Glucan chains

Henry, R.J. and Stone, B.A. (1985) Extent of b-glucan chain elongation by ryegrass (Lolium multiflorum) enzymes. Carbohydr.Polym. 5 1-12. 

Fig. 2 Mechanisms proposed for the pol3no[ierization of glucans by glucansucrases by addition of glucose to the reducing end of the glucan chain.

PF had been proposed as the terminal complex (23) and associated pores were reported on the outer membrane EF (24). Due to their proximity to the site of cellulose ribbon extrusion from the cell surface, these structures were assumed to be responsible for cellulose synthesis. A model was advanced in which cellulose synthase was localized on the outer membrane, which invoked adhesion sites between the outer and plasma membranes as a mechanism to explain the transfer of uridine-diphosphoryl-glucose (UDPG) from the cytoplasm to the cellulose synthases (25,26). However, when the outer and plasma membranes of Acetobacter were isolated separately by density-gradient centrifugation, the cellulose synthase activity was localized only in the plasma membrane fraction (27). Therefore, the linear structures observed on the Acetobacter outer membrane, while they may be associated in some manner with cellulose biosynthesis, are probably not the cellulose synthase terminal complexes. Since no ultrastructural evidence for adhesion sites between the outer and plasma membranes has been presented, a thorough investigation of the mechanism of / (1-4) glucan chain translocation from the cytoplasmic membrane to the outer membrane in Acetobacter xylinvm is now in order. 

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. 

Functionally, starch can be considered as a polysaccharide synthesized in a manner permitting its efficient degradation. Hence, biosynthesis of the starch granule is a delicate balance between efficient packing of the glucan chains and the possibility of breaking these structures at degradation. To complete this enzymatically catalyzed process in the potato tuber, a multitude of different enzyme activities are required. 

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. 

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