Lytic polysaccharide monooxygenase (LPMO)-derived saccharification of lignocellulosic biomass

Highlights

• LPMO plays a significant role in the saccharification of recalcitrant polysaccharides.

• LPMO catalyzes the oxidative cleavage of the C1 and/or C4 bonds of polysaccharides.

• Additionally, LPMO often exhibits synergistic effects with glycosidic hydrolases.

Abstract

Given that traditional biorefineries have been based on microbial fermentation to produce useful fuels, materials, and chemicals as metabolites, saccharification is an important step to obtain fermentable sugars from biomass. It is well-known that glycosidic hydrolases (GHs) are responsible for the saccharification of recalcitrant polysaccharides through hydrolysis, but the discovery of lytic polysaccharide monooxygenase (LPMO), which is a kind of oxidative enzyme involved in cleaving polysaccharides and boosting GH performance, has profoundly changed the understanding of enzyme-based saccharification. This review briefly introduces the classification, structural information, and catalytic mechanism of LPMOs. In addition to recombinant expression strategies, synergistic effects with GH are comprehensively discussed. Challenges and perspectives for LPMO-based saccharification on a large scale are also briefly mentioned. Ultimately, this review can provide insights for constructing an economically viable lignocellulose-based biorefinery system and a closed-carbon loop to cope with climate change.

Introduction

Concerns about global warming and depletion of fossil fuels have accelerated the development of diverse strategies to cope with climate change and achieve carbon neutrality. Given that biomass directly utilizes carbon dioxide through photosynthesis, not only biomass-derived biorefineries have been emerging to replace petroleum-based refineries for producing fuels, materials, and chemicals but also biomass is regarded as one of the most important feedstocks for constructing circular carbon economies (Moon et al., 2022, Park et al., 2022).

Recently, various biomasses including crops, algae, and lignocellulose have been used as renewable feedstocks in biorefineries (Moon et al., 2021, Park et al., 2022). Among them, non-edible lignocellulosic biomass is highlighted, because it circumvents the need for choosing between food and energy and can be produced in large quantities. In lignocellulose-based biorefineries, a thermochemical pretreatment (e.g., dilute acid, alkali, and organosolvent pretreatments) is a first step toward effective conversion in subsequent processes for deconstructing recalcitrant lignin components. Then, saccharification is usually performed to obtain fermentable sugars from cellulosic or hemicellulosic components. Finally, microbial fermentation utilizing fermentable sugars as carbon and energy sources produces target fuels, materials, and chemicals as metabolites (Yoo et al., 2020).

For saccharification of the pretreated lignocellulose, glycosidic hydrolases (GHs) have been widely applied for a long time. Cellulase and hemicellulase act on crystalline polysaccharides in lignocellulose to produce either oligomeric or monomeric sugars through hydrolysis. Thus, studies have focused on improving cellulase/hemicellulose production with high yields, engineering efficient and stable catalysts, developing feasible combinations for subsequent fermentation reactions, and elucidating the catalytic mechanisms (Guo et al., 2018, Min et al., 2021, Min et al., 2022). However, Vaaje-Kolstad et al. (2010) discovered for the first time that lytic polysaccharide monooxygenase (LPMO), a kind of Cu-containing chitin binding protein (CBP) with a flat binding surface, oxidatively boosts the decomposition of polysaccharides; thus, the identification of LPMO profoundly changed the understanding of enzymatic saccharification, which involves the oxidative cleavage of polysaccharides. CBP21 from Serratia marcescens can cleave glycosidic bonds in crystalline chitin, which is an analog of cellulose, and generate oxidized chain ends, thereby promoting decomposition by opening up the inaccessible polysaccharide for hydrolase-driven hydrolysis (Vaaje-Kolstad et al., 2010). Additionally, fundamental studies on the structure and catalytic mechanism of LPMO have been intensively performed to thoroughly uncover the previously unknown oxidative cleavage of cellulosic components (Eijsink et al., 2019, Forsberg et al., 2014a, Kont et al., 2020, Phillips et al., 2011, Quinlan et al., 2011, Simmons et al., 2017). Furthermore, recent studies have revealed that diverse wild-type and engineered LPMOs can contribute to the efficient saccharification of recalcitrant polysaccharides through their synergistic effects with GHs (Bernardi et al., 2021, Cheng et al., 2020, Guo et al., 2020, Ogunyewo et al., 2020, Zhou et al., 2020). Nonetheless, the application of LPMO for actual biorefinery processes has been hindered by drawbacks such as the difficulty of large-scale production and unknown operational stability.

There are many reviews describing the expression and characterization of LPMOs (Li et al., 2022a, Li et al., 2022b), whereas detailed information about their role in saccharification (e.g., synergistic effect) is still lacking thus far. Accordingly, this review mainly focuses on the role and significance of LPMOs in the saccharification of recalcitrant lignocellulosic biomass. The classification of LPMOs and information on their structure, catalytic mechanism, and production via heterologous expression are briefly introduced. In addition, recent progress and perspectives regarding feasible options are discussed including the LPMO-derived synergistic effect with GHs and practical applications in lignocellulose-based biorefineries.