Spectroscopic studies of bound cytochrome c and an iron-sulfur center in a purified reaction center complex from the green sulfur bacterium Chlorobium tepidum

Photoreduction and reoxidation of the three iron-sulfur clusters of reaction centers of green sulfur bacteria

Iron-sulfur clusters are the terminal electron acceptors of the photosynthetic reaction centers of green sulfur bacteria and photosystem I. We have studied electron-transfer reactions involving these clusters in the green sulfur bacterium Chlorobium tepidum, using flash-absorption spectroscopic measurements. We show for the first time that three different clusters, named F(X), F(1), and F(2), can be photoreduced at room temperature during a series of consecutive flashes. The rates of electron 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 F(1) and F(2). From multiple-flash experiments, it was possible to derive the intrinsic recombination rates between P840(+) and reduced iron-sulfur clusters: values of 7, 14, and 59 s(-1) were found after one, two and three electron reduction of the clusters, respectively. The implications of our results for the relative redox potentials of the three clusters are discussed.

Purification of ferredoxins and their reaction with purified reaction center complex from the green sulfur bacterium Chlorobium tepidum

Four ferredoxin (Fd) fractions, namely, FdA-D were purified from the green sulfur bacterium Chlorobium tepidum. Their absorption spectra are typical of 2[4Fe-4S] cluster type Fds with peaks at about 385 and 280 nm and a shoulder at about 305 nm. The A(385)/A(280) ratios of the purified Fds were 0.76-0.80. Analysis of the N-terminal amino acid sequences of these Fds (15-25 residues) revealed that those of FdA and FdB completely agree with those deduced from the genes, fdx3 and fdx2, respectively, found in this bacterium (Chung and Bryant, personal communication). The N-terminal amino acid sequences of FdC and FdD (15 residues) were identical, and agree with that deduced from the gene fdx1 (Chung and Bryant, personal communication). The A(385) values of these Fds were unchanged when they were stored for a month at -80 degrees C under aerobic conditions and decreased by 10-15% when they were stored for 6 days at 4 degrees C under aerobic conditions, indicating that they are not extremely unstable. In the presence of Fd-NADP(+) reductase from spinach, and a purified reaction center (RC) preparation from C. tepidum composed of five kinds of polypeptides, these Fds supported the photoreduction of NADP(+) at room temperature with the following K(m) and V(max) (in micromol NADP(+) micromol BChl a(-1) h(-1)): FdA, 2.0 microm and 258; FdB, 0.49 microM and 304; FdC, 1.13 microM and 226; FdD, 0.5 microM and 242; spinach Fd, 0.54 microM and 183. The V(max) value of FdB was more than twice that previously reported for purified RC preparations from green sulfur bacteria.