endoxylanases

Endoxylanases are enzymes that catalyze the hydrolysis of internal β-1,4-xylosidic linkages in xylan, the principal polysaccharide of plant cell walls. They are endo-acting xylanases that cut within the xylan backbone to produce shorter xylooligosaccharides, rather than removing xylose units from the ends. Endoxylanases belong mainly to glycoside hydrolase families GH10 and GH11, with related activities found in other families as well.

Substrate range and action: They act on various xylans, including beechwood and rye xylan, and can accommodate substituted xylans such as arabinoxylans. Substitutions on the backbone can influence activity, and in many cases endoxylanases work with accessory enzymes like α-L-arabinofuranosidases or acetyl xylan esterases that remove side chains to enhance hydrolysis. The enzymatic action yields a mixture of xylooligosaccharides with different degrees of polymerization.

Structure and mechanism: GH10 xylanases are generally larger and more tolerant of substitutions than GH11 xylanases, which are smaller and tend to have stricter substrate specificity. The catalytic step involves hydrolysis of the β-1,4-xylosidic bond; the exact mechanism and key residues vary by family, but both GH10 and GH11 enzymes perform internal cleavage of the xylan backbone.

Applications and production: Endoxylanases have broad industrial utility, including pulp and paper bleaching, animal feed improvement, baking and juice clarification, and the production of xylooligosaccharides for prebiotic use. They are produced by bacteria and fungi and are widely produced recombinantly for industrial use, with ongoing engineering to improve thermostability, pH tolerance, and substrate range.