Glycoside Hydrolase Family 48 Cellulase: A Key Player in Cellulolytic Bacteria for Lignocellulose Biorefinery

Abstract

Cellulases from glycoside hydrolase family 48 (GH48) are critical components of natural lignocellulose-degrading systems. GH48 cellulases are broadly distributed in cellulolytic microorganisms. With the development of genomics and metatranscriptomics, diverse GH48 genes have been identified, especially in the highly efficient cellulose-degrading ruminal system. GH48 cellulases utilize an inverting mechanism to hydrolyze cellulose in a processive mode. Although GH48 cellulases are indispensable for cellulolytic bacteria, they exhibit intrinsically low cellulolytic activity. Great efforts have been made to improve their performance. Besides, GH48 cellulases greatly synergize with the complementary endoglucanases in free cellulase systems or cellulosome systems. In this review, we summarized the studies on the diversity of GH48 cellulases, the crystal structures, the catalytic mechanism, the synergy between GH48 cellulases and endocellulases, and the strategies and progress of GH48 engineering. According to the summarized bottlenecks in GH48 research and applications, we suggest that future studies should be focused on mining and characterizing new GH48 enzymes, thoroughly understanding the progressive activity and product inhibition, engineering GH48 enzymes to improve stability, activity, and stress resistance, and designing and developing new biocatalytic system employing the synergies between GH48 and other enzymes.