Zinc is known to be functional in almost 300 enzymes representing all subgroups of the enzyme classification.1 Although it is present in organisms only about half as much as its famous congener iron, it competes with the latter in terms of versatility. In comparison, the divalent metals magnesium and calcium, being 1-2 orders of magnitude more abundant, rarely share the biochemical abilities and rarely interfere with the biological functions of zinc, and the homologues of zinc, cadmium and mercury, just manage to exist as highly poisonous impurities. The question “Why?” suggests itself. What makes zinc, the “boring element without properties”, so unique? Like all “Why?”, questions this one is difficult to answer. Definitely the redox inertness and the hard-soft properties of zinc play an important role. Unlike all other firstrow transition metals, zinc cannot get caught in a redox trap, i.e., change its properties by changing its oxidation state. Unlike calcium and magnesium, zinc likes to be coordinated by the soft donor functions in a protein environment, i.e., cysteine thiolate and histidine imidazole. Unlike cadmium and mercury, it does not hold on irreversibly to these donors. The intermediate nature of zinc in this respect, not really hard and not really soft, certainly makes it suitable to be bound to and to act in all available donor environments in living organisms. The two aforementioned properties illustrate the availability of zinc for biological purposes. When it comes to usefulness, other properties must be named. The first of these is a combination of high (kinetic) lability and low (thermodynamic) stability, which characterizes the complexes of zinc more than those of any other transition metal. Unfortunately this combination, which is the essence of catalysis, is an unpleasant symptom for a preparative chemist trying to build enzyme models. In our opinion two other properties are the most important ones in zinc enzymes, specifically for the largest part of them effecting group transfer. They are the high nucleophilicity of “relevant” anions (OH, OR, SR, OPO2X) when bound to tetrahedral zinc and the high coordinative flexibility in the ligand sphere of the metal. In an enzyme environment, the Zn-OH function which exists at neutral pH is an equivalent of free OH-, and all zinc-catalyzed group-transfer reactions involve a rapid change of coordination numbers and coordination geometries at the zinc center.1 This Account is meant to provide some information from zinc coordination chemistry related to an understanding of these latter two phenemena. Both involve atomic interactions and atomic motions in the immediate environment of the metal ion, i.e., the inorganic part of zinc-catalyzed organic or biochemical reactions. We are well aware that the properties of “biological zinc” are a consequence of its “biological environment” and that the catalytic processes can only be understood by a full analysis of all environmental effects, i.e., the interactions of the substrate with the catalyst, solvent, enzyme interior, or cofactors. But the catalytic bond breaking and bond making take place when the reagents are bound to zinc. With this in mind we put up for discussion some observations from coordination chemistry and some generalizations drawn thereof. On the basis of the fact that most zinc enzymes contain the catalytic zinc tetrahedrally coordinated by three protein side chains and the reagents (water, hydroxide, or substrate) as depicted below, we developed the “Freiburg enzyme model”, which reliably reproduces this bonding situation.2 This enzyme model mimics the protein environment by attachment of the metal to three heterocyclic nitrogen donors of a tris(pyrazolyl)borate ligand and by encapsulation of the metal by the three substituents R attached at the 3-positions of the pyrazole rings. Throughout this Account, the graphical symbol annotated as Tp*Zn will be used to represent the “enzyme” part of this model. The Tp*Zn-X complexes have been found versatile in many respects. They allow a wide variation of substituents X. They are suitable for studying the properties of single Zn-X units in a tetrahedral and hydrophobic environment, in this context the nucleophilicity of X. Due to their reasonably inert nature, they offer themselves for mechanistic studies, and the large amount of structural data gathered for them provides a basis for an evaluation of mechanistic pathways. Our contributions to pyrazolylborate-zinc chemistry started with invaluable help from the inventor of the ligands, Jerry Trofimenko. They grew complementary, in competition and in cooperation with the contributions by our colleagues G. Parkin, W. Klaui, W. Tolman, and the late N. Kitajima. Heinrich Vahrenkamp received his Dr. degree in 1967 at the University of Munchen, working under the direction of H. Noth in boron chemistry. After picking up cluster chemistry and X-ray crystallography from L. F. Dahl at the University of Wisconsin, he worked at the Universities of Marburg and Munchen on organometallic Lewis bases, becoming Privatdozent in 1972. In 1973 he was called to the chair of inorganic chemistry at the University of Freiburg where he has worked until today. After more than 20 years of research on polynuclear organometallic compounds, the main interest of his research group has now shifted to the coordination chemistry of zinc related to an understanding of its biological functions. Acc. Chem. Res. 1999, 32, 589-596

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Transitions, Transition States, Transition State Analogues: Zinc Pyrazolylborate Chemistry Related to Zinc Enzymes

Investigations of substrate specificity and reaction mechanism of several kinases using chromium(III) adenosine 5'-triphosphate and chromium(III) adenosine 5'-diphosphate.

Preparation and properties of chromium(III) adenosine 5'-triphosphate, chromium(III) adenosine 5'-diphosphate, and related chromium(III) complexes.

Mechanistic aspects of the metal ion promoted hydrolysis of nucleoside 5'-triphosphates (NTPs)

The complete coordination scheme for Mn(II) in transition-state-analogue complexes with creatine kinase has been determined by electron paramagnetic resonance (EPR) spectroscopy. Perturbations in the EPR spectra for Mn(II) due to superhyperfine coupling to 17O of selectively labeled ligands have been used to identify oxygen ligands in the first coordination sphere of the metal ion. The results show that in the complex of enzyme-MnADP-formate-creatine, Mn(II) is bound to oxygen ligands from both the alpha- and beta-phosphate groups of ADP, to an oxygen from the carboxylate group of formate, and to three water molecules. In the complex with thiocyanate replacing formate as the stabilizing anion, previous infrared experiments [Reed, G. H., Barlow, C. H., & Burns, R. A., Jr. (1978) J. Biol. Chem. 253, 4153-4158] indicated that the nitrogen from thiocyanate was bound to the Mn(II). The magnitudes of the 17O superphyperfine coupling constants from the O- ligands of the ADP phosphate groups and from the formate carboxylate are approximately equal and are larger than that for the water ligands. The symmetry of the zero-field-splitting tensor for Mn(II) indicates that the oxygens from the alpha- and beta-phosphate groups of ADP and the ligand donor atom from the anion occupy mutually cis positions in the octahedral coordination geometry. Water proton relaxation time measurements show that the three water molecules which are bound to Mn(II) are not in free exchange with the bulk solvent. Hence, an enclosed structure at the active site is indicated. The results suggest that for creatine kinase the activating metal ion is bound to all three phosphate groups in the transition state of the reaction.

Identification of the six ligands to manganese(II) in transition-state-analogue complexes of creatine kinase: oxygen-17 superhyperfine coupling from selectively labeled ligands.

J. L. Benovic

Papers

Stereospecificity of the metal--adenosine 5'-triphosphate complex in reactions of muscle pyruvate kinase.