Cellulases in the textile industry

Abstract

A primary goal of the National Energy Policy is to increase United States energy supplies using a more diverse mix of domestic resources and to reduce our dependence on imported oil. In 2002, fossil fuels, which are finite and non-renewable, supplied 86% of the energy consumed in this country. Even more alarming is that the United States imports over half (62%) of its petroleum, and dependency is increasing. In particular, gasoline and diesel constituted 98% of domestic transportation motor fuels in 2004. The United States gasoline consumption alone was about 138 billion gal/year in 2004. Corn ethanol supplies most of the remaining 2%. Bioethanol from cornstarch provides around 3 to 4 billion gallons of oxygenate that is splash-blended with gasoline to produce the common "gasohol." An Oak Ridge National Laboratory study, published in April 2005, indicated a potentially renewable feedstock base in the United States of over a billion tons per year that could generate 30% of current petroleum consumption. The feedstocks included forest thinnings, crop residues, bioenergy crops and wastes. Achieving this increase will require substantial RandD in feedstock production, harvesting, and land use. In order to efficiently utilize these lignocellulosic feedstocks, powerful new plant cell wall degrading enzymes will be required, especially cellulases. Feedstock costs will be a major component of the commodity end-product price. Therefore, yield of lignocellulose-derived sugars is perhaps of highest priority. Another impact on feedstock yield is associated with cellulases and other polysaccharide-degrading enzymes. These enzyme preparations must work efficiently to convert the dominant polysaccharides to monomers. Currently, high loadings of cellulases are needed to reach 95% conversion of cellulose in pretreated biomass in 35 days in a simultaneous saccharification and fermentation (SSF) experiment i.e. 2.2 lb (1 kg) of cellulase for 110 lb (50 kg) of cellulose (Grohmann et al., 1991). Cellulase preparations are expensive in the biorefinery context for two reasons: (1) the enzyme source, usually Trichoderma reesei, is costly to grow and induce and has limited cellulase productivity; and (2) specific enzyme performance or activity has not been improved by discovery or protein engineering in 30 years of research. A consequence of recent work announced by Genencor International was the significant breakthrough in reducing the cost to produce T. reesei cellulases from about $5/gal of ethanol to around $0.20/gal (Mitchinson et al., 2005). The biomass feedstocks most commonly considered for conversion are agricultural wastes, energy crops (perennial grasses and trees), and forest waste. The fermentable fractions of these feedstocks include cellulose (β-1,4-linked glucose) and hemicellulose, a substantial heterogeneous fraction composed of xylose and minor five- and six-carbon sugars. Although it is an abundant biopolymer, cellulose is unique because it is highly crystalline, water insoluble, and highly resistant to depolymerization. The definitive enzymatic degradation of cellulose to glucose, probably the most desirable fermentation feedstock, is generally accomplished by the synergistic action of three distinct classes of enzymes: i) The "endo-1, 4-β-glucanases" or 1,4-β-D-glucan 4-glucanohydrolases (EC 3.2.1.4), which act randomly on soluble and insoluble 1,4-β-glucan substrates and are commonly measured by detecting the reducing groups released from carboxymethylcellulose (CMC ii) The "exo-1,4-β-D-glucanases, " including both the 1,4-β-D-glucan glucohydrolases (EC 3.2.1.74), which liberate D-glucose from 1,4-β-D-glucans and hydrolyze D-cellobiose slowly, and 1,4-β-D-glucan cellobiohydrolase (EC 3.2.1.91), which liberates D-cellobiose from 1,4-β-glucans. iii) The " β-D- glucosidases" or β-D-glucoside glucohydrolases (EC 3.2.1.21), which act to release D-glucose units from cellobiose and soluble cellodextrins, as well as an array of glycosides. © 2007 Springer.