Structure's Influence on Reactivity in Metalloenzymes

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Our research program is aimed at determining how cysteinates influence function in non-heme iron enzymes. Cysteinate-ligated non-heme iron containing SOR and NHase are involved in the detoxification of superoxide radicals, and the detoxification of nitrile wastes, respectively. Superoxide has been implicated in a number of disease states, including cancer, Alzheimers, Parkinsons, and cardiac damage following a heart attack. The SOR active site closely resembles that of the heme enzyme P450 which oxidizes unactivated hydrocarbons. During this past funding period, we reported the first functional model for SOR, the first example of a thiolate-ligated Felll-OOH, and a model for the unmodified form of NHase. Neither the mechanism of superoxide reduction by SOR, nor the function and mechanism of post-translational NHase cysteinate modification, are well understood. During this funding period we plan to: ¿ investigate the mechanism of formation of our cis and trans Felll-peroxos in order to understand the proton-dependence of these reactions, and determine how the positioning of the thiolate (cis vs trans) influences the available mechanistic pathways. ¿ compare the reactivity of our cis vs. trans thiolate-ligated Felll-OOH with electrophilic and nucleophilic substrates, and H-atom donors, in order to see if an SOR model can promote P450 chemistry. ¿ determine whether thiolate ligands create favorable reaction pathways affording FelV=O, or perhaps even FeV=O species. And, determine whether a trans-thiolate is more efficient than a cis-thiolate at promoting oxidation chemistry by creating a more basic high valent FelV=0 (as was recently proposed for P450). ¿ synthesize a new thiolate-ligated (NSSPy) Fe-peroxide structurally-related to the extensively characterized nitrogen-ligated N4Py Fe-peroxides so that we can determine how thiolates influence function. ¿ synthesize a new trans thiolate-ligand that incorporates steric bulk and H-bonding residues designed to stabilize an Felll-OOH or FelV=O, and/or direct proton delivery to the distal peroxo oxygen. ¿ explore alternative functions of SOR involving SO42-, NO3-, or NO2- reduction. ¿ examine the possibility that post-translational modification of the NHase cysteinates occurs via a mechanism involving an Felll-OOH. ¿ determine how the post-translational oxygenation of two cis NHase cysteinates influences function by examining the reactivity of our unmodified NHase model with oxo-atom and proton donors.