Further Advances Needed to Improve Efficiency and Economics. Cellulase enzymes are used to break down the cellulose of plant cell walls into simple sugars that can be transformed (fermented) by microbes to fuels, primarily ethanol, as well as to chemicals, plastics, fibers, detergents, pharmaceuticals, and many other products.Like starch and sugar, cellulose is a carbohydrate (compound of carbon, hydrogen, and oxygen) made up of simple sugars (glucose) linked together in long chains called polysaccharides. These polymers form the structural portion of plant cell walls, and unraveling them is the key to economical ethanol fermentation. Technical barriers to large-scale use of cellulose technology include the low specific enzyme activity and high enzyme-production costs, as well as a general lack of understanding about enzyme biochemistry and mechanistic fundamentals. In 2004, the DOE National Renewable Energy Laboratory (NREL), working with two of the largest industrial enzyme producers (Genencor International and Novozymes Biotech), achieved a dramatic reduction in cellulase enzyme costs. Cellulases belong to a group of enzymes known as glycosyl hydrolases, which break (hydrolyze) bonds linking a carbohydrate to another molecule. The new technology involves a cocktail of three types of cellulases—endoglucanases, exoglucanases, and beta-glucosidases. These enzymes work together to attack cellulose chains, pulling them away from the crystalline structure and breaking off cellobiose molecules (two linked glucose residues), splitting them into individual glucose molecules, and making them available for further processing. This breakthrough work resulted in 20- to 30-fold cost reduction and earned NREL and collaborators an R&D 100 Award. Further cost reductions are required, however, to support an economical and robust cellulose biorefinery industry. For example, costs of amylase enzymes for converting corn grain starch to ethanol are about 1 to 2 cents per gallon of ethanol produced, but the most optimistic cost estimates for cellulase preparations now are about tenfold higher than that. Routes to improving enzyme efficiencies include the development of enzymes with more heat tolerance and higher specific activities, better matching of enzymes and plant cell-wall polymers, and development of high-solid enzymatic hydrolysis to lower capital costs. A comprehensive understanding of the structure and function of these enzymatic protein machines, how their production and activity are controlled, and changes they promote on plant cell-wall surfaces will be critical for success.
Credit or Source: Cellulase image from M. Himmel et al., Cellulase Animation, run time 11 min., National Renewable Energy Laboratory (2000).
U.S. DOE. 2006. Breaking the Biological Barriers to Cellulosic Ethanol: A Joint Research Agenda, DOE/SC/EE-0095, U.S. Department of Energy Office of Science and Office of Energy Efficiency and Renewable Energy. (p. 27) (website)