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- Hsieh, Yves S. Y., et al.
(författare)
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Genetics, Transcriptional Profiles, and Catalytic Properties of the UDP-Arabinose Mutase Family from Barley
- 2016
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Ingår i: Biochemistry. - : American Chemical Society (ACS). - 0006-2960 .- 1520-4995. ; 55:2, s. 322-334
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Tidskriftsartikel (refereegranskat)abstract
- Four members of the UDP-Ara mutase (UAM) gene family from barley have been isolated and characterized, and their map positions on chromosomes 2H, 3H, and 4H have been defined. When the genes are expressed in Escherichia coli, the corresponding HvUAM1, HvUAM2, and HvUAM3 proteins exhibit UAM activity, and the kinetic properties of the enzymes have been determined, including K-m, K-cat, and catalytic efficiencies. However, the expressed HvUAM4 protein shows no mutase activity against UDP-Ara or against a broad range of other nucleotide sugars and related molecules. The enzymic data indicate therefore that the HvUAM4 protein may not be a mutase. However, the HvUAM4 gene is transcribed at high levels in all the barley tissues examined, and its transcript abundance is correlated with transcript levels for other genes involved in cell wall biosynthesis. The UDP-L-Arap -> UDP-L-Araf reaction, which is essential for the generation of the UDP-Araf substrate for arabinoxylan, arabinogalactan protein, and pectic polysaccharide biosynthesis, is thermodynamically unfavorable and has an equilibrium constant of 0.02. Nevertheless, the incorporation of Araf residues into nascent polysaccharides clearly occurs at biologically appropriate rates. The characterization of the HvUAM genes opens the way for the manipulation of both the amounts and fine structures of heteroxylans in cereals, grasses, and other crop plants, with a view toward enhancing their value in human health and nutrition, and in renewable biofuel production.
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| 2. |
- Dimitroff, George, et al.
(författare)
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(1,3;1,4)-beta-Glucan Biosynthesis by the CSLF6 Enzyme : Position and Flexibility of Catalytic Residues Influence Product Fine Structure
- 2016
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Ingår i: Biochemistry. - : American Chemical Society (ACS). - 0006-2960 .- 1520-4995. ; 55:13, s. 2054-2061
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Tidskriftsartikel (refereegranskat)abstract
- Cellulose synthase-like F6 (CslF6) genes encode polysaccharide synthases responsible for (1,3;1,4)-beta-glucan biosynthesis in cereal grains. However, it is not clear how both (1,3)- and (1,4) -linkages are incorporated into a single polysaccharide chain and how the frequency and arrangement of the two linkage types that define the fine structure of the polysaccharide are controlled. Through transient expression in Nicotiana benthamiana leaves, two CSLF6 orthologs from different cereal species were shown to mediate the synthesis of (1,3;1,4)-beta-glucans with very different fine structures. Chimeric cDNA constructs with interchanged sections of the barley and sorghum CslF6 genes were developed to identify regions of the synthase enzyme responsible for these differences. A single amino acid residue upstream of the TED motif in the catalytic region was shown to dramatically change the fine structure of the polysaccharide produced. The structural basis of this effect can be rationalized by reference to a homology model of the enzyme and appears to be related to the position and flexibility of the TED motif in the active site of the enzyme. The region and amino acid residue identified provide opportunities to manipulate the solubility of (1,3;1,4)-beta-glucan in grains and vegetative tissues of the grasses and, in particular, to enhance the solubility of dietary fibers that are beneficial to human health.
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