Mechanistic Studies of Dinuclear Metalloenzymes - A Model Approach

With the goal to study the mechanisms of dinuclear metalloenzymes, a new line of synthetic and computational models were prepared and studied. Mixed imidazole and carboxylate multidentate complexes were synthesized to better simulate carboxylate-rich ligand environments. Of the compounds, which showed to be the most useful, were one symmetric and the other one asymmetric and in both cases they were both found to be based on a central phenoxide moiety. To study the mechanism of the dinickel enzyme urease, complexes containing two nickel ions were prepared from the two ligands. Analysis of the crystal structures of the synthesized model complexes indicated that the urea prefers the coordinatively unsaturated ion for its initial coordination. From kinetic studies comparing the complexes it became apparent that an open coordination site is important in facilitating hydrolytic catalytic activity. Computational models, based on the native crystal structure of the enzyme, were also used. The conclusions showed that the first coordination of urea most likely occurs on Ni1 trans to the carbamylated lysine ligand and that the published proposal based on the attack of the bridging hydroxy group is less likely due to the high energy barrier for the formation of the tetrahedral intermediate. When ICIMP is reacted with zinc, a tetranuclear zinc complex is formed. It can easily be dissociated to dinuclear complexes. Both the structure of the complex and the structure of the environment around the avtive site in the enzyme resemble the nickel model. The complex has also been found to be very potent as a functional model for hydrolytic enzymes. Homo- and heteronuclear iron complexes were also synthesized. As a useful precursor, a mononuclear complex could be isolated and crystallographically characterized. Based on the mononuclear system, four new heterodinuclear iron complexes were isolated

Computational Modeling of the Mechanism of Urease

In order to elucidate aspects of the mechanism of the hydrolytic enzyme urease, theoretical calculations were undertaken on a model of the active site, using density functional theory. The bridging oxygen donor that has been found in the crystal structures was determined to be a hydroxide ion. The initial coordination of urea at the active site occurs most likely through the urea oxygen to the nickel ion with the lowest coordination number. This coordination can be made without much gain in energy. The calculations also showed that weak coordination of one of the urea amine nitrogen atoms to the second nickel atom is energetically feasible. Furthermore, a proposed mechanism including a tetrahedral intermediate generated by hydrolytic attack on the urea carbon by the bridging hydroxide was modeled, and the tetrahedral intermediate was found to be energetically unfavorable relative to terminal coordination of the substrate (urea)

Head-to-head comparison of a 2-day myocardial perfusion gated SPECT protocol and cardiac magnetic resonance late gadolinium enhancement for the detection of myocardial infarction.

The aim was to determine the sensitivity and specificity of gated myocardial perfusion SPECT (MPS) with a technetium-labelled (Tc) perfusion tracer to detect myocardial infarction (MI) in a clinical population referred for assessment of stress-induced ischemia using late gadolinium enhancement cardiac magnetic resonance (CMR) as reference method

Validation of an automated method to quantify stress-induced ischemia and infarction in rest-stress myocardial perfusion SPECT.

Myocardial perfusion SPECT (MPS) is one of the frequently used methods for quantification of perfusion defects in patients with known or suspected coronary artery disease. This article describes open access software for automated quantification in MPS of stress-induced ischemia and infarction and provides phantom and in vivo validation