title: Trimeric crystal structure of the glycoside hydrolase family 42 beta-galactosidase from Thermus thermophilus A4 and the structure of its complex with galactose.
abstract: The beta-galactosidase from an extreme thermophile, Thermus thermophilus A4 (A4-beta-Gal), is thermostable and belongs to the glycoside hydrolase family 42 (GH-42). As the first known structures of a GH-42 enzyme, we determined the crystal structures of free and galactose-bound A4-beta-Gal at 1.6A and 2.2A resolution, respectively. A4-beta-Gal forms a homotrimeric structure resembling a flowerpot. Each monomer has an active site located inside a large central tunnel. The N-terminal domain of A4-beta-Gal has a TIM barrel fold, as predicted from hydrophobic cluster analysis. The putative catalytic residues of A4-beta-Gal (Glu141 and Glu312) superimpose well with the catalytic residues of Escherichia coli beta-galactosidase. The environment around the catalytic nucleophile (Glu312) is similar to that in the case of E.coli beta-galactosidase, but the recognition mechanism for a substrate is different. Trp182 of the next subunit of the trimer constitutes a part of the active-site pocket, indicating that the trimeric structure is essential for the enzyme activity. Structural comparison with other glycoside hydrolases revealed that many features of the 4/7 superfamily are conserved in the A4-beta-Gal structure. On the basis of the results of 1H NMR spectroscopy, A4-beta-Gal was determined to be a "retaining" enzyme. Interestingly, the active site was similar with those of retaining enzymes, but the overall fold of the TIM barrel domain was very similar to that of an inverting enzyme, beta-amylase.
authors.affiliation: Department of Biotechnology, The University of Tokyo, Japan.
authors.forename: Masafumi
authors.initials: M
authors.lastname: Hidaka
authors.forename: Shinya
authors.initials: S
authors.lastname: Fushinobu
authors.forename: Naomi
authors.initials: N
authors.lastname: Ohtsu
authors.forename: Hidemasa
authors.initials: H
authors.lastname: Motoshima
authors.forename: Hiroshi
authors.initials: H
authors.lastname: Matsuzawa
authors.forename: Hirofumi
authors.initials: H
authors.lastname: Shoun
authors.forename: Takayoshi
authors.initials: T
authors.lastname: Wakagi
mesh_terms: D000595:Amino Acid Sequence; D001665:Binding Sites; D002384:Catalysis; D020134:Catalytic Domain; D018360:Crystallography, X-Ray; D004926:Escherichia coli / Q000201:enzymology; D005690:Galactose / Q000737:chemistry / Q000378:metabolism*; D006860:Hydrogen Bonding; D006868:Hydrolysis; D057927:Hydrophobic and Hydrophilic Interactions; D009682:Magnetic Resonance Spectroscopy; D008670:Metals / Q000378:metabolism; D008958:Models, Molecular; D008969:Molecular Sequence Data; D020836:Protein Structure, Quaternary; D017434:Protein Structure, Tertiary; D021122:Protein Subunits; D016415:Sequence Alignment; D013237:Stereoisomerism; D016963:Thermus thermophilus / Q000201:enzymology*; D001616:beta-Galactosidase / Q000737:chemistry* / Q000145:classification / Q000378:metabolism*
chemical_list: D008670:Metals; D021122:Protein Subunits; D001616:beta-Galactosidase; D005690:Galactose