Cytochrome b₆f complex

The cytochrome b₆f complex (plastoquinol/plastocyanin reductase or plastoquinol/plastocyanin oxidoreductase;

plastoquinol + 2 oxidized plastocyanin

+ 2 H⁺ [side 1] ${\displaystyle \rightleftharpoons }$ plastoquinone + 2 reduced plastocyanin + 4 H⁺ [side 2].^[1]

The reaction is analogous to the reaction catalyzed by

The crystal structures of cytochrome b₆f complexes from Chlamydomonas reinhardtii, Mastigocladus laminosus, and Nostoc sp. PCC 7120 have been determined.[2]^{[5][6][7][8][9]}

The core of the complex is structurally similar to the cytochrome bc₁ core. Cytochrome b₆ and subunit IV are homologous to cytochrome b,^[10] and the Rieske iron-sulfur proteins of the two complexes are homologous.^[11] However, cytochrome f and cytochrome c₁ are not homologous.^[12]

Cytochrome b₆f contains seven

The inter-monomer space within the core of the cytochrome b6f complex dimer is occupied by lipids,[9] which provides directionality to heme-heme electron transfer through modulation of the intra-protein dielectric environment.^[15]

Biological function

In

The p-side quinol deprotonation-oxidation reactions within the cytochrome b6f complex have been implicated in the generation of reactive oxygen species.[20] 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.^[21]

Reaction mechanism

The cytochrome b₆f complex is responsible for "

QH₂ + 2Pc(Cu²⁺) + 2H⁺ (stroma) → Q + 2Pc(Cu⁺) + 4H⁺ (lumen)[16]

This reaction occurs through the Q cycle as in Complex III.^[22] Plastoquinol acts as the electron carrier, transferring its two electrons to high- and low-potential electron transport chains (ETC) via a mechanism called electron bifurcation.^[23] The complex contains up to three plastoquinone molecules that form an electron transfer network that are responsible for the operation of the Q cycle and its redox-sensing and catalytic functions in photosynthesis.^[24]

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^{[citation needed]}.
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

Unlike Complex III, cytochrome b₆f catalyzes another electron transfer reaction that is central to