title: Autoprocessing of Helicobacter pylori gamma-glutamyltranspeptidase leads to the formation of a threonine-threonine catalytic dyad.
abstract: Helicobacter pylorigamma-glutamyltranspeptidase (HpGT) is a glutathione-degrading enzyme that has been shown to be a virulence factor in infection. It is expressed as a 60-kDa inactive precursor that must undergo autocatalytic processing to generate a 40-kDa/20-kDa heterodimer with full gamma-glutamyl amide bond hydrolase activity. The new N terminus of the processed enzyme, Thr-380, is the catalytic nucleophile in both the autoprocessing and enzymatic reactions, indicating that HpGT is a member of the N-terminal nucleophile hydrolase superfamily. To further investigate activation as a result of autoprocessing, the structure of HpGT has been determined to a resolution of 1.9 A. The refined model contains two 40-kDa/20-kDa heterodimers in the asymmetric unit and has structural features comparable with other N-terminal nucleophile hydrolases. Autoprocessing of HpGT leads to a large conformational change, with the loop preceding the catalytic Thr-380 moving >35 A, thus relieving steric constraints that likely limit substrate binding. In addition, cleavage of the proenzyme results in the formation of a threonine-threonine dyad comprised of Thr-380 and a second conserved threonine residue, Thr-398. The hydroxyl group of Thr-398 is located equidistant from the alpha-amino group and hydroxyl side chain of Thr-380. Mutation of Thr-398 to an alanine results in an enzyme that is fully capable of autoprocessing but is devoid of enzymatic activity. Substrate docking studies in combination with homology modeling studies of the human homologue reveal additional mechanistic details of enzyme maturation and activation, substrate recognition, and catalysis.
authors.affiliation: Department of Biochemistry, University of Nebraska, Lincoln, Nebraska 68588-0664, USA.
authors.forename: Gina
authors.initials: G
authors.lastname: Boanca
authors.forename: Aaron
authors.initials: A
authors.lastname: Sand
authors.forename: Toshihiro
authors.initials: T
authors.lastname: Okada
authors.forename: Hideyuki
authors.initials: H
authors.lastname: Suzuki
authors.forename: Hidehiko
authors.initials: H
authors.lastname: Kumagai
authors.forename: Keiichi
authors.initials: K
authors.lastname: Fukuyama
authors.forename: Joseph J
authors.initials: JJ
authors.lastname: Barycki
mesh_terms: D001665:Binding Sites; D002384:Catalysis; D018360:Crystallography, X-Ray; D019281:Dimerization; D005978:Glutathione / Q000737:chemistry; D016480:Helicobacter pylori / Q000201:enzymology*; D006801:Humans; D007700:Kinetics; D008956:Models, Chemical; D008958:Models, Molecular; D011485:Protein Binding; D017434:Protein Structure, Tertiary; D013379:Substrate Specificity; D013912:Threonine / Q000737:chemistry*; D005723:gamma-Glutamyltransferase / Q000737:chemistry* / Q000378:metabolism
chemical_list: D013912:Threonine; D005723:gamma-Glutamyltransferase; D005978:Glutathione