ESR signal of the iron-sulfur center FX and its function in the homodimeric reaction center of Heliobacterium modesticaldum

Electron transfer in the membranes and the type I reaction center (RC) core protein complex isolated from Heliobacterium modesticaldum was studied by optical and ESR spectroscopy. The RC is a homodimer of PshA proteins. In the isolated membranes, illumination at 14 K led to accumulation of a stable ESR signal of the reduced iron-sulfur center F-B(-) in the presence of dithiothreitol, and an additional 20 min illumination at 230 K induced the spin-interacting F-A(-)/F-B(-) signal at 14 K. During illumination at 5 K in the presence of dithionite, we detected a new transient signal with the following values: g(z) = 2.040, g(y) = 1.911, and g(x) = 1.896. The signal decayed rapidly with a 10 ms time constant after the flash excitation at 5 K and was attributed to the F-X(-)-type center, although the signal shape was more symmetrical than that of F-X(-) in photosystem I. In the purified RC core protein, laser excitation induced the absorption change of a special pair, P800. The flash-induced P800(+) signal recovered with a fast 2-5 ms time constant below 150 K, suggesting charge recombination with F-X(-). Partial destruction of the RC core protein complex by a brief exposure to air increased the level of the P800(+)A(0)(-) state that gave a lifetime (t(1/2)) of 100 ns at 77 K. The reactions of F-X and quinone were discussed on the basis of the three-dimensional structural model of RC that predicts the conserved F-X-binding site and the quinone-binding site, which is more hydrophilic than that in the photosystem I RC.