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

Abstract

AbstractRedox, 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 Em = 380 ± 10 mV, and a split α-band (a peak at 559 nm and a shoulder at 553 nm) whereas the other has an Em = 310±10 mV and a symmetrical α-band at 556 nm. Kinetics of laser flash oxidation of the c-559553 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-559553 re-reduction. The data point to heme c-559553 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+