Spectral, redox and kinetic characteristics of high‐potential cytochrome c hemes in Rhodopseudomonas viridis reaction center

Redox, optical and kinetic characteristics of the four‐heme cytochrome c tightly bound to the reaction center complexes of Rhodopseudomonas viridis have been studied. The two high‐potential hemes, previously thought to be identical, are shown to differ in midpoint potentials, absorption spectra and kinetics of photooxidation. One heme is characterized by E m = 380 ± 10 mV, and a split α‐band (a peak at 559 nm and a shoulder at 553 nm) whereas the other has an E m = 310±10 mV and a symmetrical α‐band at 556 nm. Kinetics of laser flash oxidation of the c‐ heme by the photogenerated P‐960+ (τ ~ 0.3 μs) matches closely that of the bacteriochlorophyll reduction and precedes oxidation of the c‐556 heme, the latter occurring with τ ~ 2.5 μs concurrently with heme c‐ re‐reduction. The data point to heme c‐ being an immediate electron donor to P‐960+. Accordingly, this heme is tentatively identified with the iron‐porphyrin group proximal to the bacteriochlorophyll special pair in the three‐dimensional model of Rps. viridis reaction centers complexes [(1985) Nature 318, 618‐624]. Thus, the following reaction sequence is assumed: c‐556 → c‐559 → P‐960+.