Reduction of ferredoxins by photosynthetic reaction center complex from the green sulfur bacterium Chlorobium tepidum

Abstract

Green sulfur bacterial reaction centers (RCs) belong to an iron-sulfur type RC, and can photoreduce low potential iron-sulfur protein, ferredoxin, as does PSI. From the thermofilic green sulfur bacterium, C. tepidum, four 2[4Fe-4S] type Fd fractions were purified. The amount of each Fd fractions varied depending upon culture conditions. Absorbances at 385nm of these Fds decreased by 10-15% in 6 days at 4oC in aerobic condition, indicating that these Fds were rather stable in contrast with previous reports. Two of these Fds¿ N-terminal amino acid sequences (15 residues) were completely identical. All of these Fds¿ N-terminal amino acid sequences (15-25 residues) completely agreed to those deduced from the three fdx genes (Chung S., and Bryant D.A.). These Fds are photoreducible by purified RC from this bacterium, and support NADP+ photoreduction with spinach ferredoxin-NADP+ reductase. The Vmax and Km of electron transfer rates in the above reaction were significantly different among Fds.