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Slideshow

OXIDATIVE DEGRADATION AND REDUCTIVE REPAIR OF [4Fe-4S] CLUSTERS

Soshawn Blair
Soshawn Blair
Department of Chemistry
University of Georgia
Chemistry Building, Room 400
Inorganic Seminar

Of the three most common Fe-S clusters found in nature, the [4Fe-4S] cluster is the most abundant and accounts for the most diverse functions, ranging from electron transfer to regulation of gene expression and radical generation1. Solvent accessible [4Fe-4S] clusters are very sensitive to oxidative degradation and as such, are sometimes used as sensors of oxidative stress2. The major aim of this research project is to understand the mechanism of degradation of [4Fe-4S] clusters, when exposed to oxidative stress, by air exposure or titration with H2O2, via spectroscopic characterization of stable or semi-stable breakdown intermediates. Moreover, understanding the degradation pathway has the potential for assessing how degraded [4Fe-4S] clusters are repaired in vivo. This presentation will discuss the breakdown products of two classes of proteins containing [4Fe-4S] clusters, (de)hydratases, and radical SAM enzymes. The breakdown products of aconitase and dihydroxyacid dehydratase (DHAD) have been characterized using resonance Raman, electron paramagnetic resonance (EPR), UV-visible absorption and circular dichroism (CD) spectroscopies. While the breakdown of aconitase proceeds through a cubane [3Fe-4S]1+ cluster, followed by rapid breakdown to the apo product, the spectroscopic results indicate that the [4Fe-4S]2+ cluster of DHAD degrades to a cysteine persulfide-ligated [2Fe-2S]2+ cluster intermediate. In addition, resonance Raman spectroscopy of the radical SAM enzymes MiaB and MOCS1A show that the cluster degradation products of these proteins also contain a [2Fe-2S]2+ cluster with cysteine persulfides that can be repaired on addition of Fe2+ and a reducing agent. Spectroscopic studies of an additional radical SAM enzyme, PFL-AE, that contains only one [4Fe-4S] cluster, indicate that the degradation of its [4Fe-4S]2+ cluster proceeds through first a cubane [3Fe-4S]1+ cluster, then a [2Fe-2S]2+ cluster with cysteine persulfides, and these breakdown products can be repaired to the [4Fe-4S]2+ cluster by adding Fe2+ and a reducing agent. While the breakdown products are variable, they remain in a form that can easily repaired. Consequently, this work affords insight into the mechanism of repair of [4Fe-4S] clusters damaged by oxidative stress.

 

[1] Johnson, M. K., and Smith, A. D. (2011) Iron–Sulfur Proteins, In Encyclopedia of Inorganic and Bioinorganic Chemistry, John Wiley & Sons, Ltd.

[2] Jordan, P. A., Thomson, A. J., Ralph, E. T., Guest, J. R., and Green, J. (1997) FNR is a direct oxygen sensor having a biphasic response curve, FEBS Lett 416, 349-352.

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