Xylanase from Ceratocystis paradoxa

From rate and product studies with xylooligosaccharides it was concluded that a xylanase from Ceratocystis paradoxa requires a chain of at least five xylose residues for rapid binding and subsequent hydrolysis (18). The catalytic site is assumed to be situated asymmetrically within a row of the five binding subsites. Similar studies on a cellulase from Aspergillus niger also suggest the presence of five binding subsites (14). 

Degradation products arising from the hydrolysis of arabinoxylo-and xylo-oligosaccharides of d.p. 3-6 and 2-5, respectively, by the action of a D-xylanase (HC I) from Ceratocystis paradoxa are shown in Table XXIV. Xylotetraose (X4) was the smallest xylo-oligosaccharide attacked, and both it and X5 yielded X2 and X3. The mechanism proposed228 for the hydrolysis of X4 by HC I is as follows. 

Proposed structure (see Refs. 228, 236, and 249) from the results of the action of the endo-D-xylanases (HC I and HC II) from Ceratocystis paradoxa on this oligosaccharide. 

The molecular weights of D-xylanase preparations of different origins (see Table XXII), like those of the other hemicellulases (see Sections II, III, and IV), are relatively low, ranging from 16,000 to 38,000. The pi values reported for several D-xylanases are also shown in Table XXII, and, with the exception of the Ceratocystis paradoxa D-xylanase I (Ref. 228) (pi 9.17), are mainly acid gly-canases. 

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