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CELLULASES AND HEMICELLULASES FROM THE THERMOPHILIC FUNGUS THERMOASCUS AURANTIACUS

Objective

The objective of the proposed work was to improve the feasibility of enzymatic conversion of cellulosic materials into sugars by gaining a better understanding of the factors that effect the efficiency of conversion by the enzymes responsible. The released sugars can be used as a carbon source for fermentations generating chemicals, pharmaceuticals and fuels, as well as in modification of animal feed. Such plant cell wall degrading enzymes also find applications in fibre processing, vegetable oil extraction, flavour release, textiles, detergent and chemicals. The main problems with current commercial enzymes relate to heat-stability, activity and pH profile. In both projects reported here thermophilic (heat loving) organisms are used since these provide a source of more stable enzymes. However, the approaches differ, reflecting differences between the bacterial system (in which the enzymes are organised into a complex known as the cellulosome) and the fungal system in which a wide range of enzymes are released into the growth medium.

The primary objective is to obtain high yields of stable and efficient enzymes from a thermophilic fungus, Thermoascus aurantiacus, which produces a complete cellulase and hemicellulase enzyme system, for use in valorization of agricultural wastes of agronomic importance from colder (wheat straw) and warmer (sorghum bagasse) regions of the Community. Changes in policy concerning wheat straw burning combined with an increasing interest in sweet sorghum as a non-food crop will result in millions of tonnes of such residues becoming available. Biological treatment will yield sugars, chemicals, environmentally friendly paper pulp, peat substitutes and organic fertilizers. In a wider context the cellulolytic enzymes have applications in depolymerization of wastes from the agro-food industries and in the food industry for example in preparation of instant coffee, natural flavours and colours, and cleaning of industrial filters. Previous work on bacterial and fungal systems has established mechanisms, and exposed the limitations of the commercially available enzymes. This is the first integrated project aimed at overcoming deficiencies in enzyme yield, stability and specific activity using a thermophilic fungus, in this case T. aurantiacus.

This work will produce enzymes to be used at or above 70 ºC in industrial processes. Although these enzymes are potentially suited for commercial use, their yield and efficiency require improvement.

This will be done by protein engineering and cloning into T. reesei, whilst strain improvement of T. aurantiacus will also be attempted using UV and chemical mutagenesis. Mutants resistant to catabolite repression will be selected. Individual enzymes will be purified and characterized using novel substrates and physical techniques including crystallography. Based on their structure-function relationships, research will focus on cloning and modifying these enzymes in order to increase their yield and catalytic efficiency in collaboration with an industrial partner. Cellulase and hemicellulase production by wild and mutant strains will be optimized at pilot scale and used in trials for pulping and saccharification.
To date, a number of proteins enzymes have been isolated, purified and screened for use in saccharification, textiles, stone-washing and animal feed. Crude filtrates fractionated using ion-exchange, gel filtration, chromato-focusing and affinity chromatographies yielded seven cellulases (two b-glucosidases, two endoglucanases and four exoglucanases), six hemicellulases (two endoxylanases, one xylosidase, two acetyl esterase and one arabinofuranosidase with xylosidase activity). The major b-glucosidase, the major endoglucanase and the major endoxylanase have been characterised and the N-terminal sequence of four cellulase (major b-glucosidasemajor endoglucanase and two exoglucanases) components investigated. In addition, antibodies have been raised for six cellulase components and the amino acid composition of all seven cellulase components have been determined. Enzyme purification and characterisation was aided by the synthesis of specific affinity gels, model cello-oligosaccharides, specific ligands and inhibitors. Scale up of production and evaluation of low-cost culture medium were investigated for both submerged culture and solid state fermentation. Mutagenesis and cloning to enable the more interesting proteins to be expressed in other systems and/or to provide information which could enable the thermal stability of existing systems to be improved, as well as to increase the yield of protein and decrease end product and catabolic inhibition, have both proved more difficult than anticipated and are receiving priority in the final year. Structural studies have been initiated with production of good crystals and subsequent X-ray data-sets for the major endoglucanase and endoxylanase, which are currently being resolved.

