Cytochrome bc complexes, origin, biosynthesis etc.

Cytochrome bc complexes, origin, biosynthesis etc.

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.

Transmembrane signaling and assembly of the cytochrome b6f-lipidic charge transfer complex 1



Structure of dimeric b6f complex from M. laminosus (PDB ID: 2E74): (240,000 MW; per monomer, 


8 subunits 7 prosthetic ( additional ) groups; 8 lipids subunit organization and lipid binding sites. 
(A) View along membrane plane showing the positions of the 8 subunits. Color code: 

cytochrome f (Pet A), yellow; 
cytochrome b6 (Pet B), cyan; 
Rieske [2Fe–2S] protein (Pet C), 
orange; subunit IV/
Pet D (pink), 
Pet G (teal), 
Pet L (light brown), 
Pet M (green) and 
Pet N (gray). 

(B) Side view of M. laminosus b6f complex showing bound lipids, detergents and pigments. 

(C) Polytopic core of the b6f complex. Cyt b6 (4 TMH, cyan) and subunit IV (3 TMH, pink) form the core of the b6f monomer. The cyt b6 TMH form a four helix bundle. SubIV is organized around the bundle as a bi-partite structure, with the E-helix separated from the F and G TMH. 

(D) Peripheral four helix bundle formed by the small Pet subunits, Pet G, L, M and N. In addition, including ferredoxin-NADP+-reductase (FNR), which may mediate electron transfer from PSI to cyt b6f. ( Fig. 1) In addition, there are four soluble subunits that associate with purified b6f complex from plant or algal sources, but have not been seen in the crystal structures and have presumably dissociated and been lost during crystallization: the Pet P polypeptide seen in cyanobacteria, the light-harvesting LHCII chlorophyll protein kinase Stt7-STN7, the correlated phosphatase, and the PetO nuclear-encoded phosphorylatable subunit.

1) http://www.sciencedirect.com/science/article/pii/S0005272813000510