Degradation of larchwood xylan by enzymes of a thermophilic fungus, Thermoascus aurantiacus.

Proteins from the culture filtrates of Thermoascus aurantiacus grown on paper were found to hydrolyze larchwood xylan completely to form xylose and 4-O-methyl-alpha-D-glucuronic acid. Partial hydrolysis of xylan by a xylanase purified from the culture filtrates resulted in the formation of neutral xylooligosaccharides of dp from 2 to 6 and acidic xylooligosaccharides of dp from 5 to 8. Each of these acidic sugars contained a single molecule of 4-O-methyl-alpha-D-glucuronic acid as a branch. Extensive hydrolysis of these oligosaccharides or xylan by xylanase led to the isolation of xylose, xylobiose, and an aldotetrauronic acid as terminal products. The structure of the aldotetrauronic acid was established by NMR as (2(2)-O-alpha-D,4-O-methyl-alpha-D-glucurono)-xylotriose. A beta-glucosidase, also purified from the culture filtrates, hydrolyzed xylan and the neutral or the acidic xylooligosaccharides from the nonreducing end to release only xylose. Neither xylanase nor beta-glucosidase hydrolyzed the beta-(1----4) linkage between the xylose carrying the branch and the adjacent xylose residue on each side.