Crystallinity and polysaccharide chains of beta-glucan in white sorghum, SK(5912).
The polysaccharide chains and the crystallinity of beta-glucan in a white sorghum variety, SK(5912) were investigated using chemical and enzymic studies. Mild periodate oxidation and methylation, coupled to descending paper chromatography of products revealed the presence of unresolved non-carbohydrate moiety, 2, 4-and 2, 3-di-O-methyl D-glucose residues (molar ratio; 18:3) and 2, 4, 6-and 2, 3, 6-tri-O-methyl D-glucose residues (molar ratio; 1:14). Paper chromatography of the total acid hydrolysate also revealed a non-carbohydrate spot, identified as protein on the basis of positive Biuret and ninhydrin tests. The O-methyl D-glucose residues suggest two polysaccharide chains designated X and Y. Chain X is formed through linking of beta-D-glucopyranosyl residues by (1-->3) linkages with 85-86% (1-->6) bonds at branch points and constitute about 6-7% of the beta-glucan sample. Chain Y, which is 93-94% of the beta-glucan polysaccharide chains, constitutes beta-D-glucopyranosyl residues in (1-->4) linkages and 4-5% (1-->6) bonds at branch points. Of the 18 branch points on the X-chains in a given beta-glucan sample, about 15 are the Y chains interlinked to the X-chains through their (Y-chains) reducing ends. Both acid and enzyme hydrolyses of the beta-glucan suggest two structural organizations, a crystalline and less crystalline granules, based on two first order kinetics. This was correlated by the progress curves obtained during hydrolysis with two purified isoforms of beta-glucanases from the sorghum malt. The short and highly branched polysaccharide chains, and longer but less branched polysaccharide chains found in this beta-glucan are reminiscent of the structures of amylopectin and amylose, respectively. The K(m)s of 0.30-0.32 and 0.42-0.50 mg beta-glucan/ml for the beta-glucanase isoforms also lay credence to both the crystalline forms and the highly polymerised nature of the beta-glucan in white sorghum.
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Department of Biochemistry, Pollution Control and Biotechnology Unit, University of Nigeria, Enugu State, Nsukka, Nigeria. epseelon@aol.com
D003460:Crystallization; D002523:Edible Grain / Q000737:chemistry*; D005936:Glucans / Q000737:chemistry*; D005964:Glucosyltransferases / Q000737:chemistry; D006868:Hydrolysis; D007700:Kinetics; D008956:Models, Chemical; D010100:Oxygen / Q000378:metabolism; D011134:Polysaccharides / Q000737:chemistry*; D020033:Protein Isoforms; D017434:Protein Structure, Tertiary; D013997:Time Factors; D047071:beta-Glucans
D005936:Glucans; D011134:Polysaccharides; D020033:Protein Isoforms; D047071:beta-Glucans; C040088:1,4-glucan; C033363:beta-1,3-glucan; D005964:Glucosyltransferases; D010100:Oxygen