Crystal structure of α-galactosidase from Trichoderma reesei and its complex with galactose:: Implications for catalytic mechanism

The crystal structures of alpha-galactosidase from the mesophilic fungus Trichoderma reesei and its complex with the c mpetitive inhibitor, beta-D-galactose, have been determined at 1.54 Angstrom and 2.0 Angstrom resolution, respectively. The alpha-galactosidase structure was solved by the quick cryosoaking method using a single Cs derivative. The refined crystallographic model of the alpha-galactosidase consists of two domains, an N-terminal catalytic domain of the (beta/alpha)(8) barrel topology and a C-terminal domain which is formed by an antiparallel beta-structure. The protein contains four N-glycosylation sites located in the catalytic domain. Some of the oligosaccharides were found to participate in inter-domain contacts. The galactose molecule binds to the active site pocket located in the center of the barrel of the catalytic domain. Analysis of the alpha-galactosidase-galactose complex reveals the residues of the active site and offers a structural basis for identification of the putative mechanism of the enzymatic reaction. The structure of the alpha-galactosidase closely resembles those of the glycoside hydrolase family 27. The conservation of two catalytic Asp residues, identified for this family, is consistent with a double-displacement reaction mechanism for the alpha-galactosidase. Modeling of possible substrates into the active site reveals specific hydrogen bonds and hydrophobic interactions that could explain peculiarities of the enzyme kinetics. (C) 2004 Elsevier Ltd. All rights. reserved.