Biofuels: Fungal, Bacterial and Insect Degraders of Lignocellulose

Abstract Lignocellulose is the main structural component of plant cell walls and can be degraded into simple sugars with the help of hydrolytic enzymes. Lignocellulose is composed of three polymers: cellulose, hemicellulose and lignin. Certain fungi, bacteria and insects have evolved the ability to degrade lignocellulose. Cellulose is degraded to glucose by the synergistic action of three distinct classes of enzymes: endoglucanases, exoglucanases and β‐glucosidases. Owing to its variable structure and organisation, hemicellulose degradation requires various enzymes with diverse modes of action, which include endoxylanases, endomannanases, β‐xylosidases, β‐mannosidases, β‐galactosidases, α‐glucuronidases, α‐arabinofurnosidases, α‐galactosidases, acetyl xylan esterases, feruloyl esterases and glucuronyl esterases. Lignin degradation is much more difficult due to its complex structure and bonding to carbohydrate complexes. It requires oxidative enzymes, such as lignin peroxidases, manganese peroxidases, versatile peroxidases, laccases, and several other auxiliary enzymes. Commercial biofuel production from lignocellulosic materials can be achieved by artificially producing these cellulases, hemicellulases and lignases in recombinant form. Key Concepts: Lignocellulose is the most abundant renewable resource of energy on the Earth. Lignocellulose is a general term referring to a natural complex of three biopolymers: cellulose, hemicellulose and lignin. Simple sugar can be released from lignocellulose using three categories of enzymes: cellulases, hemicellulase and lignases. Released sugars from lignocellulose can be fermented into biofuel (ethanol). Wood‐rotting fungi such as white‐rot, brown‐rot and soft‐rot fungi can degrade lignocellulose. Aerobic and anaerobic bacteria such as Bacillus, Streptomyces, Cellulomonas and Clostridium possess lignocellulose‐degrading enzymes. Wood and plant‐feeding insects such termites, roaches, longhorn beetles and grasshoppers are rich sources of novel lignocellulases genes. Lignocellulases of fungal, bacterial and insect origin can be artificially/recombinantly produced in various protein expression systems. Fungal, bacterial and insect lignocellulases are expected to play important roles in the development of successful lignocellulose conversion technologies.