Three novel non-heme iron proteins have been isolated recently from sulfate reducing bacteria of the Desulfovibrio genus. They are: (1) desulfoferrodoxin, a monomer containing a distorted FeS4 center (center I) and an octahedrally coordinated iron center with N- and O-containing ligands (center II), (2) rubrerythrin, a dimer containing two rubredoxin-like FeS4 centers and one (may be two) mu-oxo bridged diiron cluster, and (3) isadoxin (tentatively named after Dr. Isabel Moura who discovered this protein), a monomer containing probably two redox active iron-sulfur clusters of unknown structure. This proposal is focused on the EPR and Mossbauer studies of these proteins and is an essential part of a larger, inter-laboratory research program aiming to elucidate the structure and functions of metalloproteins isolated from sulfate and nitrate reducing bacteria. The objective of this proposal is to gain structural information of the above mentioned iron centers through spectroscopic investigations. By correlating these spectroscopic results with the biological and biochemical properties to be obtained by our collaborators, we hope to acquire functional information of these interesting proteins. Certain sulfate reducing bacteria are known to be responsible for the degradation of organic matter in anaerobic marine environment. The enzyme which catalyzes the anaerobic degradation of aromatic compounds, however, has not get been found. It is therefore interesting to note that center II in desulfoferrodoxin exhibits spectroscopic properties similar to those of dioxygenases. Consequently, for the studies of desulfoferrodoxin, emphasis is on center II. The proposed experiments are specifically designed to probe the ligand environment of center II and to investigate its interaction with aromatic organic compounds and small ligands. For rubrerythrin, our focus will be on the diiron cluster. It is now realized that the mu-oxo bridged diiron cluster is involved in the catalysis of a variety of important biological functions, including oxygen transport, methane hydroxylation, phenol oxidation, and hydrolysis of phosphate esters. A specific goal of this proposed program is to obtain detailed spectroscopic characterization and to probe the ligand environment of the diiron cluster in rubrerythrin. The results should enhance our knowledge on the physical properties and coordination chemistry of this versatile biological motif. For isadoxin, our objective is to gain structural information and to obtain detailed electronic and magnetic properties of the unusual iron clusters. Our preliminary studies suggest that at least one (if not both) of the clusters is probably a 6Fe cluster with mixed S, 0, and/or N ligands, a unique structure that has never been reported previously, neither for proteins nor for model compounds. Consequently, results obtained from the proposed measurements are expected to stimulate new developments in the research of iron-sulfur proteins.
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