Thermostable microbial xylanases for pulp and paper industries: trends, applications and further perspectives

Xylanases are enzymes with biotechnological relevance in a number of fields, including food, feed, biofuel, and textile industries. Their most significant application is in the paper and pulp industry, where they are used as a biobleaching agent, showing clear economic and environmental advantages over chemical alternatives. Since this process requires high temperatures and alkali media, the identification of thermostable and alkali stable xylanases represents a major biotechnological goal in this field. Moreover, thermostability is a desirable property for many other applications of xylanases. The review makes an overview of xylanase producing microorganisms and their current implementation in paper biobleaching. Future perspectives are analyzed focusing in the efforts carried out to generate thermostable enzymes by means of modern biotechnological tools, including metagenomic analysis, enzyme molecular engineering and nanotechnology. Furthermore, structural and mutagenesis studies have revealed critical sites for stability of xylanases from glycoside hydrolase families GH10 and GH11, which constitute the main classes of these enzymes. The overall conclusions of these works are summarized here and provide relevant information about putative weak spots within xylanase structures to be targeted in future protein engineering approaches.