the physicochemical and structural properties of lignins, such as the molecular weight and the composition of aromatic and aliphatic hydroxyl groups. Moreover, the redox potential of lignins was determined with the use of large amplitude Fourier Transform alternating current cyclic voltammetry method and compared to the formal potential of the Cu (II) center in the active site of the LPMOs, providing more information about the lignin-LPMO interaction. The results demonstrated the existence of low-molecular weight lignin-derived compounds that are diffused in the reaction medium, which are able to reduce the enzyme active site and subsequently utilize additional electrons from the insoluble lignin fraction to promote the LPMO oxidative activity. Regarding the bulk lignin fractions, those isolated from the organosolv pretreated materials served as the best candidates in supplying electrons to the soluble compounds and, finally, to the enzymes. This difference, based on biomass pretreatment, was also demonstrated by the activity of LPMOs on natural substrates in the presence and absence of ascorbic acid as additional reducing agent.ConclusionsLignins can support the action of LPMOs and serve indirectly as electron donors through low-molecular-weight soluble compounds. This ability depends on their physicochemical and structural properties and is related to the biomass source and pretreatment method.Electronic supplementary materialThe online version of this article (10.1186/s13068-018-1294-6) contains supplementary material, which is available to authorized users.

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

“…Numerous reports have shown that process samples of lignocellulose refining support LPMO activity [7, 30, 31, 36–38]. The positive effect on LPMO activity has been assigned to the electron donating ability of lignin and lignin-derived, mostly phenolic compounds.…”

“…According to the originally proposed mechanism [5], LPMOs require O 2 as a co-substrate and delivery of two electrons from an external electron donor per glycosidic bond cleavage [27, 28]. A number of different compounds can support LPMOs with electrons [29], including phenolic compounds [7, 30–35] or lignin and its fragments [7, 31, 36–38], which are both expected to be present in liquid fractions emerging during thermochemical pretreatment of biomass [3, 39–45].…”

Enzymatic processing of lignocellulosic biomass: principles, recent advances and perspectives

“…Indeed, Harris et al observed that the boosting effect of an LPMO on cellulase action required the presence of other compounds in the biomass, most likely lignin-derived [134]. In retrospect, it is clear that these observations relate to the reducing power that is present in biomass but not in model cellulosic substrates such as Avicel [134,143,247]. In an important study, Eibinger et al used confocal microscopy to show that a cellulolytic LPMO from N. crassa primarily acts on surface-exposed crystalline areas of the cellulose and that LPMO treatment promoted adsorption of a CBH, TrCel7A, to these crystalline regions, resulting in more efficient hydrolysis of these cellulose crystals [96].…”

Catalytic Fast Pyrolysis of Lignin Isolated by Hybrid Organosolv—Steam Explosion Pretreatment of Hardwood and Softwood Biomass for the Production of Phenolics and Aromatics

“…The spruce and birch lignins used in this study, which were isolated by the hybrid organosolv-steam explosion method, contained a very low amount of ash (<0.1 wt.%), as well as minimal carbohydrate impurities, i.e., hemicellulose (<2 wt.%) and cellulose (<1 wt.%) [34,35]. Their molecular weights were also in the range of previously reported values for similar organosolv-type lignins, with a relatively narrow molecular mass distribution as evidenced by the low polydispersity index (PDI) values [22,85,87,90,91]. Table 1.…”