Cryo-EM structure of the four-subunit cytochrome complex in styrene maleic acid nanodiscs.

Cytochrome complexes are ubiquinol:cytochrome oxidoreductases, and as such, they are centrally important components of respiratory and photosynthetic electron transfer chains in many species of bacteria and in mitochondria. The minimal complex has three catalytic components, which are cytochrome , cytochrome , and the Rieske iron-sulfur subunit, but the function of mitochondrial cytochrome complexes is modified by up to eight supernumerary subunits. The cytochrome complex from the purple phototrophic bacterium   has a single supernumerary subunit called subunit IV, which is absent from current structures of the complex.

Cryo-EM structures of the Synechocystis sp. PCC 6803 cytochrome b6f complex with and without the regulatory PetP subunit.

In oxygenic photosynthesis, the cytochrome b6f (cytb6f) complex links the linear electron transfer (LET) reactions occurring at photosystems I and II and generates a transmembrane proton gradient via the Q-cycle. In addition to this central role in LET, cytb6f also participates in a range of processes including cyclic electron transfer (CET), state transitions and photosynthetic control. Many of the regulatory roles of cytb6f are facilitated by auxiliary proteins that differ depending upon the species, yet because of their weak and transient nature the structural details of these interactions remain unknown.