|Searching for High-Valent Iron-Oxo Transients in the Laboratory and in the Cytochrome P450 Enzymes |
Chemistry and Chemical Biology
|Martin Newcomb, University of Illinois-Chicago.|
11:00 AM, WL-Aud
The cytochrome P450 enzymes are heme-containing enzymes with thiolate from protein cysteine as the fifth ligand to iron. P450s catalyze a vast range of biological oxidations including hydroxylations of high-energy, unactivated C H bonds in substrates. In man, they perform both highly specific reactions, such as oxidations of androgens to estrogens, and broad-spectrum oxidations of drugs, pro-drugs and xenobiotics in the liver, and their expression is related to a number of disease states including breast and prostate cancers and liver disease. The oxidizing transient in P450's has been sought since the discovery of these enzymes in the 1960's. By analogy to peroxidase and catalase enzymes, the oxidant is assumed to be an oxoiron(IV) porphyrin radical cation termed Compound I, but the oxidizing transient in P450's was never detected under catalytic turnover conditions or in rapid mixing studies with attempted chemical oxidation of the enzyme.
Our group developed studies aimed at production of high valent metal-oxo species via photochemical methods that permit very rapid detection techniques. Two methods were developed, photochemical cleavage of oxo-containing ligands and photo-oxidations. The ligand cleavage methods give highly reactive porphyrin-manganese(V)-oxo species and iron-oxo transients with unprecedented reactivity that are putative porphryin-iron(V)-oxo derivatives, high energy electronic isomers of Compound I species. In the photo-oxidation reaction, an electron is photo-ejected from an oxoiron(IV) porphyrin to give a Compound I derivative, and this method has been extended to the production and kinetic studies of a Compound I in a P450 enzyme.