Exploitation

Both industrial partners obtained results capable of further development to commercial products. Sufficient information concerning potential uses of the enzymes in a number of applications as well as initial results from cloning of genes into another fungus should enable development towards commercial applications. A series of affinity materials, substrates and ligands were also developed which will be available to other researchers investigating glycolases (polysaccharide degrading enzymes). The information software developed is being developed further. The possibility of extending the Information System as an educational tool is being investigated.
Plant cell walls contain three main components: cellulose which is insoluble and forms crystalline, compact fibres which are highly resistant to degradation; hemicellulose which is amorphous and more easily degraded; and lignin. The thermophilic fungus Thermoascus aurantiacus produces active hemicellulases - in particular, xylanase.

Activities

Three approaches were taken to improve the cost-effectiveness of protein production: optimisation of growth conditions; production of mutants and cloning. In addition the links between function, structure and enhanced thermal stability were investigated as were possible applications. A PC-based information system covering many aspects of the production, science and use of plant cell wall degrading enzymes was constructed.

Methods

A number of strains of T.a were characterised and distributed to the participants. Attempts were made to optimise culture conditions, using low cost media, so that T.a produced elevated levels of cellulase and hemicellulase, in both liquid and solid culture as well as to investigate and model the production and distribution of enzymes between the cells and the media. Bulk filtrates were produced for protein purification and characterisation, using conventional techniques of ion exchange, gel filtration and isoelectric focusing as well as novel affinity gels produced during the project. Fractionation and purification procedures were established for seven cellulase (two b-glucosidases, two endoglucanases and four exoglucanases) and six hemicellulase (two endoxylanases, one xylosidase, two acetyl esterase and one arabinofuranosidase with xylosidase activity) components which were purified and characterised.

Artificial substrates, specific ligands and inhibitors, including xyloside, xylobioside, glucoside, lactoside, cellobioside and cellotrioside of 4 methyl umbelliferone (MU) were prepared where necessary (RUG) and used to characterise proteins and label crystals for X-ray analysis in structural work. In addition 4-MU derivatives of xylobiose and b-D-glucopyranosyl-xylopyranose were prepared (used to differentiate xylanases belonging to family F and family G), as were acetate esters and esters derived from dihydroferulic acid and 4-hydroxyphenylpropionic acid (used to differentiate acetyl esterase and ferulic acid esterase activities). Antibodies were raised for six cellulase components and the amino acid composition of all seven cellulase components and xylanase determined. The major endoglucanase and the major xylanase were both crystallised for structural studies resulting in a final model for xylanase, comparison of which with other family 10 xylanases, family 5 cellulases and family 17 barley glucanases suggests that hydrophobic interactions, hydrogen bonds and helix dipole stabilisation contribute to the thermostability of T.a and the ways in which substrate specificity is effected. Structure studies of the major endoglucanase from T.a progressed fairly well, resulting in a good initial map, which will provide detailed information on substrate binding once the structure is fully refined.

Enzymes were screened in various applications, including saccharification, fabric processing, animal feed and human food modification. The possibility of using some of the enzymes in catalytic synthesis was also evaluated. An integrated information system covering plant cell wall degrading enzymes was constructed, combining an `expert system' based on multimedia presentation software covering organisms, enzymes and applications with various databases and graphic presentation packages covering composition of lignocellulosic raw materials, structure of plant cell wall degrading enzymes, commercial suppliers of enzymes, glossary and reference sections, accessible through an integrated front end, which could be developed further as a CD-ROM or site on the World Wide Web.

Funding Scheme

CSC - Cost-sharing contracts

Coordinator

CPL Scientific Ltd
Address
43 Kingfisher Court Hambridge Road
RG14 5SJ Newbury
United Kingdom

Participants (5)

Cayla SARL
France
Address
Avenue De Larrieu
31094 Toulouse
Institut National de la Recherche Agronomique (INRA)
France
Address
369 Rue Jules Guesde
59651 Villeneuve-d'ascq
Institute of Food Research
United Kingdom
Address
Reading Laboratory Earley Gate Whiteknights Road
RG6 2EF Reading
NATIONAL TECHNICAL UNIVERSITY OF ATHENS
Greece
Address
Iroon Polytechniou 5
15780 Athens
RIJKSUNIVERSITEIT GENT
Belgium
Address
Sint-pietersnieuwstraat 25
9000 Gent