Exocellular structure

Xanthan is the extracellular (exocellular) polysaccharide produced by Xanthomonas campestris. As with other microbial polysaccharides, the characteristics (polymer structure, molecular weight, solution properties) of xanthan preparations are constant and reproducible when a particular strain of the organism is grown under specified conditions, as is done commercially. The characteristics vary, however, with variations in the strain of the organism, the sources of nitrogen and carbon, degree of medium oxygenation, temperature, pH, and concentrations of various mineral elements. 

Formation of L-guluronic acid, a component of the alginic acid-like polysaccharide produced by P. aeruginosa and Azotobacter vinelandii, requires special comment. In this case, a polymer built from /3-(l- 4)-linked D-mannosyluronic acid residues serves as an intermediate in the biosynthesis.204,205 Part of the D-mannosyluronic acid residues in the polymer is subjected to an epimerization at C-5 catalyzed by an exocellular enzyme of the micro-organism,205-207 producing a polysaccharide composed of structural blocks that contain only D-mannosyluronic acid or only l-gulosyluronic acid residues, as well %s some having both. The mechanism of the epimerization remains unclear. 

Other monosaccharide components (of bacterial polysaccharides) that are structurally related to D-ribose include D-riburonic acid,232 identified in the exocellular polysaccharide produced by a strain of Rhizobium meliloti, and D-arabinose, frequently present as the furanose, in polysaccharides of mycobacterial cell-wall.233,234 L-Xylose235,236 should probably be included in the group, as it may be derived from D-arabinose through epimerization at C-4. Biosynthesis of these monosaccharides was not investigated. 

The nuclei of the acellular, slime mold Physarium polycephalum contains a /3-D-gaIactan (d.p. 560) bearing phosphate (2.5%) and sulfate (9.6%) groups. One unit in every 13 is branched, but the main structural-feature is 4-O-substituted D-galactopyranosyl units.121 It resembles the exocellular polysaccharide.122... 

The acidic, exocellular polysaccharide from Tremella mesenterica, isolated by Cetavlon precipitation, has repeating structure 39 with D-... 

Kooiman separated exocellular amylose formed in liquid media at pH < 5 by Cryptococcus albidus and Cryptococcus laurentii var. flavescens NRRL Y-1401 from a polysaccharide containing D-man-nose, D-xylose, and D-glucuronic acid. X-Ray diffraction patterns of the retrograded amyloses were identical with that of tuber starch (B modification). Periodate oxidation, optical rotational measurements, and hypoiodite oxidation data suggested a linear a-D-(l—>4)-linked structure having a chain length of about 44 units for the Cr. albidus amylose. Cryptococcus neoformans produces a crystalline amylose that was isolated by the method of Schoch. It has an iodine value and alpha- and hefa-amylase hydrolysis characteristics similar to those of corn amylose. 

Llauberes, R.M., Richard, B., Lonvaud, A., Dubourdieu, D., and Foumet, B. 1990. Structure of an exocellular p-D-glucan from Pediococcus sp., a wine lactic bacteria. Carb. Res. 203, 103-107. 

R. F. Helm, Z. Huang, D. Edwards, H. Leeson, W. Peery, andM. Potts, Structural characterization of the released polysaccharide of desiccation-tolerant Nostoc commune DRH-1, J. Bacteriol., 182 (2000) 974-982 R. DePhillipis and M. Vincenzini, Exocellular polysaccharides from cyanobacteria and their possible applications, FEMS Microbiol. Rev., 22 (1998) 151-175. 

Cellulose is primarily a structural polymer in plants (especially in cotton, ramie and hemp) and trees. In the latter, cellulose is the principal structural material and constitutes about 50 weight percent of wood. Cellulose is also produced by bacteria in the form of exocellular microfibrils. In all forms, cellulose is a very highly crystalline, high molecular weight polymer, which is infusible and insoluble in all but the most aggressive, hydrogen-bond breaking solvents such as /V-methylmorpholine N-oxide. Because of its infusibility and insolubility, cellulose is usually converted into derivatives to make it processable. 

As will be discussed later (see p. 332), these data provide strong support for the argument that the microfibrils are produced by on-the-site synthesis and orientation (apposition) of the cellulosic microfibrils under the guiding influence of the living cell, rather than by a mechanism that proposes synthesis of the micro fibrils within the cell and subsequent translocation and crystallization (deposition) of microfibrils on the cell wall by exocellular factors. Further factors relevant to these opposing theories emerge from study of the fine structure of the cellulosic microfibrils, as discussed in the following Section. 

Exodextranases catalyze a stepwise hydrolysis of the dextran molecule to yield D-glucose and a residual dextran they have been extracted from cultures of species of Bacillus," Bacteroides, and Lactobacillus bifidus. These enzymes have not, however, been utilized in structural studies. Endodextranases have been isolated from exocellular fluids or cell extracts (or both) of the fungi PenidUium funiculosutn, P. lilacv-... 

Most streptocci that elaborate dextransucrases appear to secrete the enzyme(s) exocellularly, although some strains of Streptococcus mutans also elaborate structure-bound forms of the enzyme. The structure-bound enzyme activities of S. mutans may be situated at several points on the outer smface of the cell wall." Evidence has also been presented that suggests that the proportions of the structure-bound and secreted forms of the enzyme produced by individual streptococci may drange with repeated subculturing of the bacteria." Similar changes have not been detected in Leuconostoc strains. ... 

The general molecular structure of yeast exocellular mannoproteins is similar to that of mannoproteins in the cell wall (Volume 1, Section 1.2) (Villetaz et al., 1980 Llauberes et al., 1987 Llauberes, 1988). It consists of a peptide chain connected to short side chains made up of four maunose units and a high molecular weight, branched a-D-mannane (Figure 3.24). 

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