THE REALITIES OF HOMODIMER-TYPE REACTION CENTER FROM GREEN SULFUR BACTERIA AND ITS OXYGEN SUSCEPTIBILITY

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In the reaction center complex (PS-C) from the green sulfur bacterium Chlorobium tepidum, A_0 and three Fe-S clusters function on the accepter side of the electron transfer chain and two molecules of cyt c-551 on the donor side. From multiple-flash-induced spectroscopy, we found that cyt c-551 was reduced with half-lives of about 60 and 100 μs after the first and the second flash, respectively. These behaviors of cyt c-551 are explained by a model of two functionally equivalent cyt c, and are consistent with a homodimeric model on the donor side. The above apparently different rates are explained by the fact that after the first flash, two cyt c-551 are available for electron donation to P840, while only one is available in the majority of PS-C after the second flash. When PS-C was exposed to air, a rapid inactivation occurred on the acceptor side, the actual site of inactivation remaining to be investigated.Four ferredoxins (Fds) of 2[4Fe-4S] type were purified from C.tepidum. In the presence of PS-C and FNR from spinach, all of them supported efficient NADP^+ reduction. Fds from green sulfur bacteria were reported to be extremely sensitive to oxygen. However, these Fds from C.tepidum was relatively insensitive to oxygen, and inactivated by only 10-15% by storage under aerobic conditions at 4℃. We have purified two Fds from Heliobacillus mobilis. One of them was extremely sensitive to oxygen, but the other relatively insensitive to oxygen as are Fds from C.tepidum.