Self-assembly of the binuclear metal center of phosphotriesterase

The active site of the bacterial phosphotriesterase (PTE) from Pseudomonas diminuta contains two divalent metal ions and a carboxylated lysine residue. The native enzyme contains two Zn²⁺ ions, which can be replaced with Co²⁺, Cd²⁺, Ni²⁺, or Mn²⁺ without loss of catalytic activity. Carbon dioxide reacts with the side chain of lysine-169 to form a carbamate functional group within the active site, which then serves as a bridging ligand to the two metal ions. The activation of apo-PTE using variable concentrations of divalent metal ions and bicarbonate was measured in order to establish the mechanism by which the active site of PTE is self- assembled. The time courses for the activation of apo-PTE are pseudo-first- order, and the observed rate constants are directly proportional to the concentration of bicarbonate. In contrast, the apparent rate constants for the activation of apo-PTE decrease as the concentrations of the divalent cations are increased and then become constant at higher concentrations of the divalent metal ions. These results are consistent with a largely ordered kinetic mechanism for the assembly of the binuclear metal center where CO₂/bicarbonate reacts with the apo-PTE prior to the binding of the two metal ions. When apo-PTE is titrated with 0-8 equiv of Co²⁺, Cd²⁺, or Zn²⁺, the concentration of activated enzyme increases linearly until 2 equiv of metal ion is added and then remains constant at elevated levels of the divalent cations. These results are consistent with the synergistic binding of the two metal ions to the active site, and thus the second metal ion binds more tightly to the protein than does the first metal ion. Measurement of the mean dissociation constant indicates that metal binding to the binuclear metal center is strong [(KαKβ)( 1/2 ) = 6.0 x 10^-11 M and k(off) = 1.5 x 10^-3 min^-1 for Zn²⁺]. The removal of the carbamate bridge through the mutagenesis of Lys-169 demonstrates that the carbamate bridge is required for both efficient catalysis and overall stability of the metal center.