Mechanism of mixed-linkage glucan biosynthesis by barley cellulose synthase–like CslF6 (1,3;1,4)-β-glucan synthase
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Mixed-linkage (1,3;1,4)-β-glucans, which are widely distributed in cell walls of the grasses, are linear glucose polymers containing predominantly (1,4)-β-linked glucosyl units interspersed with single (1,3)-β-linked glucosyl units. Their distribution in cereal grains and unique structures are important determinants of dietary fibers that are beneficial to human health. We demonstrate that the barley cellulose synthase-like CslF6 enzyme is sufficient to synthesize a high-molecular weight (1,3;1,4)-β-glucan in vitro. Biochemical and cryo-electron microscopy analyses suggest that CslF6 functions as a monomer. A conserved "switch motif" at the entrance of the enzyme's transmembrane channel is critical to generate (1,3)-linkages. There, a single-point mutation markedly reduces (1,3)-linkage formation, resulting in the synthesis of cellulosic polysaccharides. Our results suggest that CslF6 monitors the orientation of the nascent polysaccharide's second or third glucosyl unit. Register-dependent interactions with these glucosyl residues reposition the polymer's terminal glucosyl unit to form either a (1,3)- or (1,4)-β-linkage.The yeast cell wall contains beta1,3-glucanase-extractable and beta1,3-glucanase-resistant mannoproteins. The beta1,3-glucanase-extractable proteins are retained in the cell wall by attachment to a beta1,6-glucan moiety, which in its turn is linked to beta1,3-glucan (J. C. Kapteyn, R. C. Montijn, E. Vink, J. De La Cruz, A. Llobell, J. E. Douwes, H. Shimoi, P. N. Lipke, and F. M. Klis, Glycobiology 6:337-345, 1996). The beta1,3-glucanase-resistant protein fraction could be largely released by exochitinase treatment and contained the same set of beta1,6-glucosylated proteins, including Cwp1p, as the B1,3-glucanase-extractable fraction. Chitin was linked to the proteins in the beta1,3-glucanase-resistant fraction through a beta1,6-glucan moiety. In wild-type cell walls, the beta1,3-glucanase-resistant protein fraction represented only 1 to 2% of the covalently linked cell wall proteins, whereas in cell walls of fks1 and gas1 deletion strains, which contain much less beta1,3-glucan but more chitin, beta1,3-glucanase-resistant proteins represented about 40% of the total. We propose that the increased cross-linking of cell wall proteins via beta1,6-glucan to chitin represents a cell wall repair mechanism in yeast, which is activated in response to cell wall weakening.
Glucanase
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In order to study the relationship between elongation growth inhibition induced by ultraviolet-B(UV-B) radiation and the changes of cell wall polysaccharides fraction,the stems length and contents of total sugar and uronic acid of cell wall polysaccharides fractions(pectin,hemicelluloses A,hemicelluloses B and cellulose) of pea stems irradiated with UV-B were analyzed.The results showed that with the sample of traditional pea,the stem length increased,while the amounts of cell wall polysaccharides per unit length decreased in primary 5 days,which showed a significantly negative correlation(p0.05).However,the elongation growth of stems was inhibited and amounts of cell wall polysaccharides increased with UV-B irradiated sample.Under UV-B radiation,the stem length decreased by 7.35 cm and the amounts of cell wall polysaccharides increased by 0.07 mg/cm compared with control group on the 5th day.Compared with CK,the total sugar of pectin,hemicelluloses A,hemicelluloses B,and cellulose of UV-B irradiated group increased by 22.30%,42.30%,21.47%,and 12.05%,respectively,meanwhile the related uronic acid increased by 6%,33.3%,17.24%,and 18.08%,respectively.These results can be suggested that the metabolism of cell wall polysaccharides may be regulated by UV-B radiation.The changes of cell wall structure may be involved in cell wall thickness,which will lead to the cell wall rigidified.Therefore the extensibility of cell wall is decreased and elongation growth is inhibited.
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Elongation
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Glycosyltransferases are key enzymes involved in the biosynthesis of valuable natural products providing an excellent drug-tailoring tool. Herein, we report the identification of two cooperative glycosyltransferases from the sqn gene cluster directing the biosynthesis of saquayamycins in Streptomyces sp. KY40-1: SqnG1 and SqnG2. Gene inactivation of sqnG1 leads to 50-fold decrease in saquayamycin production, while inactivation of sqnG2 leads to complete production loss, suggesting that SqnG2 acts as dual O- and C-glycosyltransferase. Gene inactivation of a third putative glycosyltransferase-encoding gene, sqnG3, does not affect saquayamycin production in a major way, suggesting that SqnG3 has no or a supportive role in glycosylation. The data indicate that SqnG1 and SqnG2 are solely and possibly cooperatively responsible for the sugar diversity observed in saquayamycins 1-7. This is the first evidence of a glycosyltransferase system showing codependence to achieve dual O- and C-glycosyltransferase activity, utilizing NDP-activated d-olivose, l-rhodinose, as well as an unusual amino sugar, presumably 3,6-dideoxy-l-idosamine.
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Nucleotide sugar
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Hydroxymethyl
Hydrolase
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Galactan
Monosaccharide
Cichorium
Xyloglucan
Hemicellulose
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This chapter discusses the methods of structural analysis of forage cell wall polysaccharides and polysaccharide structures. It presents the composition of nonstarch polysaccharides in different forages and forage fractions. In plant structures like the cell wall, nonstarch polysaccharides, together with other components, are organized in complex three dimensional structures that are neither uniform nor completely described. For structural analysis of cell wall polysaccharides, it is often necessary to isolate cell wall materials. Growing and parenchymatous tissues generally contain large amounts of intracellular compounds such as nucleic acids, phenols, lipids, proteins, and reserve polysaccharides as well as active cell wall degrading enzymes. Cereal straw, containing predominantly secondary cell walls, is rigid and resistant to microbial degradation. The barley fractions contained more glucose residues but less Klason lignin compared to the corresponding wheat fraction.
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Pectin
Xyloglucan
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Introduction Survey of Glycosyltransferases by Type of Sugar Transferred Glycosyltransferases in Oligosaccharide Biosynthesis The Use of Hlycosyltransferases in Analysis of the Structure and Structure-Function Relationships of Oligosaccharides Purification of Glycosyltransferases Concluding Remarks
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Nucleotide sugar
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Pectin
Carrageenan
Plant cell
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Summary We describe preparations of plant cell walls and polysaccharides obtained from plant cell walls that are added to food products for two purposes: as modifiers of food texture and/or as dietary fibres with potential health benefits. Although a number of different types of plant cell walls occur, only some are presently exploited. Commercial ‘fibre preparations’ range from those containing mostly primary walls to those containing mostly lignified secondary walls from which much of the lignin and non‐cellulosic polysaccharides have been removed. Preparations of cell‐wall polysaccharides are obtained from the following sources: cellulose mostly from secondary walls of cotton and wood, pectin from primary walls of dicotyledons, and (1→3),(1→4)‐ β ‐glucans and arabinoxylans from primary walls of cereal grains. Preparations of galactomannans, xyloglucans and the pectic polysaccharide rhamnogalacturonan I are obtained from non‐lignified secondary walls of certain leguminous seeds. The compositions, functionalities, uses and possible health benefits of these different preparations are discussed.
Pectin
Secondary cell wall
Xyloglucan
Food Products
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