title: 2-Hydroxychromene-2-carboxylic acid isomerase: a kappa class glutathione transferase from Pseudomonas putida.
abstract: The enzyme 2-hydroxychromene-2-carboxylic acid (HCCA) isomerase catalyzes the glutathione (GSH)-dependent interconversion (Keq = 1.5) of HCCA and trans-o-hydroxybenzylidene pyruvic acid (tHBPA) in the naphthalene catabolic pathway of Pseudomonas putida. The dimeric protein binds one molecule of GSH very tightly (Kd approximately 5 nM) and a second molecule of GSH with much lower affinity (Kd approximately 2 to 11 microM). The enzyme is unstable in the absence of GSH. The turnover number in the forward direction (47 s(-1) at 25 degrees C) greatly exceeds off rates for GSH (koff approximately 10(-3) to 10(-2) s(-1) at 10 degrees C), suggesting that GSH acts as a tightly bound cofactor in the reaction. The crystal structure of the enzyme at 1.7 A resolution reveals that the isomerase is closely related to class kappa GSH transferases. Diffraction quality crystals could only be obtained in the presence of GSH and HCCA/tHBPA. Clear electron density is seen for GSH. Electron density for the organic substrates is located near the GSH and is best modeled to include both HCCA and tHBPA at occupancies of 0.5 for each. Although there is no electron density connecting the sulfur of GSH to the organic substrates, the sulfur is located very close (2.78 A) to C7 of HCCA. Taken together, the results suggest that the isomerization reaction involves a short-lived covalent adduct between the sulfur of GSH and C7 of the substrate.
authors.affiliation: Department of Biochemistry, Center in Molecular Toxicology, and Center for Structural Biology, Vanderbilt University School of Medicine, Nashville, Tennessee 37232-0146, USA.
authors.forename: Lawrence C
authors.initials: LC
authors.lastname: Thompson
authors.forename: Jane E
authors.initials: JE
authors.lastname: Ladner
authors.forename: Simona G
authors.initials: SG
authors.lastname: Codreanu
authors.forename: Joel
authors.initials: J
authors.lastname: Harp
authors.forename: Gary L
authors.initials: GL
authors.lastname: Gilliland
authors.forename: Richard N
authors.initials: RN
authors.lastname: Armstrong
mesh_terms: D000595:Amino Acid Sequence; D000818:Animals; D001426:Bacterial Proteins / Q000737:chemistry / Q000378:metabolism; D001665:Binding Sites; D003001:Cloning, Molecular; D005978:Glutathione / Q000378:metabolism; D005982:Glutathione Transferase / Q000737:chemistry / Q000378:metabolism; D019746:Intramolecular Oxidoreductases / Q000737:chemistry / Q000378:metabolism*; D007700:Kinetics; D008958:Models, Molecular; D008969:Molecular Sequence Data; D011487:Protein Conformation; D016958:Pseudomonas putida / Q000201:enzymology*; D051381:Rats; D011994:Recombinant Proteins / Q000737:chemistry / Q000378:metabolism; D016415:Sequence Alignment; D017386:Sequence Homology, Amino Acid
chemical_list: D001426:Bacterial Proteins; D011994:Recombinant Proteins; D005982:Glutathione Transferase; D019746:Intramolecular Oxidoreductases; C078381:2-hydroxychromene-2-carboxylate isomerase; D005978:Glutathione