Crystal Structure of α-Galactosidase from Trichoderma reesei and Its Complex with Galactose: Implications for Catalytic Mechanism

• DOI: 10.1016/j.jmb.2004.03.062
• Publication Date: 2004-04-19T00:06:47Z
• Authors: A.M. Golubev; R.A.P. Nagem; J.R. Brandão Neto; K.N. Neustroev; E.V. Eneyskaya; A.A. Kulminskaya; K.A. Shabalin; A.N. Savel'ev; I. Polikarpov
• Keywords: Hydrolase; Antiparallel (mathematics)

The crystal structures of alpha-galactosidase from the mesophilic fungus Trichoderma reesei and its complex with the competitive inhibitor, beta-d-galactose, have been determined at 1.54 A and 2.0 A resolution, respectively. The alpha-galactosidase structure was solved by the quick cryo-soaking 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.