NONHEME IRON DIOXYGEN ACTIVATION IN ENZYMES AND MODELS

University Of Minnesota, Minneapolis MN

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Abstract

The activation of dioxygen[unreadable] in biological systems is a question of great importance because of its[unreadable] involvement in many important metabolic processes. Although the mechanism of[unreadable] oxygen activation in heme systems such as cytochrome P450 and peroxidases is[unreadable] becoming quite well understood, much less is known of the corresponding[unreadable] mechanisms for mononuclear non-heme iron systems. These latter enzymes are[unreadable] involved in the biosynthesis of amino acids, neurochemicals, and antibiotics[unreadable] and carry out reactions that range from oxidative cleavage of catechols,[unreadable] aliphatic and aromatic hydroxylation, to ring cyclizations involving[unreadable] heteroatoms and cis-dihydroxylation of arenes. For many mononuclear nonheme[unreadable] iron enzymes, the reaction with 02 usually occurs only after substrate or[unreadable] cofactor interacts with the active site iron, e.g. catechol for catechol[unreadable] dioxygenases, substrate thiol for isopenicillin N synthase, and the alpha-keto[unreadable] acid co-substrate for alpha-ketoglutarate-dependent oxygenases. Several of[unreadable] these enzymes have now been shown by crystallography to have an iron center[unreadable] with a common 2-His-1-carboxylate facial triad binding motif. Based on this[unreadable] motif, we have proposed a general mechanism that ties together the chemistry of[unreadable] these systems, but the details of the dioxygen activation step vary from enzyme[unreadable] to enzyme because of the differing natures of the substrates and/or cofactors[unreadable] that interact with the metal center. Important questions include, 1) how is 02[unreadable] activated by these enzymes, and 2) what is the nature of intermediates that are[unreadable] involved in the reactions? Spectroscopic (NMR, EFR, resonance Raman, Mossbauer,[unreadable] EXAFS, electrospray mass spectrometry) and mechanistic studies are proposed for[unreadable] a number of enzymes to elucidate details of the active site structure and their[unreadable] mechanisms of action. These include extradiol cleaving catechol dioxygenases[unreadable] (both Fe- and Mn-dependent forms), the alpha-ketoglutarate dependent enzymes[unreadable] that oxidize the herbicide 2,4-D and taurine, isopenicillin N synthase, and[unreadable] ACCO, the ethylene forming enzyme in plants. Model complexes will be[unreadable] synthesized to interpret spectroscopic features of the enzymes and their[unreadable] intermediates and mimic the key mechanistic steps of the oxidations. Modeling[unreadable] efforts will focus on:[unreadable] 1) extradiol cleavage of catechols, 2) the role the alpha-keto acid[unreadable] co-substrate plays in oxygenase reactions, 3) trapping of transient iron-peroxo[unreadable] and high valent iron-oxo intermediates, and 4) correlating the properties of[unreadable] these intermediates with their ability to oxidize hydrocarbon substrates. As in[unreadable] the past, the synergistic interaction of the biochemical and inorganic aspects[unreadable] of the proposal is important for the success of the program.[unreadable]