Cytochrome b6f complex
Encyclopedia
The cytochrome b6f complex (plastoquinol—plastocyanin reductase; ) is an enzyme found in the thylakoid
membrane in chloroplast
s of plants, cyanobacteria, and green algae
, catalyzing 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
. For photosynthesis
, the cytochrome b6f complex transfers electrons between the two reaction complexes from Photosystem II
to Photosystem I
.
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 complex is structurally similar to cytochrome bc1. 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 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
.
(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 chain
s (ETC) via a mechanism called electron bifurcation.
Second half of Q cycle
(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:
Thylakoid
A thylakoid is a membrane-bound compartment 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 thylakoids frequently form stacks of disks referred to as...
membrane in chloroplast
Chloroplast
Chloroplasts are organelles found in plant cells and other eukaryotic organisms that conduct photosynthesis. Chloroplasts capture light energy to conserve free energy in the form of ATP and reduce NADP to NADPH through a complex set of processes called photosynthesis.Chloroplasts are green...
s of plants, cyanobacteria, and green algae
Green algae
The green algae are the large group of algae from which the embryophytes emerged. As such, they form a paraphyletic group, although the group including both green algae and embryophytes is monophyletic...
, catalyzing the transfer of electrons from plastoquinol to plastocyanin
Plastocyanin
Plastocyanin is an important copper-containing protein involved in electron-transfer. The protein is monomeric, with a molecular weight around 10,500 Daltons, and 99 amino acids in most vascular plants...
. The reaction is analogous to the reaction catalyzed by cytochrome bc1 (Complex III) of the mitochondrial electron transport chain
Electron transport chain
An electron transport chain couples electron transfer between an electron donor and an electron acceptor with the transfer of H+ ions across a membrane. The resulting electrochemical proton gradient is used to generate chemical energy in the form of adenosine triphosphate...
. For photosynthesis
Photosynthesis
Photosynthesis is a chemical process that converts carbon dioxide into organic compounds, especially sugars, using the energy from sunlight. Photosynthesis occurs in plants, algae, and many species of bacteria, but not in archaea. Photosynthetic organisms are called photoautotrophs, since they can...
, the cytochrome b6f complex transfers electrons between the two reaction complexes from Photosystem II
Photosystem II
Photosystem II is the first protein complex in the Light-dependent reactions. It is located in the thylakoid membrane of plants, algae, and cyanobacteria. The enzyme uses photons of light to energize electrons that are then transferred through a variety of coenzymes and cofactors to reduce...
to Photosystem I
Photosystem I
Photosystem I is the second photosystem in the photosynthetic light reactions of algae, plants, and some bacteria. Photosystem I is so named because it was discovered before photosystem II. Aspects of PS I were discovered in the 1950s, but the significances of these discoveries was not yet known...
.
Enzyme structure
The cytochrome b6f complex is a dimer, with each monomerMonomer
A monomer is an atom or a small molecule that may bind chemically to other monomers to form a polymer; the term "monomeric protein" may also be used to describe one of the proteins making up a multiprotein complex...
composed of eight subunits. These consist of four large subunits: a 32 kDa cytochrome f
Cytochrome f
Cytochrome f is the largest subunit of cytochrome b6f complex . In its structure and functions, the cytochrome b6f complex bears extensive analogy to the cytochrome bc1 complex of mitochondria and photosynthetic purple bacteria...
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
Rieske protein
Rieske proteins are iron-sulfur protein components of cytochrome bc1 complexes and cytochrome b6f complexes which were first discovered and isolated by John S. Rieske and co-workers in 1964. It is a unique [2Fe-2S] cluster in that one of the two Fe atoms is coordinated by two histidine residues...
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 complex is structurally similar to cytochrome bc1. Cytochrome b6 and subunit IV are homologous to cytochrome b
Cytochrome b
Cytochrome b/b6 is the main subunit of transmembrane cytochrome bc1 and b6f complexes. In addition, it commonly refers to a region of mtDNA used for population genetics and phylogenetics.- Function :...
and the Rieske iron-sulfur proteins of the two complexes are homologous. However, cytochrome f and cytochrome c1
Cytochrome C1
Cytochrome C1 is formed in the cytosol and targeted to the mitochondrial intermembrane space. It is one of the constituents of complex III, which forms the third proton pump in the mitochondrial electron transport chain....
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
Chlorophyll a
Chlorophyll a is a specific form of chlorophyll used in oxygenic photosynthesis. It absorbs most energy from wavelengths of violet-blue and orange-red light. This photosynthetic pigment is essential for photosynthesis in eukaryotes, cyanobacteria and prochlorophytes because of its role as primary...
, β-carotene, and heme cn (also known as heme x).
Biological function
In photosynthesisPhotosynthesis
Photosynthesis is a chemical process that converts carbon dioxide into organic compounds, especially sugars, using the energy from sunlight. Photosynthesis occurs in plants, algae, and many species of bacteria, but not in archaea. Photosynthetic organisms are called photoautotrophs, since they can...
