Structural and biochemical characterization of a family 7 highly thermostable endoglucanase from the fungus Rasamsonia emersonii

Thermostable cellulases from glycoside hydrolase family 7 (GH7) are the main components of enzymatic mixtures for industrial saccharification of lignocellulose. Activity improvement of these enzymes via rational design is a promising strategy to alleviate the industrial costs, but it requires detailed structural knowledge. While substantial biochemical and structural data are available for GH7 cellobiohydrolases, endoglucanases are more elusive and only few structures have been solved so far. Here, we report a new crystal structure and biochemical characterization of a thermostable endoglucanase from the thermophilic ascomycete Rasamsonia emersonii , Re Cel7B. The enzyme was compared with the homologous endoglucanase from the mesophilic model ascomycete Trichoderma reesei ( Tr Cel7B), which unlike Re Cel7B possesses an additional carbohydrate‐binding module (CBM). With a temperature optimum of 80 °C, Re Cel7B displayed a number of differences in activity and ability to synergize with cellobiohydrolases compared to Tr Cel7B. We improved both binding and kinetics in a chimeric variant of Re Cel7B and a CBM, while we observe the opposite effect when the CBM was removed in Tr Cel7B. The crystal structure of Re Cel7B was determined at 2.48 Å resolution, with R work and R free factors of 0.182 and 0.206, respectively. Structural analyses revealed that Re Cel7B has increased rigidity in a number of peripheral loops compared to Tr Cel7B and fewer aromatics in the substrate‐binding cleft. An increased number of glycosylations were identified in Re Cel7B, and we propose a stabilizing mechanism for one of the glycans. Global structure–function interpretations of Re Cel7B highlight the differences in temperature stability, turnover, binding, and cellulose accessibility in GH7 endoglucanases. Database Structural data are available in RCSB Protein Data Bank database under the accession number 6SU8 . Enzymes Re Cel7B, endoglucanase ( EC3.2.1.4 ) from Rasamsonia emersonii ; Re Cel7A, cellobiohydrolase ( EC3.2.1.176 ) from Rasamsonia emersonii ; Tr Cel7B, endoglucanase ( EC3.2.1.4 ) from Trichoderma reesei ; Tr Cel7A, cellobiohydrolase ( EC3.2.1.176 ) from Trichoderma reesei .