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.

A GHF7 CELLULASE FROM THE PROTIST SYMBIONT COMMUNITY OF Reticulitermes flavipes ENABLES MORE EFFICIENT LIGNOCELLULOSE PROCESSING BY HOST ENZYMES

“…The presence of TFLs in translated GHF7 peptide sequences is a characteristic of some CBHs (also known as “exoglucanases”); whereas, a lack of TFLs is a characteristic of some EGases (e.g., Todaka et al., ; Zhou et al., , ; Sethi and Scharf, ). No TFLs were predicted in GHF7‐3, but two putative TFLs were present in GHF7‐5 and ‐6 (Fig.…”

“…Lignocellulose is the most sustainable form of plant biomass available on Earth. Lignocellulose consists mainly of the β‐1,4‐sugar polymers cellulose and hemicellulose, and the phenolic polymer lignin (Pettersen, ; Sethi and Scharf, ). The major obstacle to efficient usage of lignocellulose as a feedstock in biofuel production is the inefficiency of industrial lignocellulose degradation.…”

“…The major obstacle to efficient usage of lignocellulose as a feedstock in biofuel production is the inefficiency of industrial lignocellulose degradation. Therefore, it is practical to study natural systems where lignocellulose is efficiently degraded into useful monosaccharides; one such natural system is termites (Ohkuma, ; Scharf and Tartar, ; Watanabe and Tokuda, ; Sethi and Scharf, ). Subterranean termites efficiently digest lignocellulose through a coevolved mixture of host digestive enzymes and enzymes produced by microbial gut symbionts (Watanabe et al., ; Warnecke et al., ; Tartar et al., ; Scharf et al., , ).…”

“…Moreover, based on recent metatranscriptomic evidence, effective mixtures of coevolved lignocellulases should act on cellulose, hemicellulose, and lignin phenolics, as well as protect digestive enzymes and gut symbionts from xenobiotic/oxidative stress created during lignocellulose degradation (Raychoudhury et al., ; Sethi et al., ). Such enzyme mixtures from termites and their gut symbiota should thus be optimally active for degrading wood lignocellulose into fermentable monosaccharides (Sethi and Scharf, ).…”

Anaerobic co-digestion of pig manure and algae: Impact of intracellular algal products recovery on co-digestion performance

“…cell wall has been described as a rigid wall of cellulose and hemicellulose, which together with the sporopollenin-like biopolymer provides great resistance to enzymatic degradation (Mendez et al, 2014;Mussgnug et al, 2010). Lu et al (2013) reported the occurrence of Lactobacillus and Clostridia in fresh pig manure, independent of pig age and diet, while other studies have shown that Lactobacillus and Clostridia are very effective at degrading cellulosic organic matter (Calderon Santoyo et al, 2003;Li and Liu, 2012;Mussatto et al, 2008;Sethi and Scharf, 2001).…”