Cellulose three-dimensional structure

Figure 2. Three dimensional structure of the enzyme cellulase that breaks cellulose into sugar molecules10.

Polysaccharide solubility in aqueous solutions usually is dependent on polymer size and its allied three-dimensional structure. Even water-insoluble carbohydrates may be solubilized by controlled hydrolysis of o-glycosidic linkages to create smaller polysaccharide molecules. Thus, cellulose may be solubilized by heating in an alkaline solution until the polymers are broken up sufficiently to reduce their average molecular weight. Many such soluble forms of common polysaccharides are available commercially. 

Very recently 13 the three dimensional structure of the CBH II core was fully determined by X-ray diffraction. The polypeptide chain is folded in a-helices and B-strands (a,B-protein with a central B-barrel built up by seven parallel strands. Six of the )3-strands are linked by a-helices. Near the C-terminus of the enzyme is a tunnel with dimensions well suited to take up a single cellulose chain. Two aspartic acid residues (175 and 221) are probably involved in the active center. 

The major structural characteristics of cellulose I, determined in this and other studies, include extended chains stabilized alorig their lengths W two intramolecular hydrogen bonds per glucose residue (0(3)- 0(5) and 0(6)—0(2) ), the arrangement of the chains into sheets stabilized by one intermolecular hydrogen bond per residue (0(3)—0(6)), and the packing of the sheets into a three-dimensional structure marked by a parallel-chain arrai ement. These structural characteristics are illustrated in Fig. 2. 

Structure 9.1 is most commonly employed as a description of the repeat unit of cellulose but structure 9.2 more nearly represents the actual three-dimensional structure with each D-glucosyl unit rotated 180°. We will employ a combination of these two structural representations. Numbering is shown in structure 9.3 and the type of linkage is written as 1 4 since the units are connected through oxygen atoms contained in carbon 1 and 4 as shown in structure 9.3. 

The complex three-dimensional structure of these materials is determined by their carbon-based polymers (such as cellulose and lignin), and it is this backbone that gives the final carbon structure after thermal degradation. These materials, therefore, produce a very porous high-surface-area carbon solid. In addition, the carbon has to be activated so that it will interact with and physisorb (i.e., adsorb physically, without forming a chemical bond) a wide range of compounds. This activation process involves controlled oxidation of the surface to produce polar sites. 

NMR measurements to deduce conformations of sugar rings, three-dimensional structures, and the degree of conformational flexibility in various parts of N-linked oligosaccharides (Fig. 4-21). Measurement of C-O-C -C spin-coulping constants is also of value.270 Use of multidimensional NMR has permitted analysis of mixtures of cellulose oligosaccharides.269... 

FIGURE 1 Three-dimensional structures of amylose and cellulose polymers. 

Fungal cellulase enzyme systems capable of efficiently catalyzing the hydrolytic degradation of crystalline cellulose are typically composed of endo-acting cellulases (EGs), exo-acting cellulases (CBHs), and at least one cellobiase (1-6). The CBHs are typically the predominant enzymes, on a mole fraction basis, in such systems (7). Consequently, the CBHs have been the focus of many studies (8). The three-dimensional structure of prototypical CBHs is known (9-12) and their specificities are, in general, well characterized (13,14). However, mechanism-based kinetic analyses of CBH-catalyzed cellulose saccharification are rather limited (15,16). Studies of this latter type are particularly difficult owing to the inherent complexity of native cellulose substrates. 

Three-Dimensional Structures of Cellulose Molecules, Crystallites, and Fibers.32... 

THREE-DIMENSIONAL STRUCTURES OF CELLULOSE MOLECULES, CRYSTALLITES, AND FIBERS... 

Near the end of Section 5.1, there was a lengthy list of attributes needed to specify the three-dimensional structure of cellulose. The present section is concerned with descriptions of the smaller-scale structures. The information on shape comes from both theoretical and experimental methods, with the classic advantages and disadvantages of each. Theoretical methods can be used to fill the gaps between experiments and help interpret ambiguous results, but in... 

Recall from Chapter 4 that stereochemistry is the three-dimensional structure of a molecule. How important is stereochemistry Two biomolecules—starch and cellulose—illustrate how apparently minute differences in structure can result in vastly different properties. 

Cellulose is water insoluble, despite its many OH groups. Considering its three-dimensional structure, why do you think this is so ... 

Ball-and-stick models showing the three-dimensional structures of cellulose and starch were given in Figure 5.2. 

How differences in the three-dimensional structure of starch and cellulose affect their shape and function (Section 5.1) The three-dimensional structure of thalidomide, the anti-nausea dmg that caused catastrophic birth defects (Section 5.5) How mirror image isomers can have drastically different properties— the analgesic ibuprofen, the antidepressant fluoxetine, and the anti-inflammatory agent naproxen (Section 5.13)... 

Improved protein separation techniques utilizing hquid chromatography and electrophoresis coupled with X-ray diffraction and NMR studies have given insights into the three-dimensional structures of cellulolytic enzymes. This molecular architecture data coupled with DNA sequence information has given clues to the chemical mechanisms of enzymatic hydrolysis and molecular interaction between cellulose and the enzymes. 

Rouvinen J, Bergfors T, Teeri T, Knowles JKC, Jones TA (1990) Three-dimensional structure of cellobiohydrolase II from Trichoderma reesei. Science 249 380-385 Srisodsuk M, Reinikainen T, PenttUa M, Teeri TT (1993) Role of the interdomain linker peptide of Trichoderma reesei cellobiohydrolase I in its interaction with crystalline cellulose. Journal of Biological Chemistry 268 20,756-20,761... 

Cellulose is a polysaccharide found in plant cell walls. Cellulose forms the fibrous part of the plant cell wall. In terms of human diets, cellulose is indigestible, and thus forms an important, easily obtained part of dietary fiber. As compared to starch and glycogen, which are each made up of mixtures of a and (3 glucoses, cellulose (and the animal structural polysaccharide chitin) are made up of only (3 glucoses. The three-dimensional structure of the structural polysaccharides is thus constrained into straight microfibrils by the uniform nature of the glucoses, which resist the actions of enzymes (such as amylase) that breakdown storage polysaccharides (such a starch). 

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