부산대학교 도서관

The photosynthetic reaction center (RC) of green sulfur bacteria contains two [4Fe-4S] clusters named FA and FB, by analogy with photosystem I (PS I). PS I also contains an interpolypeptide [4Fe-4S] cluster named FX; however, spectroscopic evidence for an analogous iron-sulfur cluster in green sulfur bacteria remains equivocal. To minimize oxidative damage to the iron-sulfur clusters, we studied the sensitivity of FA and FB to molecular oxygen in whole cells of Chlorobium vibrioforme and Chlorobium tepidum and obtained highly photoactive membranes and RCs from Cb. tepidum by adjusting isolation conditions to maximize the amplitude of the FA−/FB− electron paramagnetic resonance signal at g=1.89 (measured at 126 mW of microwave power and 14K) relative to the P840+ signal at g=2.0028 (measured at 800μW of microwave power and 14K). In these optimized preparations we were able to differentiate FX− from FA−/FB− by their different relaxation properties. At temperatures between 4 and 9K, isolated membranes and RCs of Cb. tepidum show a broad peak at g=2.12 and a prominent high-field trough at g=1.76 (measured at 126 mW of microwave power). The complete g-tensor of FX−, extracted by numerical simulation, yields principal values of 2.17, 1.92, and 1.77 and is similar to FX in PS I. An important difference from PS I is that because the bound cytochrome is available as a fast electron donor in Chlorobium, it is not necessary to prereduce FA and FB to photoaccumulate FX−.