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.

Keywords: bioethanol; insects; protozoa; termites; recombinant enzymes; bioenergy; biomass; biorefining; renewable energy

Figure 1.

Scheme of biofuel production from lignocellulosic biomass. Degradation of lignocellulose into simple hexose and pentose sugars can be achieved by treatment with lignocellulases of fungal, bacterial and insect origin. Subsequently, the sugars can be fermented into ethanol, using yeast and bacteria. Modified from Rubin with permission from Nature Publishing Group .

Figure 2.

Structure of lignocellulose and schematic view of cellulose degradation (Genome Management Information System/Oak Ridge National Laboratory, USA).

Figure 3.

Structural components of hemicellulose and the hemicellulases responsible for their degradation. Modified from Shallom and Shoham with permission from Elsevier.

Figure 4.

Structure of lignin and schematic view of its degradation. Lignin is composed of three major phenolic components, namely p‐coumaryl alcohol (H), coniferyl alcohol (G) and sinapyl alcohol (S).

Figure 5.

Ribbon structures of typical endoglucanases and exoglucanases. The catalytic domain of an endoglucanase possesses an open groove to attack the cellulose chain at any point along its length, whereas the catalytic domain of an exoglucanase possesses four loops covering the tunnel through which only one of the terminal glucose unit of a cello‐oligosaccharide can be threaded. Modified from Davies and Henrissat with permission from Elsevier.

Figure 6.

Assembly of a typical cellulosome. Cellulosomes contain dockerins appended to enzymes and noncatalytic carbohydrate‐binding modules (CBM). Dockerins bind the cohesins to a noncatalytic scaffoldin. Scaffoldin possesses a cellulose‐specific binding domain (CBD) and a C‐terminal dockerin that attaches the cellulosome to cellulose and the bacterial cell membrane, respectively. Modified from Doi and Kosugi with permission from Nature Publishing Group.

Figure 7.

(a) Worker caste of the eastern subterranean termite, Reticulitermes flavipes. The arrows point the highly visible fermentation chamber of the hindgut. (b) Different regions of the worker termite gut. Modified from Scharf and Tartar .

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Further Reading

Carroll A and Somerville C (2009) Cellulosic Biofuels. Annual Review of Plant Biology 60: 165–182.

Decker SR, Siika‐Aho M and Viikari L (2009) Enzymatic depolymerization of plant cell wall hemicelluloses. In: Himmel ME (ed.) Biomass Recalcitrance: Deconstructing the Plant Cell wall for Bioenergy, pp. 352–373. Oxford, UK: Blackwell Publishing Ltd.

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Javier PFI, Oscar G, Sanz‐Aparicio J et al. (2007) Xylanases: molecular properties and applications. In: Polaina J and MacCabe AP (eds) Industrial Enzymes, pp. 65–82. Germany: Springer.

Koseki T, Fushinobu S, Ardiansyah   et al. (2009) Occurrence, properties, and applications of feruloyl esterases. Applied Microbiology and Biotechnology 84: 803–810.

Lundell TK, Mäkelä MR and Hildén K (2010) Lignin‐modifying enzymes in filamentous basidiomycetes – ecological, functional and phylogenetic review. Journal of Basic Microbiology 50: 5–20.

Martínez AT, Ruiz‐Dueñas FJ, Martínez MJ et al. (2009) Enzymatic delignification of plant cell wall: from nature to mill. Current Opinion in Biotechnology 20: 348–357.

Xu Q, Adney WS, Ding S‐Y and Himmel ME (2007) Cellulases for biomass conversion. In: Polaina J and MacCabe AP (eds) Industrial Enzymes, pp. 35–50. Germany: Springer.

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Sethi, Amit, and Scharf, Michael E(Feb 2013) Biofuels: Fungal, Bacterial and Insect Degraders of Lignocellulose. In: eLS. John Wiley & Sons Ltd, Chichester. http://www.els.net [doi: 10.1002/9780470015902.a0020374]