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Cellulose Structure and Hydrolysis Challenges

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Within the plant cell wall, chains of cellulose molecules associate with other polymers to form linear structures of high tensile strength known as microfibrils. Layers upon layers of microfibrils make up the cell wall. Each microfibril is about 10 to 20 nm in diameter and may consist of up to 40 cellulose chains. A microfibril's crystalline and paracrystalline (amorphous) cellulose core is surrounded by hemicellulose, a branched polymer composed of a mix of primarily pentose sugars (xylose, arabinose), and some hexoses (mannose, galactose, glucose). In addition to cross-linking individual microfibrils, hemicellulose also forms covalent associations with lignin, a rigid aromatic polymer. Lignin is not pictured since its structure and organization within the cell wall are poorly understood. Pretreatment of biomass with enzymes or acids is necessary to remove the surrounding matrix of hemicellulose and lignin from the cellulose core prior to hydrolysis. The crystallinity of cellulose presents another challenge to efficient hydrolysis. The high degree of hydrogen bonding that occurs among the sugar subunits within and between cellulose chains forms a 3D lattice-like structure. The highly ordered, water-insoluble nature of crystalline cellulose makes access and hydrolysis of the cellulose chains difficult for the aqueous solutions of enzymes. Paracrystalline cellulose lacks this high degree of hydrogen bonding, thus giving it a structure that is less ordered. Each cellulose molecule is a linear polymer of thousands of glucose residues. Cellobiose, which consists of a pair of glucose residues (one right side up and one upside down) is the repeating unit of cellulose. [Microfibril portion of this figure adapted from J. K. C. Rose and A. B. Bennett, Cooperative Disassembly of the Cellulose-Xyloglucan Network of Plant Cell Walls: Parallels Between Cell Expansion and Fruit Ripening, Trends Plant Sci. 4, 176-83 (1999).]

Credit or Source: Office of Biological and Environmental Research of the U.S. Department of Energy Office of Science. science.energy.gov/ber/

Citation(s):

US DOE. 2005. Genomics:GTL Roadmap, DOE/SC-0090, U.S. Department of Energy Office of Science. (p. 204) (website)

Prepared by the Biological and Environmental Research Information System, Oak Ridge National Laboratory, genomicscience.energy.gov/ and genomics.energy.gov/.