title: Salicylate biosynthesis in Pseudomonas aeruginosa. Purification and characterization of PchB, a novel bifunctional enzyme displaying isochorismate pyruvate-lyase and chorismate mutase activities.
abstract: Isochorismate pyruvate-lyase (IPL), the second enzyme of pyochelin biosynthesis and the product of the pchB gene, was purified to homogeneity from Pseudomonas aeruginosa. In the reaction catalyzed by this enzyme, isochorismate --> salicylate + pyruvate, no cofactors appear to be required. At the pH optimum (pH 6.8), the enzyme displayed Michaelis-Menten kinetics, with an apparent K(m) of 12.5 microm for isochorismate and a kcat of 106 min(-1), calculated per monomer. The native enzyme behaved as a homodimer, as judged by molecular sieving chromatography, electrophoresis under nondenaturing conditions, and cross-linking experiments. PchB has approximately 20% amino acid sequence identity with AroQ-class chorismate mutases (CMs). Chorismate was shown to be converted to prephenate by purified PchB in vitro, with an apparent K(m) of 150 microm and a kcat of 7.8 min(-1). An oxabicyclic diacid transition state analog and well characterized inhibitor of CMs competitively inhibited both IPL and CM activities of PchB. Moreover, a CM-deficient Escherichia coli mutant, which is auxotrophic for phenylalanine and tyrosine, was functionally complemented by the cloned P. aeruginosa pchB gene for growth in minimal medium. A mutant form of PchB, in which isoleucine 88 was changed to threonine, had no detectable IPL activity, but retained wild-type CM activity. In conclusion, the 11.5-kDa subunit of PchB appears to contain a single active site involved in both IPL and CM activity.
authors.affiliation: Laboratoire de Biologie Microbienne, Université de Lausanne, CH-1015 Lausanne, Switzerland.
authors.forename: Catherine
authors.initials: C
authors.lastname: Gaille
authors.forename: Peter
authors.initials: P
authors.lastname: Kast
authors.forename: Dieter
authors.initials: D
authors.lastname: Haas
mesh_terms: D000595:Amino Acid Sequence; D001426:Bacterial Proteins / Q000737:chemistry* / Q000235:genetics / Q000302:isolation & purification*; D001667:Binding, Competitive; D015153:Blotting, Western; D002384:Catalysis; D002851:Chromatography, High Pressure Liquid; D003432:Cross-Linking Reagents / Q000494:pharmacology; D019281:Dimerization; D004305:Dose-Response Relationship, Drug; D004591:Electrophoresis, Polyacrylamide Gel; D004926:Escherichia coli / Q000378:metabolism; D005816:Genetic Complementation Test; D006863:Hydrogen-Ion Concentration; D007532:Isoleucine / Q000737:chemistry; D007700:Kinetics; D008956:Models, Chemical; D008958:Models, Molecular; D008969:Molecular Sequence Data; D009154:Mutation; D011485:Protein Binding; D017433:Protein Structure, Secondary; D017434:Protein Structure, Tertiary; D011550:Pseudomonas aeruginosa / Q000201:enzymology*; D012459:Salicylates / Q000378:metabolism*; D013696:Temperature
chemical_list: D001426:Bacterial Proteins; D003432:Cross-Linking Reagents; D012459:Salicylates; C097122:pchB protein, Pseudomonas aeruginosa; D007532:Isoleucine