title: Structure of the type I L-asparaginase from the hyperthermophilic archaeon Pyrococcus horikoshii at 2.16 angstroms resolution.
abstract: The crystal structure of the L-asparaginase from the hyperthermophilic archaeon Pyrococcus horikoshii (PhA) was determined by the multiwavelength anomalous diffraction (MAD) method and was refined to a resolution of 2.16 angstroms with a crystallographic R factor and free R factor of 21.1 and 25.3%, respectively. This is the first report of the three-dimensional structure of a type I L-asparaginase. These enyzmes are known as cytosolic L-asparaginases with lower affinities for substrate than the type II L-asparaginases. Although the overall fold of PhA was closely related to the structure of the well characterized type II L-asparaginase, structural differences were also detected. PhA forms a homodimer that corresponds to half the homotetramer of type II L-asparaginases. Structure comparison at the active site reveals that most catalytic residues are conserved except for two residues that recognize the amino group of the substrate. Additionally, a remarkable structural difference is found in the so-called 'active-site flexible loop'. In PhA this loop is stabilized by beta-hairpin formation and by elaborate interactions with the type-I-specific alpha-helical region derived from the other subunit forming the PhA dimer. The flexible loop of the type II enzyme is considered to serve as a mobile gate to the active site. Therefore, the loop stabilization observed in the PhA structure may cause limitation of the access of the substrate to the active site.
authors.affiliation: Division of Biological Sciences, Graduate School of Science, Hokkaido University, Kita-10, Nishi-8, Kita-ku, Sapporo, Hokkaido 060-0810, Japan.
authors.forename: Min
authors.initials: M
authors.lastname: Yao
authors.forename: Yoshiaki
authors.initials: Y
authors.lastname: Yasutake
authors.forename: Hazuki
authors.initials: H
authors.lastname: Morita
authors.forename: Isao
authors.initials: I
authors.lastname: Tanaka
mesh_terms: D000595:Amino Acid Sequence; D001215:Asparaginase / Q000737:chemistry*; D001665:Binding Sites; D003460:Crystallization; D018360:Crystallography, X-Ray; D019281:Dimerization; D008958:Models, Molecular; D008969:Molecular Sequence Data; D011487:Protein Conformation; D043902:Pyrococcus horikoshii / Q000201:enzymology*; D017386:Sequence Homology, Amino Acid
chemical_list: D001215:Asparaginase