D-Glucan chains

Submicrofibril and triple-stranded left-hand helical microfibrils are found in tobacco primary cell wall and bacterial A. xylinum cellulose. We suspect from our results and the literature survey outlined in reference (1) that the triple stranded structures are prominent in the primary plant cell wall. The highly crystalline cellulose of plant and algae secondary cell wall appears by X-ray fiber diffraction (18,19) and TEM lattice imaging (20-23) to be largely crystalline arrays of planar straight chains of (l-4)-/3-D-glucan chains. 

Estimation of the degree of polymerization of the D-glucan chains within the cellulose fibrils is complicated by the necessity of first solubilizing the D-glucans, and this process is likely to break the chains. One estimate put the degree of polymerization at 6000 to 7000 for cellulose chains derived from cotton fibers221 (see also, Ref. 217 and references cited therein). 

It is possible that the D-glucan chains of cellulose have no natural ends that is, once a chain is initiated, it never ends, except when a fibril is physically separated from its synthetic enzymes. This idea is supported by the electron-microscope observation that the cellulose fibrils do not appear to have natural termination-points.4,2174,218 It is also possible that the fibrils have an unlimited length, but that the individual D-glucan chains within the fibrils have a finite length the ends of the D-glucan chains may overlap, and thus result in fibrils of indeterminate length. 

The enzymes involved are nucleoside diphosphohexosetransferases, probably GDP-D-mannosyltransferase (EC 2.4.1.32) and UDP-D-galactosyl-transferase. For a structurally related polymer, (1 - 6)-a-D-xylosyl-(l - 4)-/3-D-glucan, synthesis was shown to involve concurrent incorporation of D-xylose and D-glucose, and not substitution by D-xylose of a preformed (1 - 4)-/3-D-glucan chain.34 The insolubility of (1 - 4)-/3-D-glucan and man-nan would make the latter mechanism unlikely. 

Figure 6.8 A cluster model of amylopectin proposed by Hizukuri with A, and B1-B3 chains. The chain carrying the reducing end (0) is the C chain, (1 —>4)-a-D-glucan chain a-(1 —>-6) linkage.263...

Amylose and amylopectin are the main carbohydrate components of the starch granule. Amylose is an essentially linear a-1, 4-D-glucan chain. About 25-55% of the... 

Polymerization of the D-glucan chains occurs by way of a multi-subunit, enzyme complex embedded in the plasma membrane an almost simultaneous association, by means of hydrogen bonds, of the newly formed chains results in formation of partially crystalline microfibrils. This mechanism of polymerization and crystallization results in the creation of microfibrils whose chains are oriented parallel (cellulose I). In A. xylinum, the complex is apparently immobile, but, in cells in which cellulose is deposited as a cell-wall constituent, it seems probable that the force generated by polymerization of the relatively rigid microfibrils propels the complex through the fluid-mosaic membrane. The direction of motion may be guided through the influence of microtubules. 

Figure 10-5 Diagrammatic Representation of Amylopectin Structure. Lines represent a-D-glucan chains linked by 1,4-bonds. The branch points are 1,6-a glucosidic bonds. Source From J.J. Marshall, Starch Degrading Enzymes, Old and New, Starke, Vol. 27, pp. 377-383, 1975.

Comparison of projection drawings and calculated chain dimensions for amylose, linear "dextran", and pullulan chains shows that the a-1,6-linkage is a source of considerable conformation freedon in D-glucan chains v ich incorporate this linkage. Experimental studies of the coil dimensions of pullulan and other polysaccharides containing a-1,6-linkages will be required to refine the parameters of the theoretical model for this linkage. 

The action of Q-enzyme on a model amylodextrin which contained a singlebranch linkage has been studied.It was concluded that the enzymatic process for synthesizing the branch linkage in amylopectin is a random action of the Q-enzyme on a complex, possibly a double helix, formed between two (1- 4)-linked a-D-glucan chains. The action pattern predicts a novel arrangement of the unit chains in amylopectin. 

In a study of the production of oi-D-glucans by yeasts and yeast-like organisms, most of a total of 177 strains were found able to utilize (1 4)-0f-D-glucans by means of enzymes acting on the reducing ends of the outer D-glucan chains. 

Potato amylose (DP ca. 800) has been oxidized with sodium metaperiodate and used as a substrate for glucoamylase free of a-amylase activity.The Michaelis constant and maximal velocity for the enzyme were found to decrease with increasing oxidation of the amylose substrate. These observations were explained on the basis of competitive inhibition by the oxidized non-reducing end of the (1 4)-Q -D-glucan chain. 

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