Glycoside Hydrolases in Plant Cell Wall Proteomes: Predicting Functions That Could Be Relevant for Improving Biomass Transformation Processes

Abstract

Glycoside hydrolases (GHs) are enzymes that are able to rearrange the plant cell wall polysaccharides, being developmental-and stress-regulated. Such proteins are used in enzymatic cocktails for biomass hydrolysis in the second-generation ethanol (E2G) production. In this chapter, we investigate GHs identified in plant cell wall proteomes by predicting their functions through alignment with homologous plant and microorganism sequences and identification of functional domains. Up to now, 49 cell wall GHs were identified in sugarcane and 114 in Brachypodium distachyon. We could point at candidate proteins that could be targeted to lower biomass recalcitrance. We more specifically addressed several GHs with predicted cellulase, hemicellulase, and pectinase activities, such as β-xylosidase, α and β-galactosidase, α-N-arabinofuranosidases, and glucan β-glucosidases. These enzymes are among the most used in enzymatic cocktails to deconstruct plant cell walls. As an example, the fungi arabinofuranosidases belonging to the GH51 family, which were also identified in sugarcane and B. distachyon, have already been associated to the degradation of hemicellulosic and pectic polysaccharides, through a peculiar mechanism, probably more efficient than other GH families. Future research will benefit from the information available here to design plant varieties with self-disassembly capacity, making the E2G more cost-effective through the use of more efficient enzymes.

Plant Cell Wall Proteomics: A Focus on Monocot Species, Brachypodium distachyon, Saccharum spp. and Oryza sativa

“…GHs are proteins involved in cell wall carbohydrates remodeling and can be regulated during development. These families are used in enzymatic cocktails for biomass degradation in second-generation ethanol production [49]. Literature data suggests that grasses present fewer GH1, GH16, GH28 and GH35 members than dicots and more GH5, GH13, GH18 and GH51 members [33,50], which is consistent with our comparative analysis on monocots vs .…”

“…GH5 are more numerous in monocots (Os05g0244500, Os10g0370500, Bradi2g31690, Bradi5g13550 and Bradi5g13560), and this data may correlate with the fact that (1,3) (1,4)-β- d -glucans are their putative substrates [53]. O. sativa studies identified several GH13 (Os08g0473600, Os09g0457800, Os08g0473900, Os09g0457400, Os06g0713800, Os02g0765600 and Os09g0457600), all α-amylases that are capable of hydrolizing 1,4-α-glucosidic linkages [49], which is consistent with O. sativa endogenous metabolism. Starch is a plant carbohydrate often linked to storage organs, and its breakdown is mediated by amylases.…”

“…Saccharum spp. GH17, such as GH1, are probably β-glucosidases [49]. The proportion of GH17 was also higher in monocots (see Supplementary Table S1 for accession numbers).…”

“…More members of GH51 were found in B. distachyon (Bradi4g26270, Bradi1g63990, Bradi4g43710 and Bradi1g57017) than in A. thaliana . These GH51 are probably α- l -arabinofuranosidases [49], a type of enzyme used for monocots biomass deconstruction, as they are rich in arabinoxylans. α- l -arabinofuranosidases act on hydrolysis of α- l -arabinofuranoside in α- l -arabinosides, together with hemicellulases, resulting in hemicellulose hydrolysis [49].…”

“…These GH51 are probably α- l -arabinofuranosidases [49], a type of enzyme used for monocots biomass deconstruction, as they are rich in arabinoxylans. α- l -arabinofuranosidases act on hydrolysis of α- l -arabinofuranoside in α- l -arabinosides, together with hemicellulases, resulting in hemicellulose hydrolysis [49]. It is important to mention that more studies related to substrate specificities and protein structure are necessary to establish the function of these proteins.…”