Comparison of electron transfer properties of bound iron-sulfur clusters between green sulfur bacterial reaction center and photosystem I

Abstract

The green sulfur bacterial reaction center (RC) belongs to an Fe-S type RC as PSI. In contrast with PSI, it binds two molecules of monoheme cyt c.and involvement of quinone in e- transfer chain is controversial. Electron transfer kinetics involving three Fe-S clusters in RC from C. tepidum were studied using flash-absorption spectroscopic measurements. Three different clusters, named FX, F1 and F2, can be photoreduced at room temperature during a series of consecutive flashes. The rates of e- escape to exogenous acceptors depend strongly upon the number of reduced clusters. When two or three clusters are reduced, the escape is biphasic, with the fastest phase being 12/14-fold faster than the slowest phase, which is similar to that observed after single reduction. This is explained by assuming that escape involves mostly the second reducible cluster. Evidence is thus provided for a functional asymmetry between the two terminal acceptors F1 and F2. The intrinsic recombination rates between P840+ and reduced clusters were estimated to be 7, 14 and 59 s-1 after one, two and three e- reduction of the clusters, respectively. The charge recombination rates were much slower than those found in PSI.