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Nitrogen Oxides and Heme

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Iron plays significant roles in the chemistry of nitrogen oxides in the biosphere. The simplest nitrogen oxide, NO (nitric oxide), is a small paramagnetic molecule that binds to heme iron to result in various biological events ranging from vasodilation to the enabling of light flashes in fireflies. Heme iron is intricately involved in the mammalian biosynthesis of NO and in physiological signaling by NO. The diverse biological effects of NO when bound to heme raises intriguing questions regarding the identities and chemical properties of the FeNO moieties. The related nitrite anion is now recognized to play key roles in NO biochemistry, and may be the major storage form of NO in mammals. Interestingly, myoglobin (Mb) and hemoglobin have recently been proposed to act as nitrite reductases under hypoxic conditions, generating NO in the oxygen-deficient environment. In this proposed research, we will utilize a combination of synthesis, characterization, spectroscopy, small molecule and protein crystallography, mutagenesis, and theoretical calculations to probe fundamental issues concerning the nature of the interaction of NO, nitrite, and related nitrogen oxides with biologically relevant heme iron. We will prepare, in Specific Aim 1, a representative set of N-base and thiolate liganded heme model compounds with NO ligands and characterize them by spectroscopy (IR, EPR, rR, UV-vis), electrochemistry, and crystallography. We will utilize DFT calculations to provide further insight into the electronics of the FeNO moieties. MbNO and its mutants will be examined as well. In Specific Aim 2, we will characterize the reaction pathways (NO linkage isomerism, reactions with H-donors, NO coupling reactions, etc) available to the FeNO group in heme and heme model compounds. Nitrite adducts of heme models and Mb (and distal pocket mutants) will be prepared and characterized in Specific Aim 3, and they will be examined for their ability to convert nitrite to NO. Relevance to public health: Mammals controls the production of nitric oxide, a cellular signaling agent, by a complex set of reactions that either generate NO or consume it. Much of this control is by iron in vivo. A multi-faceted approach (chemical, biochemical, structural, theoretical) is proposed to obtain a clearer understanding of the iron-NO and the related iron-nitrite interactions central to good health.
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