Protonation of the binuclear metal center within the active site of phosphotriesterase.
Phosphotriesterase (PTE) is a binuclear metalloenzyme that catalyzes the hydrolysis of organophosphates, including pesticides and chemical warfare agents, at rates approaching the diffusion controlled limit. The catalytic mechanism of this enzyme features a bridging solvent molecule that is proposed to initiate nucleophilic attack at the phosphorus center of the substrate. X-band EPR spectroscopy is utilized to investigate the active site of Mn/Mn-substituted PTE. Simulation of the dominant EPR spectrum from the coupled binuclear center of Mn/Mn-PTE requires slightly rhombic zero-field splitting parameters. Assuming that the signal arises from the S = 2 manifold, an exchange coupling constant of J = -2.7 +/- 0.2 cm(-)(1) (H(ex) = -2JS(1) x S(2)) is calculated. A kinetic pK(a) of 7.1 +/- 0.1 associated with loss in activity at low pH indicates that a protonation event is responsible for inhibition of catalysis. Analysis of changes in the EPR spectrum as a function of pH provides a pK(a) of 7.3 +/- 0.1 that is assigned as the protonation of the hydroxyl bridge. From the comparison of kinetic and spectral pK(a) values, it is concluded that the loss of catalytic activity at acidic pH results from the protonation of the hydroxide that bridges the binuclear metal center.
Samples
Cynthia R
CR
Department of Chemistry, P.O. Box 30012, Texas A&M University, College Station, Texas 77842-3012, USA.
Howard
Timothy
T
Raushel
Frank M
FM
DeRose
Victoria J
VJ
D001419:Bacteria / Q000201:enzymology*; D001426:Bacterial Proteins / Q000737:chemistry* / Q000378:metabolism; D001665:Binding Sites / Q000502:physiology; D004578:Electron Spin Resonance Spectroscopy; D007700:Kinetics; D008345:Manganese / Q000737:chemistry* / Q000378:metabolism; D044345:Phosphoric Triester Hydrolases / Q000737:chemistry* / Q000378:metabolism; D011485:Protein Binding / Q000502:physiology; D017434:Protein Structure, Tertiary
D001426:Bacterial Proteins; D008345:Manganese; D044345:Phosphoric Triester Hydrolases