Orientation and assignment of the four cytochrome hemes in Rhodopseudomonas viridis reaction centers

Abstract

Low temperature absorption and linear dichroism measurements on oriented reaction centers of Rhodopseudomonas viridis at different redox potentials allow the α-bands of the four hemes to be spectrally resolved. The high-potential heme C556 presents an α-band split into two components absorbing at 555 and 551 nm. The corresponding linear dichroism spectrum shows two transitions of opposite sign and equal amplitudes. Comparison of our experimental results with the linear dichroism values calculated from the atomic coordinates leads us to assign unambiguously the high-potential C556 cytochrome with heme 2, the third heme away from the primary donor. Taken together with other available spectroscopic observations, this result implies the following sequence for the redox centers in Rhodopseudomonas viridis: C554 C556 C552 C559 P.