Cadmium(II) complexes of the glycerophosphodiester-degrading enzyme GpdQ and a biomimetic N,O ligand

The glycerophosphodiester-degrading enzyme GpdQ from Enterobacter aerogenes is a promising bioremediator owing to its ability to degrade some organophosphate pesticides and diester products originating from the hydrolysis of nerve agents such as VX. Here, the cadmium derivative of GpdQ was prepared by reconstituting the apoenzyme. Catalytic measurements with (Cd ²⁺)₂-GpdQ and the phosphodiester substrate bis(4-nitrophenyl)phosphate yield k _cat = 15 s^-1. The pK _a of 9.4, determined from the pH dependence of the catalytic activity, implicates a hydroxide ligand as the catalytic nucleophile. Also prepared was the cadmium-containing biomimetic [Cd₂((HP) ₂B)(OAc)₂(OH₂)](PF₆) (where (HP)₂B is [2,6-bis([(2-pyridylmethyl)(2-hydroxyethyl)amino]methyl)-4- methylphenol]), which mimics the asymmetry of the metal ion coordination in the active site of GpdQ. The phosphoesterase-like activity of [Cd ₂((HP)₂B)(OAc)₂(OH₂)](PF ₆) was studied using the substrate bis(2,4-dinitrophenyl)phosphate, yielding a kinetically relevant pK _a of 8.9, with k _cat = 0.004 s^-1. In summary, the model is both an adequate structural and a reasonable functional mimic of GpdQ.