Correlation of Structure and Function in Oligonuclear Zinc(II) Model Phosphatases

A series of pyrazolate-based dizinc(II) complexes has been synthesized and investigated as functional models for
phosphoesterases, focusing on correlations between hydrolytic activity and molecular parameters of the bimetallic
core. The Zn···Zn distance, the (bridging or nonbridging) position of the Zn-bound hydroxide nucleophile, and
individual metal ion coordination numbers are controlled by the topology of the compartmental ligand scaffold.
Species distributions of the various dizinc complexes in solution have been determined potentiometrically, and
structures in the solid state have been elucidated by X-ray crystallography. The hydrolysis of bis(p-nitrophenyl)phosphate (BNPP) promoted by the dinuclear phosphoesterase model complexes has been investigated in DMSO/buffered water (1:1) at 50 °C as a function of complex concentration, substrate concentration, and pH. Coordination
of the phosphodiester has been followed by ESI mass spectrometry, and bidentate binding could be verified
crystallographically in two cases. Drastic differences in hydrolytic activity are observed and can be attributed to
molecular properties. A significant decrease of the pKa of zinc-bound water is observed if the resulting hydroxide
is involved in a strongly hydrogen-bonded intramolecular O2H3 bridge, which can be even more pronounced than
for a bridging hydroxide. Irrespective of the pKa of the Zn-bound water, a hydroxide in a bridging position evidently
is a relatively poor nucleophile, while a nonbridging hydroxide position is more favorable for hydrolytic activity.
Additionally, the metal array has to provide a sufficient number of coordination sites for activating both the substrate
and the nucleophile, where phosphate diesters such as BNPP preferentially bind in a bidentate fashion, requiring
a third site for water binding. Product inhibition of the active site by the liberated (p-nitrophenyl)phosphate is observed,
and the product-inhibited complex could be characterized crystallographically. In that complex, the phosphate
monoester is found to cap a rectangular array of four zinc ions composed of two bimetallic entities.