HEME-IRON MONO-OXYGENASES: SPECTROSCOPY AND MECHANISM

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This proposal presents a program of spectroscopic studies of porphyrin model complexes with magnetic circular dichroism (MCD) spectroscopy and of spectroscopic and mechanistic studies of four oxygen and peroxide metabolizing heme iron proteins: cytochrome P-450, secondary amine mono-oxygenase (SAMO), indoleamine dioxygenase (IDO) and chloroperoxidase (CPO). There are three overall objectives: (1) By investigating model porphyrin and heme protein ligand complexes of defined structure, the utility of MCD spectroscopy as a probe of heme iron electronic and ultimately, physical structure will be ascertained. We will examine synthetic Fe+3 (porph)(RS-)(X) and Fe+2 (porph)(X)(CO) complexes with variable biomimetic legands, X. In addition, we will study Fe+2/+3 (porph) imidazolate complexes and unusual six-coordinate high-spin Fe+3 porphyrin complexes. Experiments with heme iron complexes are designed to extablish spectroscopic fingerprints for each ligand and/or complex type for use in structure determination. We will also examine zinc porphyrin complexes where the effects of axial ligation can be more directly assessed. Next we will investigate IDO, CPO and SAMO and their ligand complexes in an effort to better define their coordination structure. (2) Studies designed to define the sturctural requirements for heme iron catalyzed hydroxylation of organic substrates by further characterization of the active site structures of P-450 and SAMO will be carried out. Studies from our laboratory indicate that they have different metal ion environments and that SAMO differs from myoglobin as well. Spectroscopic studies designed to better define its legand environment will continue. The sixth ligand to ferric P-450 may be an oxygen donor; to look for direct evidence for a water ligand, we will examine its EPR spectrum in 170 labelled water. Using a specific inhibitor, we will attempt to trap-out the key hydroxylation intermediate just beyond oxy-P-450. With NMR spectroscopy we will also try to define the distance relationship between the cnetral iron and the substrate. (3) We will test current hypotheses about the mechanism of N-dealkylation of P-450 and SAMO by doing 180 labelling experiments with substrates that form stable carbinolamine and related products rather than dealkylating. The stable product can then be tested for label incorporation from either dioxygen or water.