Figure 3.

Electron-proton transfer pathways in the cytochrome b₆f complex. The dashed box represents the cytochrome b₆f transmembrane core. On the p side, anionic semiquinone (PQ^•-) (highlighted in red) is produced by deprotonation-oxidation of the reduced plastoquinol (PQH₂), or back-transfer of an electron to the oxidized PQ from the reduced heme b_p. The anionic semiquinone has been implicated in the reduction of molecular oxygen to superoxide (O₂^•-). FNR, ferredoxin-NADP⁺-oxidoreductase; Fd, ferredoxin. To see this figure in color, go online.