Study of the high-potential iron sulfur protein in Halorhodospira halophila confirms that it is distinct from cytochrome c as electron carrier.

The role of high-potential iron sulfur protein (HiPIP) in donating electrons to the photosynthetic reaction center in the halophilic gamma-proteobacterium Halorhodospira halophila was studied by EPR and time-resolved optical spectroscopy. A tight complex between HiPIP and the reaction center was observed. The EPR spectrum of HiPIP in this complex was drastically different from that of the purified protein and provides an analytical tool for the detection and characterization of the complexed form in samples ranging from whole cells to partially purified protein. The bound HiPIP was identified as iso-HiPIP II. Its Em value at pH 7 in the form bound to the reaction center was approximately 100 mV higher (+140 +/- 20 mV) than that of the purified protein. EPR on oriented samples showed HiPIP II to be bound in a well defined geometry, indicating the presence of specific protein-protein interactions at the docking site. At moderately reducing conditions, the bound HiPIP II donates electrons to the cytochrome subunit bound to the reaction center with a half-time of < or =11 micros. This donation reaction was analyzed by using Marcus's outer-sphere electron-transfer theory and compared with those observed in other HiPIP-containing purple bacteria. The results indicate substantial differences between the HiPIP- and the cytochrome c2-mediated re-reduction of the reaction center.