Fig. 3.

The overview of the sequence of electron transfer reactions in the bacterial type II reaction centers. The reaction starts with a dimer of bacteriochlorophylls (P) excited by light. Electron transfer from the excited state (P*) leads to reduction of a nearby bacteriopheophytin (H). The reduced H^•− is used to reduce the primary quinone, Q_A, which in turn reduces the secondary quinone, Q_B, to the anionic semiquinone (Q_B^•−). Absorption of a second photon again leads to formation of Q_A^•− which now reduces Q_B^•− to the quinol, QH₂, which dissociates from the protein and quinone is rebound. The Q_B charge in parenthesis is found on the second turnover. The order in which electrons and protons are added to Q_B is described in Fig. 4 B. In the membrane of the purple non-sulfur photosynthetic bacteria the cytochrome c is rereduced and the QH₂ reoxidized by the cytochrome bc₁ oxidoreductase with the concomitant increase in the transmembrane proton gradient