Heat- and Alkaline-Stable Xylanases: Application, Protein Structure and Engineering

Xylan is the most abundant hemicellulosic component, and its degradation is the major step in a wide variety of industrial processes. Xylanases, which are capable of randomly hydrolyzing the β-1,4-xylosidic linkages, constitute the key enzymes in xylan decomposition. As xylanases hold a high economic potential in the marketplace, great efforts have been made in search of suitable candidates for the development into commercial products with better performances such as higher specific activity, better thermostability, higher operating temperatures, and a better pH profile. To achieve these goals, the three-dimensional structural information of xylanases with the extremophilic properties is of pivotal importance. In this review, the current knowledge about heat- and alkaline-stable xylanase structures is summarized, including protein engineering studies and focusing on the glycoside hydrolase families 10 and 11 enzymes. These structures pave the way to understanding the catalytic mechanisms of xylanases and provide an important engineering basis for their industrial applications.