Glucans linear

Mclntire, T. M., Brant, D. A. (1998). Observations of the (1- 3)—D-glucan linear triple helix to macrocycle interconversion using noncontact atomic force microscopy, /. Am. Chem. Soc., 120 6909. 

Glucans linear or branched polysaccharides composed of D-glucose. The glycosidic linkages may be a-1,4 as in amylose and bacterial dextran, p-1,4 as in cellulose, P-1,3 as in leucosin and callose, or 1,6 as in pustulan. Branched glucans include amylopectin (a-1,4 and a-1,6 bonds), dextran, laminarin and lichenin. 

PuUulan [9057-02-7] first described in detail in 1959, is a water-soluble extracellular a-D-glucan elaborated by the fungus yiureobasidiumpullulans (formerly Eullulariapullulans) (285). It is a linear polymer of maltotriose units linked from the reducing end of one trisaccharidic unit to the nonreducing end of the next trisaccharidic unit by a(l — 6) linkages (286) ... 

Chitosan, having a similar chemical backbone as cellulose, is a linear polymer composed of a partially deacety-lated material of chitin [(l-4)-2-acetamide-2-deoxy-/3-D-glucan]. Grafting copolymer chains onto chitosan can improve some properties of the resulting copolymers [48-50]. Yang et al. [16] reported the grafting reaction of chitosan using the Ce(IV) ion as an initiator, but no detailed mechanism of this initiation has been published so far. 

Extensive studies have been performed on the (1- 6)-)8-D-glucan (pustulan) and the (l- 4)-a-D-glucan (amylose). These are linear polysaccharides that may exist as helical polymers in aqueous solution, as demonstrated by c.d. spectroscopy. Characteristic of the helical structure of these glucans is a negative band at 182 nm, a crossover at 177 nm, and a more intensely positive band at shorter wavelengths (see Figs. 8 and 9). 

Figure 2 compares the conformational transition curves of wild-type yeast glucan (branch frequency = 0.20) and PGG (branch frequency = 0.50). Wild-type yeast glucan required approximately 0.1M NaOH to disrupt the triple helical conformation, whereas this transition is observed at approximately 0.04 M NaOH with PGG. This trend is consistent with the observation that curdlan, an entirely linear p-D(l-3)-linked glucan, requires approximately 0.25M NaOH to disrupt the ordered conformation (76). Hence, it is concluded that the highly branched PGG molecules only form weak inter-chain associations resulting in the formation of predominantly single-helical zones. 

Figure 2 Effect of branch frequency on glucan conformation. Conformational characterization of glucans was carried out as described in the experimental section. Curdlan is a linear p(l-3)linked glucan Yeast glucan has a 30% P(l-6) branch frequency and PGG-R glucan has a 50% p(l-6) branch frequency. The Congo Red-single/triple helix complex absorption maxima are indicated.

The synthesis of linear 4 —> 1-a-D-glucans from D-glucopyranosyl phosphate by the action of phosphorylases has been shown by comparison of results of methylation and end-group assay and viscosity determination,209 and by potentiometric, iodine titrations82 on the product. The chain length of the enzymic product (100 to 200 D-glucose units) is less than that of the natural component. Whether this is due to impure enzymes cannot yet... 

Klarzynski O, Plesse B, Joubert JM, Yvin JC, Kopp M, Kloareg B, Fritig B (2000) Linear beta-1, 3-glucans are elicitors of defense responses in tobacco. Plant Physiol 124 1027-1038 Klebanoff SJ (1980) Oxygen metabolism and the toxic properties of phagocytes. Ann Intern Med 93 480-489... 

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. 

In the field of polymer science, the most extensively used transferase is phosphorylase (systematic name (1 4)-a-D-glucan phosphate a-D-glucosyltransferase EC 2.4.1.1). Although this enzyme is responsible for the depolymerization of linear a-( 1 4) glycosidic chains in vivo it can also be used to synthesize linear a-( 1 4) glycosidic chains (amylose) in vitro. 

In vivo linear a-l,4-glucans are synthesized from ADP-glucose by the enzyme glycogen synthase [94-97]. The enzyme, as well as the monomer, are quite sensitive and therefore most researchers (at least in the field of polymer science) prefer to use phosphorylase for the synthesis of amylose. 

Vacuum-ultraviolet, circular dichroism (v.u.c.d.) measurements have been made on films and solutions of linear D-glucan in a study of the conformation of dextran and its oligomers.123 Film formation of the linear dextran was accompanied by crystallization, and the v.u.c.d. band was observed at 165 nm, in contrast to nonlinear dextran films displaying a band at 177 nm. The difference was ascribed to hydrogen bonding of the ring-oxygen atom in the crystalline state. 

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