We have determined the crystal structure of Trichoderma reesei (Hypocrea jecorina) beta-galactosidase (Tr-beta-gal) at a 1.2 angstrom resolution and its complex structures with galactose, IPTG and PETG at 1.5, 1.75 and 1.4 angstrom resolutions, respectively. Tr-beta-gal is a potential enzyme for lactose hydrolysis in the dairy industry and belongs to family 35 of the glycoside hydrolases (GH-35). The high resolution crystal structures of this six-domain enzyme revealed interesting features about the structure of Tr-beta-gal. We discovered conformational changes in the two loop regions in the active site, implicating a conformational selection-mechanism for the enzyme. In addition, the Glu200, an acid/base catalyst showed two different conformations which undoubtedly affect the pK(a) value of this residue and the catalytic mechanism. The electron density showed extensive glycosylation, suggesting a structure stabilizing role for glycans. The longest glycan showed an electron density that extends to the eighth monosaccharide unit in the extended chain. The Tr-beta-gal structure also showed a well-ordered structure for a unique octaserine motif on the surface loop of the fifth domain. (C) 2010 Elsevier Inc. All rights reserved.