1. Field of the Invention
The present invention relates to methods for degrading lignocellulosic materials.
2. Description of the Related Art
The majority of carbohydrates in plants are in the form of lignocellulose, which is composed of mainly cellulose, hemicellulose, pectin, and lignin. Lignocellulose is found, for example, in the stems, leaves, hulls, husks, and cobs of plants. Hydrolysis of these polymers releases a mixture of neutral sugars including glucose, xylose, mannose, galactose, and arabinose.
Cellulose is a polymer of the simple sugar glucose covalently bonded by beta-1,4-linkages. Many microorganisms produce enzymes that hydrolyze beta-linked glucans. These enzymes include endoglucanases, cellobiohydrolases, glucohydrolases and beta-glucosidases. Endoglucanases digest the cellulose polymer at random locations, opening it to attack by cellobiohydrolases. Glucohydrolases liberate molecules of glucose from the ends of the cellulose polymer. Cellobiohydrolases sequentially release molecules of cellobiose from the ends of the cellulose polymer. Cellobiose is a water-soluble beta-1,4-linked dimer of glucose. Beta-glucosidases hydrolyze cellobiose to glucose.
Hemicelluloses are short branched chain heteropolysaccharides that are composed of various hexoses (glucose, mannose and galactose), pentoses (D-xylose and L-arabinose), uronic acids, acetic acid, and other minor sugars. Similar to cellulose degradation, hemicellulose hydrolysis requires coordinated action of many enzymes, which can be placed into three general categories, the endo-acting enzymes that attack internal bonds within the polysaccharide chain, the exo-acting enzymes that act processively from either the reducing or nonreducing end of the polysaccharide chain, and the accessory enzymes (acetylesterases and esterases that hydrolyze lignin glycoside bonds).
Lignocellulosic materials, such as wood, herbaceous material, agricultural residues, corn fiber, waste paper, pulp and paper mill residues can be used to produce ethanol. No known natural organism can rapidly and efficiently metabolize all carbohydrate polymers in lignocellulosic biomass into ethanol. The conversion of lignocellulosic feedstocks into ethanol has the advantages of the ready availability of large amounts of feedstock, the desirability of avoiding burning or land filling the materials, and the cleanliness of the ethanol fuel. Once the cellulose is converted to glucose, the glucose is easily fermented by yeast into ethanol.
However, an obstacle to commercialization is the cost of enzymes to convert the lignocellulosic material to glucose and other fermentable sugars. There is a need in the art to improve the ability of cellulolytic enzymes to degrade lignocellulosic materials to useful organic products or to intermediates to useful end-products.
It is an object of the present invention to improve the ability of cellulolytic enzymes to degrade lignocellulosic materials.