Abstract
Feruloyl esterases (FAEs) have an important role in the enzymatic conversion of lignocellulosic biomass by decoupling plant cell wall polysaccharides and lignin. Moreover, FAEs release anti-oxidative hydroxycinnamic acids (HCAs) from biomass. As a plethora of FAE candidates were found in fungal genomes, FAE classification related to substrate specificity is an indispensability for selection of most suitable candidates. Hence, linking distinct substrate specificities to a FAE classification, such as the recently classified FAE subfamilies (SF), is a promising approach to improve the application of these enzymes for a variety of industrial applications. In total, 14 FAEs that are classified members of SF1, 5, 6, 7, 9, and 13 were tested in this research. All FAEs were investigated for their activity toward a variety of substrates: synthetic model substrates, plant cell wall-derived substrates, including lignin, and natural substrates. Released HCAs were determined using reverse phase-ultra high performance liquid chromatography coupled to UV detection and mass spectrometry. Based on this study, FAEs of SF5 and SF7 showed the highest release of FA, pCA, and diFAs over the range of substrates, while FAEs of SF6 were comparable but less pronounced for diFAs release. These results suggest that SF5 and SF7 FAEs are promising enzymes for biorefinery applications, like the production of biofuels, where a complete degradation of the plant cell wall is desired. In contrast, SF6 FAEs might be of interest for industrial applications that require a high release of only FA and pCA, which are needed as precursors for the production of biochemicals. In contrast, FAEs of SF1, 9 and 13 showed an overall low release of HCAs from plant cell wall-derived and natural substrates. The obtained results substantiate the previous SF classification as a useful tool to predict the substrate specificity of FAEs, which eases the selection of FAE candidates for industrial applications.
Highlights
Plant polysaccharides within lignocellulosic biomass are considered a sustainable and green resource for the production of biobased chemicals and fuels after depolymerization (Mood et al, 2013; Loqué et al, 2015)
feruloyl esterase (FAE) of SF5 and SF7 showed the highest release of ferulic acid (FA), p-coumaric acid (pCA), and diferulic acid (diFA) over the range of substrates, while FAEs of SF6 were comparable but less pronounced for diFAs release
These results suggest that SF5 and SF7 FAEs are promising enzymes for biorefinery applications, like the production of biofuels, where a complete degradation of the plant cell wall is desired
Summary
Plant polysaccharides within lignocellulosic biomass are considered a sustainable and green resource for the production of biobased chemicals and fuels after depolymerization (Mood et al, 2013; Loqué et al, 2015). The major component of lignocellulosic biomass consists of the plant cell wall (PCW) – a network of cellulose, hemicellulose and lignin. This complex structure hinders the depolymerization of the PCW by, for example, fungal enzymes (Kabel et al, 2007; Pauly and Keegstra, 2008). The secondary layer is mainly composed of cellulose, which is embedded in a network consisting of hemicellulosic xylan and/or mannan, and the aromatic polymer lignin (Carpita and McCann, 2008; Harris and Stone, 2008). Beyond the complexity of the PCW polymers, the accessibility of enzymes is hindered by various cross-links, linking either two polysaccharide chains with each other or a polysaccharide chain to lignin. One of the major cross-links in the PCW is formed via one or two hydroxycinnamic acid (HCA) units, in particular ferulic acid (FA) (Harris and Stone, 2008)
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