Glucan structural model

A series of per-O-acetylated cellooligomers have been examined by CP/MAS NMR spectroscopy to obtain a structural model for cellulose triacetate in the solid state. Molecular modelling experiments with trisaccharides 23 and 24 indicated that the (1 6)-a-linked side chains did not interfere significantly with the stereochemistry of the a-(1 3) backbone of (l- 3)-a-D-glucans. ... 

Since the local polymer conformation in solution is sometimes related to the crystalline structure, the complex structure in solution should be represented by a triple helix similar to that of the original P-(1 3)-glucans. Based on the above discussion, we can construct a structural model for the complex with computer chemistry. First, we constructed the triple helix of schizophyllan (a right-handed 6i triple helix with a 17.4 A) according to the literatures," and took one chain... 

Structural Characterization. 13C-NMR Spectra of PGG glucan preparations (15 mg/mL in 0.5 M NaOD) were recorded with a Bruker Model AC 200 at 50.3 MHz and all chemical shifts were expressed in parts per million downfield from an internal tetramethylsilane (TMS) standard. 

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. 

Since the 5-position of successive saccharide units occur on the outside surface of the triple-strand molecule it was evident after the discovery of such a model for B-(1-3)-D-xylan that a similar structure would be feasible for e-(l-3)-D-glucan. 

Amylopectin molecules are large flattened disks consisting of (1 —4)-linked a-glucan chains joined by frequent 6)-branch points. Many models of amylopectin structure have been proposed. The most satisfactory models are those proposed by Robin et al.,18 Manners and Matheson,19 and Hizukuri 20 all are known as cluster models. Reviews by Morrison and Karkalis21 and Hizukuri22 discuss in detail the chemical and physical aspects of the starch granule and its components, amylose and amylopectin. 

The main structural constituents of Saccharomyces cerevisiae yeast cell wall are glucans and mannans with a minor proportion of chitin (Walker 1998). Manno-proteins are located in the outer layer of the yeast cell wall and determine most of the surface properties of the wall. Vasserot et al. (1997) studied the capacity of yeast lees to adsorb anthocyanins in an attempt to reduce the detrimental effects of charcoal on the color of red musts and wines. Experiments based on model wine solutions revealed that yeast lees possess a greater affinity for anthocyanins than... 

This brief comparative resume shows clearly that a number of facets of the action of 0-glucan endo-hydrolases may be explained by reference to an enzyme model based on the structure found for lysozyme. The crystallographic data which provide the basis for the three-dimensional model of lysozyme have been supplemented by information on enzyme-substrate interactions in solution (4, 19, 39, 41, 47, 51, 52, 181, 196). Since it is not yet possible to examine the 0-glucan endo-hydrolases crystallographically, the methods which have been used to study lysozyme-substrate interactions in solution will be of primary importance in the elucidation of the action of these enzymes. Already certain 0-1,4-glucan endo-hydrolases have been examined using some of these techniques (57, 171, 172) but wider application of such methods is needed. 

Chain Conformation. From computer modeling studies of cellulose ( ), we know that the cellulose molecule has limited flexibility. Only a few regular (Internally symmetric) conformations are allowed for cellulose, a beta 1,4-llnked glucan, and structures similar to the traditional 2-fold helix are the only ones In accord with the observed z axis spacing. Such chains resemble flat ribbons. 

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