Biomedical subjects

Katrina Peariso

Publications and source records attributed to Katrina Peariso.

6 recordsLinked to original sources

PubMed2006

Testing bridge-mediated differences in dinuclear valence tautomeric behavior.

Two structurally characterized dinuclear valence tautomers are described. Cobalt ions are bridged by p- and m-phenylene units connected to 2,2'-bipyridines. X-ray crystal structures show that the molecules are in the [(Co(III))(Co(III))] forms at ca. 125 K, while spectroscopic studies show that both molecules can achieve the [(Co(II))(Co(II))] form above 400 K and confirm the [(Co(III))(Co(III))] form below 10 K. Magnetic susceptibility studies are also included. Our results highlight the necessity of studying both crystalline and amorphous samples to distinguish the effects of intrinsic electronic structure and intermolecular forces on valence tautomeric behavior.

Journal Article↗

PubMed2003

Synthesis and EPR characterization of new models for the one-electron reduced molybdenum site of sulfite oxidase.

Deconvoluting the different contributions of thiolate and ene-1,2-dithiolate donors to the underlying electronic structure of the Mo site in sulfite oxidase (SO) has proven to be a difficult task. One way in which these differences might be illuminated is by selectively substituting Se for S in model complexes which possess multiple sulfur donor ligand environments. Here we report the synthesis and structures of new oxo-Mo(V) complexes as effective models for the one-electron reduced active site of SO. We have used the tridentate heteroscorpionate ligand (2-dimethylethanethiol)bis(3,5-dimethylpyrazolyl)methane (L3SH) in order to model the constrained cysteinyl sulfur (S(Cys)) ligand environment observed in the crystal structure of the enzyme, and benzene-1,2-dithiol (bdt) as a mimic of the ene-1,2-dithiolate chelate. [(L3S)MoO(bdt)] and [(L3S)MoO(SPh)(2)] have been structurally characterized by X-ray crystallography, and as such, [(L3S)MoO(bdt)] is only the second known model compound that closely approximates the active site structure of reduced forms of SO. Additionally, benzenethiol (SPh) and benzeneselenol (SePh) have been used to perturb the equatorial ligand environment of [(L3S)MoO(bdt)].) This has provided much needed insight into the electronic structure of the one-electron reduced SO site and has allowed for increased understanding of the individual roles played by these different thiolate donors in the oxidative half-reaction of the enzyme. Interestingly, the EPR spectra of [(L3S)MoO(bdt)], [(L3S)MoO(SPh)(2)], and [(L3S)MoO(SePh)(2)] closely resemble that of both high pH (hpH) and low pH (lpH) SO, except for the fact that the magnitude of g(1) is found to be consistently higher in the model spectra compared to that of the enzyme. It is suggested that this derives from an increase in Mo-S covalency in the models relative to hpH and lpH SO.

Crystallography, X-Ray↗

PubMed2003

EXAFS studies of the zinc sites of UDP-(3-O-acyl)-N-acetylglucosamine deacetylase (LpxC).

Extended X-ray absorption fine structure (EXAFS) spectroscopy has been used to determine the structure of the Zn(II) sites in UDP-(3-O-acyl)-N-acetylglucosamine deacetylase (LpxC) from Aquifex aeolicus and Pseudomonas aeruginosa. The active site Zn(II) is four coordinate, with exclusively low-Z (nitrogen and oxygen) ligation in both enzymes. The amplitude of the outer-shell scattering from the histidine ligands is best fit using two histidine ligands, suggesting a ZnO(2)(His)(2) site, where O most likely represents a conserved aspartate and a solvent molecule. The same structure was found for Co(II)-substituted A. aeolicus LpxC, although in this case it is possible that the coordination sphere may expand to include a fifth low-Z ligand. EXAFS data were also measured for the Escherichia coli LpxC enzyme. When a single Co(II) is substituted for Zn(II) in the active site of E. coli LpxC, EXAFS data show the same ligand environment as is found for the P. aeruginosa and A. aeolicus enzymes. However, the EXAFS data for E. coli LpxC with two zinc ions bound per protein, with the second Zn(II) acting as an inhibitory metal, demonstrates that the inhibitory metal is bound to at least two high-Z (sulfur, presumably thiolate, or chlorine) ligands. Results of the outer-shell scattering analysis, combined with previous studies of the LpxC enzyme, indicate a novel zinc binding motif not found in any previously studied zinc metalloproteins.

