Cytochrome b6f complex

Enzyme structure

The cytochrome b₆f complex is a dimer, with each monomer composed of eight subunits. These consist of four large subunits: a 32 kDa cytochrome f with a c-type cytochrome, a 25 kDa cytochrome b₆ with a low- and high-potential heme group, a 19 kDa Rieske iron-sulfur protein containing a [2Fe-2S] cluster, and a 17 kDa subunit IV; along with four small subunits (3-4 kDa): PetG, PetL, PetM, and PetN. The total molecular weight is 217 kDa.

The crystal structure of cytochrome b₆f complexes from Chlamydomonas reinhardtii, Mastigocladus laminosus, and Nostoc sp. PCC 7120 have been determined.

The core of the complex is structurally similar to cytochrome bc₁ core. Cytochrome b₆ and subunit IV are homologous to cytochrome b and the Rieske iron-sulfur proteins of the two complexes are homologous. However, cytochrome f and cytochrome c₁ are not homologous.

Cytochrome b₆f contains seven prosthetic groups. Four are found in both cytochrome b₆f and bc₁: the c-type heme of cytochrome c₁ and f, the two b-type hemes (b_p and b_n) in bc₁ and b₆f, and the [2Fe-2S] cluster of the Rieske protein. Three unique prosthetic groups are found in cytochrome b₆f: chlorophyll a, β-carotene, and heme c_n (also known as heme x).

The inter-monomer space within the core of the cytochrome b6f complex dimer is occupied by lipids, which provides directionality to heme-heme electron transfer through modulation of the intra-protein dielectric environment.

Biological function

In photosynthesis, the cytochrome b₆f complex functions to mediate the transfer of electrons between the two photosynthetic reaction center complexes, from Photosystem II to Photosystem I, while transferring protons from the chloroplast stroma across the thylakoid membrane into the lumen. Electron transport via cytochrome b₆f is responsible for creating the proton gradient that drives the synthesis of ATP in chloroplasts.

In a separate reaction, the cytochrome b₆f complex plays a central role in cyclic photophosphorylation, when NADP⁺ is not available to accept electrons from reduced ferredoxin. This cycle results in the creation of a proton gradient by cytochrome b₆f, which can be used to drive ATP synthesis. It has also been shown that this cycle is essential for photosynthesis, in which it is proposed to help maintain the proper ratio of ATP/NADPH production for carbon fixation.

The p-side quinol deprotonation-oxidation reactions within the cytochrome b6f complex have been implicated in the generation of reactive oxygen species. An integral chlorophyll molecule located within the quinol oxidation site has been suggested to perform a structural, non-photochemical function in enhancing the rate of formation of the reactive oxygen species, possibly to provide a redox-pathway for intra-cellular communication.

Reaction mechanism

The cytochrome b₆f complex is responsible for "non-cyclic" (1) and "cyclic" (2) electron transfer between two mobile redox carriers, plastoquinone (QH₂) and plastocyanin (Pc):

Cytochrome b₆f catalyzes the transfer of electrons from plastoquinol to plastocyanin, while pumping two protons from the stroma into the thylakoid lumen:

This reaction occurs through the Q cycle as in Complex III. Plastoquinone acts as the electron carrier, transferring its two electrons to high- and low-potential electron transport chains (ETC) via a mechanism called electron bifurcation.

Q cycle

First half of Q cycle

1. QH₂ binds to the positive 'p' side (lumen side) of the complex. It is oxidized to a semiquinone (SQ) by the iron-sulfur center (high-potential ETC) and releases two protons to the thylakoid lumen.
2. The reduced iron-sulfur center transfers its electron through cytochrome f to Pc.
3. In the low-potential ETC, SQ transfers its electron to heme b_p of cytochrome b6.
4. Heme b_p then transfers the electron to heme b_n.
5. Heme b_n reduces Q with one electron to form SQ.

Second half of Q cycle

1. A second QH₂ binds to the complex.
2. In the high-potential ETC, one electron reduces another oxidized Pc.
3. In the low-potential ETC, the electron from heme b_n is transferred to SQ, and the completely reduced Q²⁻ takes up two protons from the stroma to form QH₂.
4. The oxidized Q and the reduced QH₂ that has been regenerated diffuse into the membrane.

Cyclic electron transfer

In contrast to Complex III, cytochrome b₆f catalyzes another electron transfer reaction that is central to cyclic photophosphorylation. The electron from ferredoxin (Fd) is transferred to plastoquinone and then the cytochrome b₆f complex to reduce plastocyanin, which is reoxidized by P700 in Photosystem I. The exact mechanism for how plastoquinone is reduced by ferredoxin is still under investigation. One proposal is that there exists a ferredoxin:plastoquinone-reductase or an NADP dehydrogenase. Since heme x does not appear to be required for the Q cycle and is not found in Complex III, it has been proposed that it is used for cyclic photophosphorylation by the following mechanism:

1. Fd (red) + heme x (ox) → Fd (ox) + heme x (red)
2. heme x (red) + Fd (red) + Q + 2H⁺ → heme x (ox) + Fd (ox) + QH₂