Abstract
Biomass includes cellulose, hemicelluloses, pectin and lignin; constitutes the components of dietary fibre of plant and alge origins in animals and humans; and can potentially provide inexhaustible basic monomer compounds for developing sustainable biofuels and biomaterials for the world. Development of efficacious cellulases is the key to unlock the biomass polymer and unleash its potential applications in society. Upon reviewing the current literature of cellulase research, two characterized and/or engineered glycosyl hydrolase family-5 (GH5) cellulases have displayed unique properties of processive endoglucanases, including GH5-tCel5A1 that was engineered and was originally identified via targeted genome sequencing of the extremely thermophilic Thermotoga maritima and GH5-p4818Cel5_2A that was screened out of the porcine hindgut microbial metagenomic expression library. Both GH5-tCel5A1 and GH5-p4818Cel5_2A have been characterized as having small molecular weights with an estimated spherical diameter at or < 4.6 nm; being monomodular without a required carbohydrate-binding domain; and acting as processive β-1,4-endoglucanases. These two unique GH5-tCel5A1 and GH5-p4818Cel5_2A processive endocellulases are active in hydrolyzing natural crystalline and pre-treated cellulosic substrates and have multi-functionality towards several hemicelluloses including β-glucans, xylan, xylogulcans, mannans, galactomannans and glucomannans. Therefore, these two multifunctional and monomodular GH5-tCel5A1 and GH5-p4818Cel5_2A endocellulases already have promising structural and functional properties for further optimization and industrial applications.
Highlights
Biomass primarily of plant cell wall and alge polysaccharides, including cellulose, hemicelluloses, pectin and lignin, constitutes the major constituents of dietary fibre [1,2].As an important dietary polymer nutrient, dietary fibre is utilized by various species of animals [2,3]
The highest wild-type bacterial free cellulase activity was reported at about 2.5 U/mg protein on Avicel for a GH9 processive endoglucanase originated from Clostridium phytofermentans [78] and a glycosyl hydrolase family-5 (GH5) bacterial endoglucanase screened from the Buffalo rumen metagenomic library [55]
Another layer of biomass resistance is the restricted nanoporosity, which reflects the crosslinks formed by hemicellulose & pectic polysaccharides embedding each elementary fibril via non-covalent bonds and the lignin polymers surrounding the exterior of each microfibril via covalent bonds, and is related to free space across microfibrils and macrofibrils in the biomatrix [109]
Summary
Biomass primarily of plant cell wall and alge polysaccharides, including cellulose, hemicelluloses, pectin and lignin, constitutes the major constituents of dietary fibre [1,2]. Lignin, representing a large group of aromatic polymers, and native cellulose of the quasi-crystalline structure in the lignocellulose composite are mainly responsible for the recalcitrance of plant cell wall by limiting accessibility to biomass degrading microbial enzymes [4,5,6,7,8]. These research reports suggest that a unique and efficient microbial fibre degradation system that adapts to a faster rate of food digesta passage has evolved in the pig intestinal system for the discovery of novel and efficacious fibre degradation enzymes
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