W0297

Structure and Mechanism of Endo/Exocellulase E4 from Thermomonospora Fusca. Joshua Sakon, P. Andrew Karplus, Chemistry and Biochemistry, University of Arkansas, Fayetteville, AR 72701 USA

Cellulase E4 from Thermomonospora fusca is unusual in that it has characteristics of both exo- and endo-cellulases. Here we report the crystal structure of a 68 kDa fragment of E4 (E4-68) at 1.9 Å resolution. E4-68 contains both a family 9 catalytic domain, exhibiting an ([alpha]/[alpha])₆ barrel fold, and a family III cellulose binding domain, having an antiparallel [beta]-sandwich fold. While neither of these folds are novel, E4-68 provides the first cellulase structure having interacting catalytic and cellulose binding domains. The complexes of E4-68 with cellopentaose, cellotriose and cellobiose reveal conformational changes associated with ligand binding and allow us to propose a catalytic mechanism for family 9 enzymes. We also provide evidence that E4 has two novel characteristics: first it combines exo- and endo- activities and second when it functions as an exo-cellulase, it cleaves off cellotetraose units.