PURIFICATION, CHARACTERIZATION, AND PROPERTIES OF 2 XYLANASES FROM HUMICOLA-INSOLENS

Two endoxylanases (EC 3.2.1.8), xyl1 and xyl2, were purified by subseq uent anion-exchange, size-exclusion, and cation-exchange chromatograph y from a commercial enzyme preparation derived from the thermophilic f ungus Humicola insolens. The homogeneous proteins had molecular masses of 6 and 21 kDa (SDS polyacrylamide gel electrophoresis) and isoelect ric points of 9.0 and 7.7, respectively. The low molecular weight of x yl1 was confirmed by mass spectrometry. Both enzymes had similar pH an d temperature optima (pH 6-6.5 and 55-60 degrees C) but their stabilit y at various pH and temperatures differed. The molar activity towards xylans from beech, birch, larch, and arabinoxylans from wheat was high er for xyl2. Both xylanases had remarkably lower molar activities towa rd the isolated insoluble fractions of these xylans or toward the esse ntially insoluble beech xylan, but the decrease was relatively less pr onounced with xyl2. These findings might be explained by differences i n specific adsorption: xyl2 adsorbed strongly onto insoluble beech xyl an while the affinity of xyl1 was much lower. In contract to xyl1, xyl 2 was markedly inhibited by a number of metal ions. The reaction produ cts formed during hydrolysis of different xylans and the end products (xylobiose, xylotriose, minor amounts of monomeric xylose, and substit uted [(4-o-methyl)glucurono]arabino-xylooligomers) were equal for both enzymes, but their relative proportions differed slightly. (C) 1997 b y Elsevier Science Inc.