Evidence for the coupling reaction between bc complex and the P798 RC in Heliobacterium gestii

Abstract

Heliobacteria are strictly anaerobic gram-positive bacteria according to the 16S rRNA analysis. Their P798 RC complex is classified as iron-sulfur type (type-1) RCs and has been considered to form a homodimeric structure. Since heliobacteria have no outer membranes, there is no soluble cytochrome c which functions on the donor side of the P798 RC. We here studied the flash-induced absorption changes of membrane-bound cytochrome c in Heliobacterium gestii using whole cells and isolated membranes. Although the electron transfer rate from cytochrome c to P798, which showed two kinetic phases in whole cells with t1/2 = 10 and 300 ms, respectively, decreased dramatically upon disruption of cells, the divalent cation, Mg2+, was found to recover its slowed rate (t1/2 = 3 ms) to the almost original level. The rate also showed high temperature dependency exhibiting the apparent activation energy of 88.2 kJ/mol in whole cells and 58.9 kJ/mol in isolated membranes, respectively. Membrane-bound cytochrome c, therefore, donates electron to P798+ in a collisional mode as expected for the electron donation from soluble proteins. Furthermore, the addition of stigmatellin decreased the rereduction rate of the flash-oxidized cytochrome c, indicating the cooperative electron transfer from the bc complex to the P798 RC. This situation resembles the case of the tight coupling of the electron transfer reaction between the bc and the RC complexes in green sulfur bacteria (Oh-oka et al., Biochemistry, 1998, 37: 12293-12300).