Structural studies of sapote (Sapota achras) gum

Chemical structure and rheological studies of arabinoglucuronoxylans from the Cercidium praecox exudate brea gum

The structure of the arabinoglucuronoxylans from brea gum was elucidated through an chemical and NMR spectroscopical analysis. They are composed of xylose, arabinose, glucuronic acid and 4-O-methylglucuronic acid in a molar ratio 1:0.44:0.16:0.22. The structure consists of a central chain of (1→4)-β-d-xylopyranose of which ca.70% are susbstituted in C2 with single stubs of others sugars (β-d-Xylp, α-d-GlcpA and 4-O-Me-α-d-GlcpA), with disaccharides (α-l-Arap-(1→2)-4-O-Me-α-d-GlcpA-(1→, α-l-Arap-(1→2)-α-d-GlcpA-(1→, β-l-Araf-(1→3)-α-l-Araf-(1→ and α-l-Araf-(1→3)-α-l-Araf-(1→5), and possibly with trisaccharides of xylose. The determination of the location of the acetyl groups and their quantification in these arabinoglucuronoxylans has been achieved for the first time. Brea gum presents a higher thickening effect than gum arabic in 5% aqueous solution, demonstrating its potential usefulness for food and pharmaceutical applications.

Purification, properties, and mode of action of hemicellulase I produced by Ceratocystis paradoxa

A culture isolate (CP2) of the fungal plant pathogen Ceratocystis paradoxa produces at least five extra-cellular hemicellulases when grown on a medium containing a commercial hemicellulose as inducer. One of the five enzymes, hemicellulase I (HC-I), was purified by ammonium sulphate preceipitation, ion-exchange chromatography (DEAE-Sephadex and then Cellex-CM), and iso-electric focusing at pH 3-10 and 8-10. HC-I behaves as a single protein on a electrophoresis at pH 6.0 and 8.4. The enzyme degrades hemicellulose B (an arabino-4-O-methylglucurono-xylan) and arabinoxylanto arabinose, xylose, xylobiose (Xyl2; beta-D-Xylp-(1 leads to 4)-D-Xyl), and a mixture of arabinose-xylose and xylose oligosaccharides (AraXyln and Xyln, where n=3, 4, or 5). The enzyme is deduced to be an endo-enzyme. Xylotetraose (Xyl4) was the lowest homologue of the xylose oligosaccharides attacked, yielding xylobiose and xylotriose (Xyl3) only. A mechanism is postulated for this reaction. AraXyl5 were slowly hydrolysed to arabinose and the respective xylose saccharide (Xyl2-Xyl5), and thence to Xyl2 and Xyl3. Hydrolysis of the arabinofuranosyl linkage probably does not occur at the same active site as for the xylose oligosaccharides. Hemicellulose B fractions from different sources appeared to be degraded by HC-I. The enzyme showed optimum activity at pH 5.5 and 40 degrees, and Km was 4.24 mg of hemicellulose/ml.