Contribution of the active-site metal cation to the catalytic activity and to the conformational stability of phosphotriesterase: temperature- and pH-dependence.
Phosphotriesterase (PTE) detoxifies nerve agents and organophosphate pesticides. The two zinc cations of the PTE active centre can be substituted by other transition metal cations without loss of activity. Furthermore, metal-substituted PTEs display differences in catalytic properties. A prerequisite for engineering highly efficient mutants of PTE is to improve their thermostability. Isoelectric focusing, capillary electrophoresis and steady-state kinetics analysis were used to determine the contribution of the active-site cations Zn2+, Co2+ or Cd2+ to both the catalytic activity and the conformational stability of the corresponding PTE isoforms. The three isoforms have different pI values (7.2, 7.5 and 7.1) and showed non-superimposable electrophoretic titration curves. The overall structural alterations, causing changes in functional properties, were found to be related to the nature of the bound cation: ionic radius and ion electronegativity correlate with Km and kcat respectively. In addition, the pH-dependent activity profiles of isoforms were different. The temperature-dependent profiles of activity showed maximum activity at T < or =35 degrees C, followed by an activation phase near 45-48 degrees C and then inactivation which was completed at 60 degrees C. Analysis of thermal denaturation of the PTEs provided evidence that the activation phase resulted from a transient intermediate. Finally, at the optimum activity between pH 8 and 9.4, the thermostability of the different PTEs increased as the pH decreased, and the metal cation modulated stability (Zn2+-, Co2+- and Cd2+-PTE showed different T (m) values of 60.5-67 degrees C, 58-64 degrees C and 53-64 degrees C respectively). Requirements for optimum activity of PTE (displayed by Co2+-PTE) and maximum stability (displayed by Zn2+-PTE) were demonstrated.
Rochu
Daniel
D
Unité d'Enzymologie, Centre de Recherches du Service de Santé des Armées, BP 87, 38702 La Tronche cedex, France. danielrochu@crssa.net
Viguié
Nathalie
N
Renault
Frédérique
F
Crouzier
David
D
Froment
Marie-Thérèse
MT
Masson
Patrick
P
D001426:Bacterial Proteins / Q000737:chemistry / Q000378:metabolism; D001665:Binding Sites / Q000502:physiology; D002384:Catalysis; D002413:Cations, Divalent / Q000378:metabolism*; D019075:Electrophoresis, Capillary / Q000379:methods; D004795:Enzyme Stability; D006863:Hydrogen-Ion Concentration; D007525:Isoelectric Focusing / Q000379:methods; D007526:Isoelectric Point; D007527:Isoenzymes / Q000737:chemistry / Q000378:metabolism; D008670:Metals / Q000378:metabolism*; D044345:Phosphoric Triester Hydrolases / Q000737:chemistry* / Q000378:metabolism*; D011487:Protein Conformation; D011549:Pseudomonas / Q000201:enzymology; D013696:Temperature
D001426:Bacterial Proteins; D002413:Cations, Divalent; D007527:Isoenzymes; D008670:Metals; D044345:Phosphoric Triester Hydrolases