Cytochrome b6f complex

The **cytochrome *b***6***f* complex** (plastoquinol—plastocyanin reductase; EC 1.10.99.1) is an enzyme found in the thylakoid membrane in chloroplasts of plants, cyanobacteria, and green algae, that catalyzes the transfer of electrons from plastoquinol to plastocyanin. The reaction is analogous to the reaction catalyzed by cytochrome bc1 (Complex III) of the mitochondrial electron transport chain. During photosynthesis, the cytochrome b6f complex is one step along the chain that transfers electrons from Photosystem II to Photosystem I, and at the same time pumps protons into the thylakoid space that contribute to create an electrochemical (energy) gradient which is later used to synthesize ATP from ADP.

## 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 (bp and bn) 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 cn (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 *b*6*f* 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:

QH2 + 2Pc(Cu2+) + 2H+ (stroma) → Q + 2Pc(Cu+) + 4H+ (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 bp of cytochrome b6.
4. Heme bp then transfers the electron to heme bn.
5. Heme bn 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 bn 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