Model Complexes of Cytochrome P450 Nitric Oxide Reductase.

Abstract

It came as quite a surprise when it was discovered in the 1980s that the toxic molecule nitric oxide (NO) is a signaling molecule in mammals responsible for nerve signal transduction, blood pressure control, and immune response. Unfortunately, overproduction of NO in the blood stream due to a bacterial infection can lead to septic shock and organ degradation, both of which can be fatal. Thus, the need to develop a method by which to detoxify NO from biological systems is of critical importance. Cytochrome P450nor, a NO reductase, represents one way to achieve this detoxification. The active site of this fungal cytochrome P450-type enzyme contains a ferric heme with proximal cysteinate ligation. P450nor plays a vital role in the fungal denitrification by catalyzing the reduction of NO to N2O through a two electron reduction of the initially formed ferric heme-nitrosyl by NADH. In order to fully elucidate the mechanism of this enzyme, stable model complexes are necessary. In this dissertation, small molecule synthetic models of intermediates in the catalytic cycle of P450nor have been prepared. First, a series of ferric heme-nitrosyl complexes with thiolate coordination have been synthesized employing substituted porphyrins and a series of thiolate ligands. This has allowed us to determine the key characteristics required to form these highly unstable ferric nitrosyls. In conjunction with density functional theory (DFT) calculations, we have now been able to gain detailed insight into the electronic structures and spectroscopic properties of these species as a function of the axial ligand donor strength. The second intermediate in the catalytic cycle of P450nor, a ferrous heme-nitroxyl complex, has been prepared and its fundamental properties and reactivity explored. Then, using a bis-picket fence porphyrin we work towards synthesis of high-valent iron complexes with N-based ligands as models for the key intermediate in the catalytic cycle of P450nor responsible for the critical N-N bond formation in the reduction of NO to N2O. Finally, the trans effect of HNO in ferrous heme complexes and the implications for soluble guanylate cyclase activity has been investigated using DFT calculations.