The cytochrome ''b''₆''f'' complex (plastoquinol—plastocyanin reductase; ) is an enzyme found in the

thylakoid Thylakoids are membrane-bound compartments inside chloroplasts and cyanobacteria. They are the site of the light-dependent reactions of photosynthesis. Thylakoids consist of a thylakoid membrane surrounding a thylakoid lumen. Chloroplast thyl ...

membrane in

chloroplast A chloroplast () is a type of membrane-bound organelle known as a plastid that conducts photosynthesis mostly in plant and algal cells. The photosynthetic pigment chlorophyll captures the energy from sunlight, converts it, and stores it in ...

s of plants,

cyanobacteria Cyanobacteria (), also known as Cyanophyta, are a phylum of gram-negative bacteria that obtain energy via photosynthesis. The name ''cyanobacteria'' refers to their color (), which similarly forms the basis of cyanobacteria's common name, blu ...

, and

green algae The green algae (singular: green alga) are a group consisting of the Prasinodermophyta and its unnamed sister which contains the Chlorophyta and Charophyta/Streptophyta. The land plants (Embryophytes) have emerged deep in the Charophyte alga as ...

, that catalyzes the transfer of electrons from

plastoquinol Plastoquinone (PQ) is an isoprenoid quinone molecule involved in the electron transport chain in the light-dependent reactions of photosynthesis. The most common form of plastoquinone, known as PQ-A or PQ-9, is a 2,3-dimethyl-1,4-benzoquinone mol ...

plastocyanin Plastocyanin is a copper-containing protein that mediates electron-transfer. It is found in a variety of plants, where it participates in photosynthesis. The protein is a prototype of the blue copper proteins, a family of intensely blue-colored m ...

. The reaction is analogous to the reaction catalyzed by cytochrome bc₁ (Complex III) of the

mitochondria A mitochondrion (; ) is an organelle found in the Cell (biology), cells of most Eukaryotes, such as animals, plants and Fungus, fungi. Mitochondria have a double lipid bilayer, membrane structure and use aerobic respiration to generate adenosi ...

electron transport chain An electron transport chain (ETC) is a series of protein complexes and other molecules that transfer electrons from electron donors to electron acceptors via redox reactions (both reduction and oxidation occurring simultaneously) and couples th ...

. During

photosynthesis Photosynthesis is a process used by plants and other organisms to convert light energy into chemical energy that, through cellular respiration, can later be released to fuel the organism's activities. Some of this chemical energy is stored i ...

, the cytochrome b₆f complex is one step along the chain that transfers

electrons The electron ( or ) is a subatomic particle with a negative one elementary electric charge. Electrons belong to the first generation of the lepton particle family, and are generally thought to be elementary particles because they have no ...

from

Photosystem II Photosystem II (or water-plastoquinone oxidoreductase) is the first protein complex in the light-dependent reactions of oxygenic photosynthesis. It is located in the thylakoid membrane of plants, algae, and cyanobacteria. Within the photosystem ...

Photosystem I Photosystem I (PSI, or plastocyanin–ferredoxin oxidoreductase) is one of two photosystems in the photosynthetic light reactions of algae, plants, and cyanobacteria. Photosystem I is an integral membrane protein complex that uses ...

, and at the same time pumps protons into the thylakoid space, contributing to the generation of an electrochemical (energy) gradient that is later used to synthesize ATP from ADP.

Enzyme structure

The cytochrome b₆f complex is a dimer, with each

monomer In chemistry, a monomer ( ; ''mono-'', "one" + '' -mer'', "part") is a molecule that can react together with other monomer molecules to form a larger polymer chain or three-dimensional network in a process called polymerization. Classification Mo ...

composed of eight subunits. These consist of four large subunits: a 32 kDa

cytochrome f Cytochrome ''f'' is the largest subunit of cytochrome ''b''6''f'' complex (plastoquinol—plastocyanin reductase; ). In its structure and functions, the cytochrome b6f complex bears extensive analogy to the cytochrome bc1 complex of mitochondr ...

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 Fe-2Scluster, 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 structures 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 the cytochrome bc₁ core. Cytochrome b₆ and subunit IV are homologous to

cytochrome b Cytochrome b within both molecular and cell biology, is a protein found in the mitochondria of eukaryotic cells. It functions as part of the electron transport chain and is the main subunit of transmembrane cytochrome bc1 and b6f complexes. F ...

, 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 A cofactor is a non-protein chemical compound or metallic ion that is required for an enzyme's role as a catalyst (a catalyst is a substance that increases the rate of a chemical reaction). Cofactors can be considered "helper molecules" that ass ...

