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

Abstract

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.