Protein‐Assisted Formation and Stabilization of Catalytically Active Polyoxometalate Species

Abstract

The effect of the protein environment on the formation and stabilization of an elusive catalytically active polyoxometalate (POM) species, K [Hf(α -P W O )] (1), is reported. In the co-crystal of hen egg-white lysozyme (HEWL) with 1, the catalytically active monomeric species is observed, originating from the dimeric 1:2 POM form, while it is intrinsically unstable under physiological pH conditions. The protein-assisted dissociation of the dimeric POM was rationalized by means of DFT calculations. The dissociation process is unfavorable in bulk water, but becomes favorable in the protein-POM complex due to the low dielectric response at the protein surface. The crystal structure shows that the monomeric form is stabilized by electrostatic and water-mediated hydrogen bonding interactions with the protein. It interacts at three distinct sites, close to the aspartate-containing hydrolysis sites, demonstrating high selectivity towards peptide bonds containing this residue.

Noncovalent Complexes Formed between Metal‐Substituted Polyoxometalates and Hen Egg White Lysozyme

“…By analyzing the refined co‐crystal structures (Table ), a clear side‐by‐side comparison of POM‐HEWL interaction sites could be made. Similarly to the previously reported structures of HEWL with Zr IV ‐Keggin and Hf IV ‐Wells–Dawson POMs, three distinct interaction sites were observed in all crystals (Figure ) , . Each interaction site consists of two slightly different POM binding modes with HEWL, due to its aforementioned special position in the crystal (Figure ).…”

“…HEWL was mixed with four Wells–Dawson type POMs: a dimeric [Zr IV (α 2 ‐P 2 W 17 O 61 ) 2 ] 16– and three monomeric POMs [Co II (α 2 ‐P 2 W 17 O 61 )] 10– ( 2 ), [Ni II (α 2 ‐P 2 W 17 O 61 )] 10– ( 3 ) and [Cu II (α 2 ‐P 2 W 17 O 61 )] 10– ( 4 ), resulting in four structural models. Similarly to previously observed,, during the crystallization process the dimeric 1:2 Zr IV ‐Wells–Dawson POM dissociated into monomeric 1:1 Zr IV ‐Wells–Dawson anion [Zr IV (α 2 ‐P 2 W 17 O 61 )] 8– ( 1 ) (Scheme ), which crystalized in a noncovalent complex with HEWL.…”

“…This implies that dimeric 1:2 [Zr(α 2 ‐P 2 W 17 O 61 ) 2 ] 16– needs to dissociate into catalytically active monomeric 1:1 complex [Zr IV (α 2 ‐P 2 W 17 O 61 )] 8– which has more open sites for the interaction with the peptide bond. The co‐crystal structures of Zr IV ‐substituted Keggin and Hf IV ‐substituted Wells–Dawson POM bound to HEWL, have shown that although HEWL was mixed with the hydrolytically inactive 1:2 forms of the POM, the resulting co‐crystals contained only the monomeric 1:1 forms. These crystal structures provided the first structural proof for the existence of monomeric Zr IV ‐Keggin and Hf IV ‐Wells–Dawson POMs under neutral pH conditions, as these species have previously only been detected through spectroscopic studies .…”

Chemical Mimics of Aspartate‐Directed Proteases: Predictive and Strictly Specific Hydrolysis of a Globular Protein at Asp−X Sequence Promoted by Polyoxometalate Complexes Rationalized by a Combined Experimental and Theoretical Approach

“…The X-ray structures of the complexeso fH EWL with 1:1Z r-K and 1:1H f-WD POMs [34] as well as MD simulations [22] showed that metal-substituted POMs prefer to bind to the protein by pointingtheir catalytically activemetal centers towards the solvent due to the hydrophilicity of the catalytically active M-(OH 2 ) n site. Consequently, the dynamic hydrolysis process should involver eorientation of the POM, in order to bind a backbonea mide oxygen atom (required for the electrophilic activation of the peptideb ond), with the corresponding energy penalty.Ap ossible mono-or bidentatec oordination of the Asp COO À group to the active metal of the POM can, in principle, compensate to some extent for the energy loss due to the POM reorientation and eventual migration from the optimal binding site on the protein surface.…”

“…The results obtained by docking are in line with previous experimental studies on protein-POM systems showing that cleavages ites are often located in the vicinity of the binding sites. [34] Elucidating the molecularo rigins of AspÀXselectivity of Zr-POMs…”