The Cytochrome b6-f Complex Connects Photosystem II to Photosystem I
The electrons extracted from water by photosystem II are transferred to plastoquinol, a strong electron donor similar to ubiquinol in mitochondria. This quinol, which can diffuse rapidly in the lipid bilayer of the thylakoid membrane, transfers its electrons to the cytochrome b6-f complex, whose structure is homologous to the cytochrome c reductase in mitochondria. The cytochrome b6-f complex pumps H+ into the thylakoid space using the same Q cycle that is utilized in mitochondria, thereby adding to the proton gradient across the thylakoid membrane. The cytochrome b6-f complex forms the connecting link between photosystems II and I in the chloroplast electron-transport chain. It passes its electrons one at a time to the mobile electron carrier plastocyanin (a small copper-containing protein that takes the place of the cytochrome c in mitochondria), which will transfer them to photosystem I (Figure 14–50).
Photosystem I then harnesses a second photon of light to further energize the electrons that it receives.
The structure and function of the cytochrome b6 f complex
Molecular mechanisms of photosynthesis, Robert Blankenship, page 120
The cytochrome b6 f complex is an essential player in noncyclic and cyclic electron flow. The cytochrome b6 f complex is similar in most ways to the cytochrome bc1 complex. However, there are some important differences. The structure of the cytochrome b6 f complex is shown in Fig. 7.7.
Table 7.2 gives the identity and masses of the proteins of the cytochrome b6 f complex.
In addition to the proteins, the complex contains chlorophyll and carotenoid molecules of unknown function. Cytochrome f is a c-type cytochrome that serves a similar functional role to cytochrome c1 in the cytochrome bc1 complex. However, the two cytochromes have very different structural features.