In this paper, we determine the mutant W203F structure of TFs beta-glucanase, which contains aromatic residue Trp203 at the active site of the enzyme. Residue Trp203 is stacked with the glucose product of cellotriose. Further analysis reveals that two extra calcium ions and a Tris molecule bind to the mutant structure. A Iris molecule, bound to the catalytic residues of Glu56 and Glu60, was found at the position normally taken by substrate binding at the -1 subsite. In addition, a second Ca2+ ion was found near the residues Phe152 and Glu154 on the protein's surface, and a third one near the active site residue Asp202. Kinetic experiments reveal that both Iris and imidazole are competitive inhibitors, while calcium is a noncompetitive inhibitor for TFs beta-glucanase. The two types of enzymatic inhibition are first-time descriptions for the glycosyl hydrolase family 16. (C) 2011 Elsevier Inc. All rights reserved.