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J Biol Chem, Vol. 273, Issue 25, 15404-15411, June 19, 1998

The Two [4Fe-4S] Clusters in Chromatium vinosum Ferredoxin Have Largely Different Reduction Potentials
STRUCTURAL ORIGIN AND FUNCTIONAL CONSEQUENCES

Panayotis Kyritsis, Oliver M. Hatzfeld^§, Thomas A. Link^§, and Jean-Marc Moulis

From the  CEA, Département de Biologie Moléculaire et Structurale, Laboratoire Métalloprotéines, 17 rue des Martyrs, 38054 Grenoble Cedex 9, France and ^§ Institut für Biochemie I, Molekulare Bioenergetik, Universitätklinikum Frankfurt, ZBC, D-60590 Frankfurt/Main, Germany

The 2[4Fe-4S] ferredoxin from Chromatium vinosum arises as one prominent member of a recently defined family of proteins found in very diverse bacteria. The potentiometric circular dichroism titrations of the protein and of several molecular variants generated by site-directed mutagenesis have established that the reduction potentials of the two clusters differ widely by almost 200 mV. This large difference has been confirmed by electrochemical methods, and each redox transition has been assigned to one of the clusters. The unusually low potential center is surprisingly the one that displays a conventional CX¹X²CX³X⁴C (Xⁿ, variable amino acid) binding motif and a structural environment similar to that of clusters having less negative potentials. A comparison with other ferredoxins has highlighted factors contributing to the reduction potential of [4Fe-4S] clusters in proteins. (i) The loop between the coordinating cysteines 40 and 49 and the C terminus -helix of C. vinosum ferredoxin cause a negative, but relatively moderate, shift of ~60 mV for the nearby cluster. (ii) Very negative potentials, below 600 mV, correlate with the presence of a bulky side chain in position X⁴ of the coordinating triad of cysteines. These findings set the framework in which previous observations on ferredoxins can be better understood. They also shed light onto the possible occurrence and properties of very low potential [4Fe-4S] clusters in less well characterized proteins.

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