. 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 Fe-2Scluster of the Rieske protein. Three unique prosthetic groups are found in cytochrome b₆f: chlorophyll a,

β-carotene β-Carotene is an organic, strongly coloured red-orange pigment abundant in fungi, plants, and fruits. It is a member of the carotenes, which are terpenoids (isoprenoids), synthesized biochemically from eight isoprene units and thus having 40 ...

, 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

Tobacco (Nicotiana tabacum) cyt6bf mutant

, the cytochrome b₆f complex functions to mediate the transfer of electrons and of energy between the two photosynthetic reaction center complexes,

and

, while transferring protons from the chloroplast stroma across the

membrane into the lumen.

Electron transport An electron transport chain (ETC) is a series of protein complexes and other molecules that transfer electrons from electron donors to electron acceptors via redox reactions (both reduction and oxidation occurring simultaneously) and couples thi ...

via cytochrome b₆f is responsible for creating the

proton gradient An electrochemical gradient is a gradient of electrochemical potential, usually for an ion that can move across a membrane. The gradient consists of two parts, the chemical gradient, or difference in solute concentration across a membrane, and th ...

that drives the synthesis of ATP in chloroplasts. In a separate reaction, the cytochrome b₆f complex plays a central role in

cyclic photophosphorylation In the process of photosynthesis, the phosphorylation of ADP to form ATP using the energy of sunlight is called photophosphorylation. Cyclic photophosphorylation occurs in both aerobic and anaerobic conditions, driven by the main primary source of ...

, when NADP⁺ is not available to accept electrons from reduced

ferredoxin Ferredoxins (from Latin ''ferrum'': iron + redox, often abbreviated "fd") are iron–sulfur proteins that mediate electron transfer in a range of metabolic reactions. The term "ferredoxin" was coined by D.C. Wharton of the DuPont Co. and applied t ...

. This cycle, driven by the energy of

P700 P700, or photosystem I primary donor, is the reaction-center chlorophyll ''a'' molecular dimer associated with photosystem I in plants, algae, and cyanobacteria. Etymology Its name is derived from the word “pigment” (P) and the presence ...

⁺, contributes to the creation of a proton gradient that can be used to drive ATP synthesis. It has been shown that this cycle is essential for photosynthesis, helping to maintain the proper ratio of ATP/NADPH production for

carbon fixation Biological carbon fixation or сarbon assimilation is the process by which inorganic carbon (particularly in the form of carbon dioxide) is converted to organic compounds by living organisms. The compounds are then used to store energy and as ...

. 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 Cycle, cycles, or cyclic may refer to: Anthropology and social sciences * Cyclic history, a theory of history * Cyclical theory, a theory of American political history associated with Arthur Schlesinger, Sr. * Social cycle, various cycles in soc ...

" (2) electron transfer between two mobile redox carriers,

(QH₂) and

(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 The Q cycle (named for ''quinol'') describes a series of reactions that describe how the sequential oxidation and reduction of the lipophilic electron carrier, Coenzyme Q (CoQ), between the ubiquinol and ubiquinone forms, can result in the net mov ...

as in Complex III. Plastoquinol acts as the electron carrier, transferring its two electrons to high- and low-potential

s (ETC) via a mechanism called electron bifurcation. 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.

Q cycle

First half of Q cycle # QH₂ binds to the positive 'p' side (lumen side) of the complex. It is oxidized to a

semiquinone Semiquinone (or ubisemiquinone) is a free radical resulting from the removal of one hydrogen atom with its electron during the process of dehydrogenation of a hydroquinone, such as hydroquinone itself or catechol, to a quinone or alternatively the ...

(SQ) by the iron-sulfur center (high-potential ETC) and releases two protons to the thylakoid lumen. # The reduced iron-sulfur center transfers its electron through cytochrome f to Pc. # In the low-potential ETC, SQ transfers its electron to heme b_p of cytochrome b6. # Heme b_p then transfers the electron to heme b_n. # Heme b_n reduces Q with one electron to form SQ. Second half of Q cycle # A second QH₂ binds to the complex. # In the high-potential ETC, one electron reduces another oxidized Pc. # 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. # 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

. The electron from

(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 of the reduction of plastoquinone 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: # Fd (red) + heme x (ox) → Fd (ox) + heme x (red) # heme x (red) + Fd (red) + Q + 2H⁺ → heme x (ox) + Fd (ox) + QH₂

References

External links

Structure-Function Studies of the Cytochrome ''b''₆''f'' Complex
- Current research on cytochrome ''b''₆''f'' in William Cramer's Lab at Purdue University, USA * - Calculated positions of b6f and related complexes in membranes * {{DISPLAYTITLE:Cytochrome b₆f complex Hemoproteins Iron–sulfur proteins Light reactions Integral membrane proteins EC 1.10.99