Microbial exo-xylanases: a mini review.

Exo-xylanases are a class of glycosyl hydrolases and play an important role in hydrolysis of xylan to xylose. They belong to glycosyl hydrolase (GH) family 8 with a characteristic (α/α)6 barrel fold in their molecular structures. These enzymes are generally produced by bacteria. Exceptionally, the endo-xylanases from Trichoderma reesei Rut C-30 and a few bacterial strains also show considerable exo-xylanase activities. Exo-xylanases are active on natural xylan substances, hydrolyzing long-chain xylo-oligomers from the reducing end to produce short-chain xylo-oligomers and xylose. Exo-xylanases usually show multiple enzyme functions such as β-xylosidase, exo-glucanase, β-glucosidase, and arabinofuranosidase activities, which are helpful for more efficient hydrolysis of xylan. The combined use of exo- and endo-xylanases can increase the xylose yield compared to using either of them alone. Screening new exo-xylanase-producing microbes, mining the enzyme coding sequences, genetically engineering the enzymes, and producing them in a large scale are recommended for their commercial applications in lignocellulose-based biorefinery.