, the cytochrome b6f complex functions to mediate the transfer of electrons between the two photosynthetic reaction center complexes, from Photosystem II
Photosystem II
Photosystem II is the first protein complex in the Light-dependent reactions. It is located in the thylakoid membrane of plants, algae, and cyanobacteria. The enzyme uses photons of light to energize electrons that are then transferred through a variety of coenzymes and cofactors to reduce...
to Photosystem I
Photosystem I
Photosystem I is the second photosystem in the photosynthetic light reactions of algae, plants, and some bacteria. Photosystem I is so named because it was discovered before photosystem II. Aspects of PS I were discovered in the 1950s, but the significances of these discoveries was not yet known...
, while transferring protons from the chloroplast stroma across the thylakoid
Thylakoid
A thylakoid is a membrane-bound compartment 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 thylakoids frequently form stacks of disks referred to as...
membrane into the lumen
Lumen
Lumen can mean:* Lumen , the SI unit of luminous flux* Lumen , the cavity or channel within a tubular structure* Thylakoid lumen, the inner membrane space of the chloroplast* Phenobarbital...
. Electron transport via cytochrome b6f is responsible for creating the proton gradient that drives the synthesis of ATP
Adenosine triphosphate
Adenosine-5'-triphosphate is a multifunctional nucleoside triphosphate used in cells as a coenzyme. It is often called the "molecular unit of currency" of intracellular energy transfer. ATP transports chemical energy within cells for metabolism...
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
Ferredoxin
Ferredoxins 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...
. 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
Carbon fixation
In biology, carbon fixation is the reduction of carbon dioxide to organic compounds by living organisms. The obvious example is photosynthesis. Carbon fixation requires both a source of energy such as sunlight, and an electron donor such as water. All life depends on fixed carbon. Organisms that...
.
Reaction mechanism
The cytochrome b6f complex is responsible for "non-cyclic" (1) and "cyclic" (2) electron transfer between two mobile redox carriers, plastoquinolPlastoquinone
Plastoquinone is a quinone molecule involved in the electron transport chain in the light-dependent reactions of photosynthesis. Plastoquinone is reduced , forming plastoquinol...
(QH2) and plastocyanin
Plastocyanin
Plastocyanin is an important copper-containing protein involved in electron-transfer. The protein is monomeric, with a molecular weight around 10,500 Daltons, and 99 amino acids in most vascular plants...
(Pc):
| H2O | → | photosystem II | → | QH2 | → | Cyt b6f | → | Pc | → | photosystem I | → | NADPH | (1) |
| QH2 | → | Cyt b6f | → | Pc | → | photosystem I | → | Q | (2) |
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
Q cycle
- History :The Q cycle describes a series of reactions first proposed by Peter Mitchell that describe how the sequential oxidation and reduction of the lipophilic electron carrier, ubiquinol-ubiquinone , can result in the net pumping of protons across a lipid bilayer...
as in Complex III. Plastoquinone
Plastoquinone
Plastoquinone is a quinone molecule involved in the electron transport chain in the light-dependent reactions of photosynthesis. Plastoquinone is reduced , forming plastoquinol...
acts as the electron carrier, transferring its two electrons to high- and low-potential electron transport chain
Electron transport chain
An electron transport chain couples electron transfer between an electron donor and an electron acceptor with the transfer of H+ ions across a membrane. The resulting electrochemical proton gradient is used to generate chemical energy in the form of adenosine triphosphate...
s (ETC) via a mechanism called electron bifurcation.
Q cycle
First half of Q cycle- QH2 binds to the positive 'p' side (lumen side) of the complex. It is oxidized to a semiquinoneSemiquinoneSemiquinone is a free radical resulting from the removal of one hydrogen atom with its electron during the process of dehydrogenation of a hydroquinone to quinone or alternatively the addition of a single H atom to a quinone....
(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 bp of cytochrome b6.
- Heme bp then transfers the electron to heme bn.
- Heme bn 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 bn 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
In contrast to Complex III, cytochrome b6f catalyzes another electron transfer reaction that is central to cyclic photophosphorylation. The electron from ferredoxinFerredoxin
Ferredoxins 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...
(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:
- Fd (red) + heme x (ox) → Fd (ox) + heme x (red)
- heme x (red) + Fd (red) + Q + 2H+ → heme x (ox) + Fd (ox) + QH2
External links
- 1Q90 - PDBProtein Data BankThe Protein Data Bank is a repository for the 3-D structural data of large biological molecules, such as proteins and nucleic acids....
structure of cytochrome b6f complex from Chlamydomonas reinhardtii - 1VF5 - PDB structure of cytochrome b6f complex from Mastigocladus laminosus
- 2D2C - PDB structure of cytochrome b6f complex from Mastigocladus laminosus
- 2E74 - PDB structure of cytochrome b6f complex from Mastigocladus laminosus
- 2E75 - PDB structure of cytochrome b6f complex from Mastigocladus laminosus
- 2E76 - PDB structure of cytochrome b6f complex from Mastigocladus laminosus
- 2ZT9 - PDB structure of cytochrome b6f complex from Nostoc sp. PCC 7120
- Structure-Function Studies of the Cytochrome b6f Complex - Current research on cytochrome b6f in William Cramer's Lab at Purdue University, USA - Calculated positions of b6f and related complexes in membranes