Modeling Zinc Enzyme Inhibition with Functional Thiolate Ligands

Ligand Basicity and Chelate Effects on Sulfur Insertion vs. Sulfur Reduction by Zinc Thiolate Complexes

4- and 5-coordinate zinc thiolate complexes supported either by bis(carboxamide)pyridine frameworks or by substituted tris(pyrazolyl)borate ligands react with elemental sulfur (S8) following two distinct pathways. Some zinc thiolate moieties insert sulfur atoms to form zinc polysulfanide complexes, while others reduce sulfur and oxidize the thiolate. Here, we compare the effects of ligand electronics, strain, and sterics for selecting the respective reaction pathway. These results show that chelating and electron-deficient thiolate ligands better stabilize persistent zinc-bound polysulfanide species.

How the Local Environment of Functional Sites Regulates Protein Function

Proteins form complex biological machineries whose functions in the cell are highly regulated at both the cellular and molecular levels. Cellular regulation of protein functions involves differential gene expressions, post-translation modifications, and signaling cascades. Molecular regulation, on the other hand, involves tuning an optimal local protein environment for the functional site. Precisely how a protein achieves such an optimal environment around a given functional site is not well understood. Herein, by surveying the literature, we first summarize the various reported strategies used by certain proteins to ensure their correct functioning. We then formulate three key physicochemical factors for regulating a protein's functional site, namely, (i) its immediate interactions, (ii) its solvent accessibility, and (iii) its conformational flexibility. We illustrate how these factors are applied to regulate the functions of free/metal-bound Cys and Zn sites in proteins.

Exploring the influence of the protein environment on metal-binding pharmacophores

The binding of a series of metal-binding pharmacophores (MBPs) related to the ligand 1-hydroxypyridine-2-(1H)-thione (1,2-HOPTO) in the active site of human carbonic anhydrase II (hCAII) has been investigated. The presence and/or position of a single methyl substituent drastically alters inhibitor potency and can result in coordination modes not observed in small-molecule model complexes. It is shown that this unexpected binding mode is the result of a steric clash between the methyl group and a highly ordered water network in the active site that is further stabilized by the formation of a hydrogen bond and favorable hydrophobic contacts. The affinity of MBPs is dependent on a large number of factors including donor atom identity, orientation, electrostatics, and van der Waals interactions. These results suggest that metal coordination by metalloenzyme inhibitors is a malleable interaction and that it is thus more appropriate to consider the metal-binding motif of these inhibitors as a pharmacophore rather than a "chelator". The rational design of inhibitors targeting metalloenzymes will benefit greatly from a deeper understanding of the interplay between the variety of forces governing the binding of MBPs to active site metal ions.

'Unconventional' coordination chemistry by metal chelating fragments in a metalloprotein active site

The binding of three closely related chelators: 5-hydroxy-2-methyl-4H-pyran-4-thione (allothiomaltol, ATM), 3-hydroxy-2-methyl-4H-pyran-4-thione (thiomaltol, TM), and 3-hydroxy-4H-pyran-4-thione (thiopyromeconic acid, TPMA) to the active site of human carbonic anhydrase II (hCAII) has been investigated. Two of these ligands display a monodentate mode of coordination to the active site Zn²⁺ ion in hCAII that is not recapitulated in model complexes of the enzyme active site. This unprecedented binding mode in the hCAII-thiomaltol complex has been characterized by both X-ray crystallography and X-ray spectroscopy. In addition, the steric restrictions of the active site force the ligands into a ‘flattened’ mode of coordination compared with inorganic model complexes. This change in geometry has been shown by density functional computations to significantly decrease the strength of the metal–ligand binding. Collectively, these data demonstrate that the mode of binding by small metal-binding groups can be significantly influenced by the protein active site. Diminishing the strength of the metal–ligand bond results in unconventional modes of metal coordination not found in typical coordination compounds or even carefully engineered active site models, and understanding these effects is critical to the rational design of inhibitors that target clinically relevant metalloproteins.

S-nitrosothiol and nitric oxide reactivity at zinc thiolates

S-Nitrosothiols undergo reversible transnitrosation reactions at tris(pyrazolyl)boratozinc thiolates (iPr2)TpZn-SR. These zinc thiolates are unreactive toward anaerobic NO but rapidly react with NO in the presence of O(2) or anaerobically with NO(2) to release the S-nitrosothiol RSNO with formation of the corresponding zinc nitrate.

Thioester hydrolysis promoted by a mononuclear zinc complex

The mononuclear zinc complex [(bpta)Zn](ClO(4))(2).0.5H(2)O promotes the hydrolysis of the thioester PhCH(OH)C(O)SCD(3) when dissolved in CH(3)CN:H(2)O (50:50 buffered at pH 9.0). This reaction results in the formation of a mixture of CD(3)SH and a zinc thiolate complex, the latter of which can be protonated to generate additional CD(3)SH. Kinetic studies revealed an overall second-order reaction with an activation energy that is similar to that found for aqueous OH(-) promoted thioester hydrolysis. These studies represent the first investigation of chemistry relevant to that occurring in the monozinc-containing form of human glyoxalase II.

Synthesis and Characterization of Thiolate-Oxo Ligated Zinc Alkyl Derivatives for Production of Zn-Based Nanoparticles

A series of mercapto-oxo containing reagents [3-mercaptopropionic acid (H2-3MPA), 4-mercaptophenol (H2-4MP), 2-mercaptopyridine-N-oxide (H-2MPO)] was reacted with diethyl zinc (ZnEt2) in hexanes/pyridine (py) to yield {(μ4-3MPA)[Zn(Et)(py)]4}∞ (1), [(py)2(Et)Zn(μ3-4MP)Zn(Et)(py)]2 (2), and (2MPO)Zn(Et)py (3). For polymeric 1, each of the functional sites of the 3MPA was bound to four tetrahedral (Td) coordinated Zn(Et)(py) subunits. The sulfur of the 3MPA bridges two of the 'Zn(Et)(py)' subunits, which are also bridged by the two carboxylate oxygens of another 3MPA to propagate the chain. In contrast, 2 forms a discrete tetranuclear species consisting of two Zn(Et)(py) moieties bridged by the oxygens of two 4MP ligands with the thiolate sites of each terminated by Zn(Et)(py)2 moieties. Compound 3 adopts a monomeric species using a chelating 2MPO, a terminal Et, and a bound py to fill the Td coordination of the Zn metal center. Compounds 1 - 3 were then used to generate nanoparticles via solution precipitation and solvothermal routes to determine the effect these precursors have on the morphology and composition of the final materials produced. Compounds 1 - 3 were found to form zincite, zinc metal, or mixed zincite/wurtzite phases from solution precipitation or solvothermal routes; however, no routes yielded the mixed anion (i.e., ZnO x S y ) materials.