Cellulose chain structure

The intermolecular interactions stabilise the helices and greatly influence the properties of exopolysaccharides in solution, ie solubility, viscosity and gel-formation. A strong interaction or good-fit between molecules will lead to insolubility, whereas poor interaction will lead to solubility of exopolysaccharides. The interactions between molecules is influenced by the presence of side-chains. For example, cellulose is insoluble but introduction of a three monosaccharide side-chain into the cellulose chain gives the soluble xanthan. Small changes in the structure of the side-chains can alter the molecular interactions and thus properties of the exopolysaccharide. 

Plant cell walls are made of bundles of cellulose chains laid down in a cross-hatched pattern that gives cellulose strength in all directions. Hydrogen bonding between the chains gives cellulose a sheetlike structure. 

At the same time K. H. Meyer and Mark (69) proposed an important structure for cellulose which is best described as a compromise between the aggregates of the association theory and Standinger s macromolecules. In an extensive paper, they carefully developed the idea of cellulose chains consisting of so called "primary valence chains". They further proposed that the primary valence chains were aggregated by molecular forces such as hydrogen bonding and van der Waal s forces. 

From these considerations there evolved the concept of "primary valence chains" in cellulose, held together in bundles, or micelles (crystallites) by secondary forces, as propounded by Meyer and Mark (5). This view was then extended to encompass other high polymers as well. It should be noted however, that Freudenberg had already proposed a chain structure for cellulose, based on degradation experiments (6). If the micelles were to... 

By forming intramolecular and intermolecular hydrogen bonds between OH groups within the same cellulose chain and the surrounding cellulose chains, the chains tend to be arranged in parallel and form a crystalline supermolecular stracture. Then, bundles of linear cellulose chains (in the longitudinal direction) form a microfibril which is oriented in the cell wall structure. Cellulose is insoluble in most solvents and has a low accessibility to acid and enzymatic hydrolysis (Demirbas, 2008b). 

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. 

As the above results show, the gross features of the cellulose I crystal structure predicted by various methods do not differ appreciably, but the accompanying deviations in the R -factors are significant. When these predictions are used to assess, for example, whether the cellulose I crystal structure is based on parallel- or antmarallel-chains, the range in the R"-factors seen for the parallel models (cf. Table II) is comparable to that between the two different polarity models. As shown in Fig. 5, the most probable parallel- and antiparallel-chain structures of cellulose I, refined by minimizing the function O, differ in R -factors by approximately the same extent as the three predictions for the parallel model shown in Fig. 4 and Table II. 

Cellulose, a polysaccharide consisting of linear 1,4-/ -D-anhydroglucopyra-nose chains laterally associated by hydrogen bonds, is the most abundant and commercially important plant cell wall polymer (1). Consequently, cellulose is also one of the most thoroughly investigated plant cell wall polymers. However, it is enigmatic in the sense that significant elements of cellulose physical structure and the mechanism of cellulose biosynthesis still are not well understood. Since these subjects have been reviewed recently (2-10), this review will update topics covered previously and provide a new analysis of selected topics of contemporary interest. 

Divne, C., Stahlberg, J., Teeri, T. and Jones, T. (1998) High-resolution crystal structures reveal how a cellulose chain is bound in the 50 Angstrom long tuimel of cellobiohydrolase I from Trichoderma reesei. J. Mol. Biol., 275, 309-325. 

It is shown that ether oxygen atoms must participate in the hydrogen bonding in all three modifications. The symmetry of the cellulose chain and the relation of the two chains in the unit-cell are discussed with reference to the number of distinguishable 0—H groups. Structures consistent with the infrared data and stereochemical considerations are described. 

FIGURE 20-32 A plausible model for the structure of cellulose synthase. The enzyme complex includes a catalytic subunit with eight transmembrane segments and several other subunits that are presumed to act in threading cellulose chains through the catalytic site and out of the cell, and in the crystallization of 36 cellulose strands into the paracrystalline microfibrils shown in Figure 20-29. 

On the basis of X-ray measurements it is assumed that a micelle composed of 100-170 simple cellulose chains has a length of at least 600A and a width of 50-60A. An outline of the micellar structure of cellulose, according to Meyer and Mark [31], is sketched in Fig. 77. The secondary valencies (a) unite individual... 

Structural features met in some cellulases include an a,a barrel111 similar to that of glucoamylase (Fig. 2-29) and, in a cellobiohydrolase,101 a 5-nm-long tunnel into which the cellulose chains must enter. Ten well-defined subsites for glycosyl units are present in the tunnel.101 A feature associated with this tunnel is processive action, movement of the enzyme along the chain without dissociation,105 a phenomenon observed long ago for amylases (see Section 9) and often observed for enzymes acting on nucleic acids. 

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