title: Insight into the protein and solvent contributions to the reduction potentials of [4Fe-4S]2+/+ clusters: crystal structures of the Allochromatium vinosum ferredoxin variants C57A and V13G and the homologous Escherichia coli ferredoxin.
abstract: The crystal structures of the C57A and V13G molecular variants of Allochromatium vinosum 2[4Fe-4S] ferredoxin (AlvinFd) and that of the homologous ferredoxin from Escherichia coli (EcFd) have been determined at 1.05-, 1.48-, and 1.65-A resolution, respectively. The present structures combined with cyclic voltammetry studies establish clear effects of the degree of exposure of the cluster with the lowest reduction potential (cluster I) towards less negative reduction potentials (E degrees ). This is better illustrated by V13G AlvinFd (high exposure, E degrees = -594 mV) and EcFd (low exposure, E degrees = -675 mV). In C57A AlvinFd, the movement of the protein backbone, as a result of replacing the noncoordinating Cys57 by Ala, leads to a +50-mV upshift of the potential of the nearby cluster I, by removal of polar interactions involving the thiolate group and adjustment of the hydrogen-bond network involving the cluster atoms. In addition, the present structures and other previously reported accurate structures of this family of ferredoxins indicate that polar interactions of side chains and water molecules with cluster II sulfur atoms, which are absent in the environment of cluster I, are correlated to the approximately 180-250 mV difference between the reduction potentials of clusters I and II. These findings provide insight into the significant effects of subtle structural differences of the protein and solvent environment around the clusters of [4Fe-4S] ferredoxins on their electrochemical properties.
authors.affiliation: Institute of Physical Chemistry, NCSR Demokritos, Aghia Paraskevi, Athens, Greece.
authors.forename: Emmanuel
authors.initials: E
authors.lastname: Saridakis
authors.forename: Petros
authors.initials: P
authors.lastname: Giastas
authors.forename: Georgios
authors.initials: G
authors.lastname: Efthymiou
authors.forename: Vladimiros
authors.initials: V
authors.lastname: Thoma
authors.forename: Jean-Marc
authors.initials: JM
authors.lastname: Moulis
authors.forename: Panayotis
authors.initials: P
authors.lastname: Kyritsis
authors.forename: Irene M
authors.initials: IM
authors.lastname: Mavridis
mesh_terms: D000595:Amino Acid Sequence; D001426:Bacterial Proteins / Q000737:chemistry*; D002841:Chromatiaceae / Q000737:chemistry*; D018360:Crystallography, X-Ray; D004563:Electrochemistry; D004926:Escherichia coli / Q000737:chemistry*; D005288:Ferredoxins / Q000737:chemistry* / Q000235:genetics; D008958:Models, Molecular; D008969:Molecular Sequence Data; D016297:Mutagenesis, Site-Directed; D011487:Protein Conformation; D016415:Sequence Alignment
chemical_list: D001426:Bacterial Proteins; D005288:Ferredoxins