The enzymatic degradation of cellulose is an important process, both ecologically and commercially. The three-dimensional structure of a cellulase, the enzymatic core of CBHII from the fungus Trichoderma reesei reveals an alpha-beta protein with a fold similar to but different from the widely occurring barrel topology first observed in triose phosphate isomerase. The active site of CBHII is located at the carboxyl-terminal end of a parallel beta barrel, in an enclosed tunnel through which the cellulose threads. Two aspartic acid residues, located in the center of the tunnel are the probable catalytic residues.
Rouvinen, J., Bergfors, T., Teeri, T., Knowles, J., & Jones, A. (1990). Three dimensional structure of cellobiohydrolase II from Trichoderma reesei. Science, 249(4967), 380-386. https://doi.org/10.1126/science.2377893