Amidohydrolases↗

PubMed2003

The PcoC copper resistance protein coordinates Cu(I) via novel S-methionine interactions.

The E. coli copper resistance protein PcoC enhances survival of a bacterium under conditions of extreme copper stress. This small protein has no cysteines, but does have an unusual methionine-rich sequence motif, suggesting that methionine ligation may be important in Cu binding. It is shown that PcoC binds both Cu(I) and Cu(II), in addition to binding Hg(II) and Ag(I). Previously crystallographic studies of PcoC had shown that the apo protein adopts a beta-barrel fold typical of that seen for blue-copper electron-transfer proteins. However, in contrast with electron-transfer proteins, where the Cu(I) and Cu(II) structures are nearly identical, X-ray absorption spectra show that the structures of the Cu site in reduced and oxidized PcoC are dramatically different. Cu(II) PcoC has a tetragonal Cu structure in which the Cu is coordinated to O or N ligands, including at least two histidine ligands. Cu(I) PcoC has a trigonal site with two methionine ligands. This is the first well-characterized example of a methionine-rich protein Cu binding site, demonstrating a new type of biological Cu coordination chemistry.

Copper↗

PubMed2003

Recent applications of MCD spectroscopy to metalloenzymes.

Magnetic circular dichroism (MCD) continues to be a powerful probe of metalloenzyme electronic and geometric structure, in addition to playing a major role in the determination of heme enzyme coordination geometries. Excited state electronic structure contributions to enzyme activity have been gleaned from C-term MCD studies, which are usually interpreted in the context of other spectroscopies, including electronic absorption and resonance Raman. The recent development of sophisticated methods for the analysis of variable-temperature, variable-field MCD have allowed the ground states of metalloenzyme active sites to be studied in detail, providing information on the electronic and geometric structure of the site. This has been especially informative for non-heme iron enzymes. In the past two years X-ray MCD has been shown to be a promising technique for the study of metalloenzymes.

Animals↗

PubMed2002

Active-site stereochemical control of oxygen atom transfer reactivity in sulfite oxidase.

A number of both experimental and computational studies have recently been reported for symmetric, six-coordinate dioxomolybdenum(VI) complexes as models of the fully oxidized form of the molybdopterin enzyme sulfite oxidase (SO). Such studies have suggested that the two terminal oxo donors in SO are electronically equivalent. However, the consensus structure of the catalytically competent Mo(VI) active site in SO is five-coordinate square pyramidal, possessing two terminal oxo donors, an ene-1,2-dithiolate chelate and a cysteine sulfur donor ligand. Computational studies at the density functional level of theory have been performed on a minimal model of the SO active site, [Mo(VI)O2(S2C2Me2)(SCH3)]-, in C1 symmetry to evaluate the composition of the LUMO, which is the putative electron acceptor orbital in the oxygen atom transfer (OAT) reaction with the sulfite substrate. The LUMO in this model is principally composed of a Mo dxy - ppi* interaction between the Mo and the equatorial oxygen (Oeq), while the axial oxygen (Oax) possesses no contribution to this orbital. In fact, the LUMO+1 orbital which possesses a substantial amount of Oax character lies nearly 1 eV higher in energy than the LUMO. It has also been suggested that changes in the Oax-Mo-Sthiolate-C torsion angle during the course of enzyme catalysis may aid in selection of Oeq for OAT. Calculations were performed in which this torsion angle was varied by 20 degrees through 360 degrees . These calculations demonstrate that the Mo dxy -Oeq ppi* interaction, and therefore the Oeq atom character, always dominates the LUMO. The results presented here suggest that oxygen atom selection and activation are a direct function of the low-symmetry structure of the oxidized SO active site and provide a role for the ene-1,2-dithiolate in promoting OAT reactivity through its kinetic trans effect on the equatorial oxo donor.

Binding Sites↗