Transition from octahedral to tetrahedral geometry causes the activation or inhibition by Zn2+ of Pseudomonas aeruginosa phosphorylcholine phosphatase

Pseudomonas aeruginosa phosphorylcholine phosphatase (PchP) catalyzes the hydrolysis of phosphorylcholine, which is produced by the action of hemolytic phospholipase C on phosphatidylcholine or sphyngomielin, to generate choline and inorganic phosphate. Among divalent cations, its activity is dependent on Mg²⁺ or Zn²⁺. Mg²⁺ produced identical activation at pH 5.0 and 7.4, but Zn²⁺ was an activator at pH 5.0 and became an inhibitor at pH 7.4. At this higher pH, very low concentrations of Zn²⁺ inhibited enzymatic activity even in the presence of saturating Mg²⁺ concentrations. Considering experimental and theoretical physicochemical calculations performed by different authors, we conclude that at pH 5.0, Mg²⁺ and Zn²⁺ are hexacoordinated in an octahedral arrangement in the PchP active site. At pH 7.4, Mg²⁺ conserves the octahedral coordination maintaining enzymatic activity. The inhibition produced by Zn²⁺ at 7.4 is interpreted as a change from octahedral to tetrahedral coordination geometry which is produced by hydrolysis of the [ textZn ²⁺ textL ₂ ^{- 1} textL ₂⁰ ( textH ₂ textO ) ₂ ] left[ {{text{Zn}}^{ 2+ } {text{L}}_{ 2}^{ - 1} {text{L}}_{ 2}^{0} left( {{text{H}}_{ 2} {text{O}}} right)_{ 2} } right] complex.