A bifunctional beta-xylosidase-xylose isomerase from Streptomyces sp EC 10

beta-Xylosidase (1,4-beta-D-xylan xylohydrolase EC 3.2.1.37) and xylose iso merase (D-xylose ketol-isomerase EC 5.3.1.5) produced by Streptomyces sp. s train EC 10, were cell-bound enzymes induced by xylan, straw, and xylose. E nzyme production was subjected to a form of carbon catabolite repression by glycerol. beta-Xylosidase and xylose isomerase copurified strictly, and th e preparation was found homogeneous by gel electrophoresis after successive chromatography on DEAE-Sephacel and gel filtration on Biogel A. Streptomyc es sp. produced apparently a bifunctional beta-xylosidase-xylose isomerase enzyme. The molecular weight of the enzyme was measured to be 163,000 by ge l filtration and 42,000 by SDS-PAGE, indicating that the enzyme behaved as a tetramer of identical subunits. The Streptomyces sp. beta-xylosidase was a typical glycosidase acting as an exoenzyme on xylooligosaccharides, and w orking optimally at pH 7.5 and 45 degrees C. The xylose isomerase optimal t emperature was 70 degrees C and maximal activity was observed in a broad ra nge pH (5-8). Enhanced saccharification of arabinoxylan caused by the addit ion of the enzyme to endoxylanase suggested a cooperative enzyme action. Th e first 35 amino acids of the N-terminal sequence of the enzyme showed stro ng analogies with N-terminal sequences of xylose isomerase produced by othe r microorganisms but not with other published N-terminal sequences of beta- xylosidases. (C) 2000 Elsevier Science Inc. All rights reserved.