EPR AND REDOX CHARACTERIZATION OF FERREDOXIN-I AND FERREDOXIN-II FROMDESULFOVIBRIO-VULGARIS MIYAZAKI

Detailed redox titrations monitored by EPR and W-visible spectroscopie s have been carried out on the dimeric ferredoxins I and II from Desul fovibrio vulgaris Miyazaki. Ferredoxin II contains a unique [4Fe-4S] c luster per subunit characterized by a midpoint potential of -417 mV at 24 degrees C. The enthalpic and entropic contributions to the redox f ree energy variation of this cluster have been determined from the tem perature dependence of the midpoint potential and compared to the data reported for other iron-sulfur The molecular arrangement of the two s ubunits is such that two [4Fe-4S](1+) clusters are magnetically couple d in the fully reduced state of the protein. Ferredoxin I contains one [3Fe-4S] and one [4Fe-4S] cluster per subunit, whose spectral and red ox properties are very similar to those of the same clusters in ferred oxin III from Desulfovibrio africanus, The strong heterogeneity in the redox properties of the [3Fe-4S] center supports a bridging position between the N-terminal and C-terminal parts of the protein. (C) 1995 A cademic Press, Inc.