Electron donation from membrane-bound cytochrome c to the photosynthetic reaction center in whole cells and isolated membranes of Heliobacterium gestii

Abstract

The reaction between membrane-bound cytochrome c and the reaction center bacteriochlorophyll g dimer P798 was studied in the whole cells and isolated membranes of Heliobacterium gestii. In the whole cells, the flash-oxidized P798⁺ was rereduced in multiple exponential phases with half times (t _1/2s) of 10 μs, 300 μs and 4 ms in relative amplitudes of 40, 35 and 25%, respectively. The faster two phases were in parallel with the oxidation of cytochrome c. In isolated membranes, a significantly slow oxidation of the membrane-bound cytochrome c was detected with t _1/2 = 3 ms. This slow rate, however, again became faster with the addition of Mg²⁺. The rate showed a high temperature dependency giving apparent activation energies of 88.2 and 58.9 kJ/mol in the whole cells and isolated membranes, respectively. Therefore, membrane-bound cytochrome c donates electrons to the P798⁺ in a collisional reaction mode like the reaction of water-soluble proteins. The rereduction of the oxidized cytochrome c was suppressed by the addition of stigmatellin both in the whole cells and isolated membranes. This indicates that the electron transfer from the cytochrome bc complex to the photooxidized P798⁺ is mediated by the membrane-bound cytochrome c. The multiple flash excitation study showed that 2–3 hemes c were connected to the P798. By the heme staining after the SDS-PAGE analysis of the membraneous proteins, two cytochromes c were detected on the gel indicating apparent molecular masses of 17 and 30 kDa, respectively. The situation resembles the case in green sulfur bacteria, that is, the membrane-bound cyotochrome c _z couples electron transfer between the cytochrome bc complex and the P840 reaction center complex.