title: Aspergillus aculeatus beta-1,4-galactanase: substrate recognition and relations to other glycoside hydrolases in clan GH-A.
abstract: The three-dimensional structure of Aspergillus aculeatus beta-1,4-galactanase (AAGAL), an enzyme involved in pectin degradation, has been determined by multiple isomorphous replacement to 2.3 and 1.8 A resolution at 293 and 100 K, respectively. It represents the first known structure for a polysaccharidase with this specificity and for a member of glycoside hydrolase family 53 (GH-53). The enzyme folds into a (beta/alpha)(8) barrel with the active site cleft located at the C-terminal side of the barrel consistent with the classification of GH-53 in clan GH-A, a superfamily of enzymes with common fold and catalytic machinery but diverse specificities. Putative substrate-enzyme interactions were elucidated by modeling of beta-1,4-linked galactobioses into the possible substrate binding subsites. The structure and modeling studies identified five potential subsites for the binding of galactans, of which one is a pocket suited for accommodating the arabinan side chain in arabinogalactan, one of the natural substrates. A comparison with the substrate binding grooves of other Clan GH-A enzymes suggests that shape complementarity is crucial in determining the specificity of polysaccharidases.
authors.affiliation: Centre for Crystallographic Studies, Department of Chemistry, University of Copenhagen, Universitetsparken 5, DK-2100 Copenhagen, Denmark.
authors.forename: Carsten
authors.initials: C
authors.lastname: Ryttersgaard
authors.forename: Leila
authors.initials: L
authors.lastname: Lo Leggio
authors.forename: Pedro M
authors.initials: PM
authors.lastname: Coutinho
authors.forename: Bernard
authors.initials: B
authors.lastname: Henrissat
authors.forename: Sine
authors.initials: S
authors.lastname: Larsen
mesh_terms: D001230:Aspergillus / Q000201:enzymology*; D001426:Bacterial Proteins; D001665:Binding Sites; D020134:Catalytic Domain; D003198:Computer Simulation; D017124:Conserved Sequence; D018360:Crystallography, X-Ray; D004187:Disaccharides / Q000737:chemistry; D005656:Fungal Proteins / Q000737:chemistry* / Q000145:classification; D005685:Galactans / Q000737:chemistry; D006026:Glycoside Hydrolases; D008759:Methylglycosides; D008958:Models, Molecular; D005810:Multigene Family; D011487:Protein Conformation; D017433:Protein Structure, Secondary; D013379:Substrate Specificity; D001616:beta-Galactosidase / Q000737:chemistry* / Q000145:classification*
chemical_list: D001426:Bacterial Proteins; D004187:Disaccharides; D005656:Fungal Proteins; D005685:Galactans; D008759:Methylglycosides; C056853:6-O-galactopyranosylgalactose; C034207:methyl lactoside; D006026:Glycoside Hydrolases; D001616:beta-Galactosidase; C094192:arabinogalactan endo-1,4-beta-galactosidase; C109490:galactanase A