Purification and characterization of endo‐xylanases from Aspergillus niger. II. An enzyme of pl 4.5

Abstract

A homogeneous endo‐xylanase (1,4‐β‐D‐xylan xylanohydrolase, EC 3.2.1.8) was obtained from a crude Aspergillus niger pentosanase by chromatography with Ultrogel AcA 54, SP‐Sephadex C‐25 at pH 4.5, DEAE‐Sephadex A‐25 at pH 5.4, Sephadex G‐50, and SP‐Sephadex C‐25 with a gradient from pH 2.8 to pH 4.6. It was much more active on soluble than on insoluble xylan, yielding large amounts of unreacted xylan and a mixture of oligosaccharides with chain lengths from two to six. No xylose or L‐arabinose was produced. There was high activity on a xylopentaose through xylononaose mixture, but not on xylobiose, xylotriose, or xylotetraose. The enzyme had slight activity on untreated cellulose, carboxymethylcellulose, and pectin. Molecular weight was ca. 1.4 × 10⁴, with an isoelectric point of 4.5 and an amino acid profile high in acidic but low in sulfur‐containing residues. In a 25‐min assay at pH 4.7, this endo‐xylanase was most active at 45°C, with an activation energy from 5 to 35°C of 33.3 kJ/mol. The optimum pH for activity was 4.9. Decay in buffer was first order, with an activation energy at pH 4.7 from 48 to 53°C of 460 kJ/mol. Optimum pH for stability was about 5.6, where the half‐life at 48°C in buffer was ca. 40 h.