cytochrome b6f complex

The cytochrome b₆f complex is a dimer, with each monomer composed of eight subunits.{{cite journal | vauthors = Whitelegge JP, Zhang H, Aguilera R, Taylor RM, Cramer WA | title = Full subunit coverage liquid chromatography electrospray ionization mass spectrometry (LCMS+) of an oligomeric membrane protein: cytochrome b(6)f complex from spinach and the cyanobacterium Mastigocladus laminosus | journal = Molecular & Cellular Proteomics | volume = 1 | issue = 10 | pages = 816–27 | date = Oct 2002 | pmid = 12438564 | doi = 10.1074/mcp.m200045-mcp200 | doi-access = free }} 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.{{cite book | last1 = Voet | first1 = Donald J. | last2 = Voet | first2 = Judith G. | name-list-style = vanc | title = Biochemistry | year = 2011 | publisher = Wiley, J | location = New York, NY | isbn = 978-0-470-57095-1 }} 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.{{cite journal | vauthors = Stroebel D, Choquet Y, Popot JL, Picot D | title = An atypical haem in the cytochrome b(6)f complex | journal = Nature | volume = 426 | issue = 6965 | pages = 413–8 | date = Nov 2003 | pmid = 14647374 | doi = 10.1038/nature02155 | s2cid = 130033 }}{{cite journal | vauthors = Yamashita E, Zhang H, Cramer WA | title = Structure of the cytochrome b6f complex: quinone analogue inhibitors as ligands of heme cn | journal = Journal of Molecular Biology | volume = 370 | issue = 1 | pages = 39–52 | date = Jun 2007 | pmid = 17498743 | pmc = 1993820 | doi = 10.1016/j.jmb.2007.04.011 }}{{cite journal | vauthors = Baniulis D, Yamashita E, Whitelegge JP, Zatsman AI, Hendrich MP, Hasan SS, Ryan CM, Cramer WA | title = Structure-Function, Stability, and Chemical Modification of the Cyanobacterial Cytochrome b6f Complex from Nostoc sp. PCC 7120 | journal = The Journal of Biological Chemistry | volume = 284 | issue = 15 | pages = 9861–9 | date = Apr 2009 | pmid = 19189962 | pmc = 2665108 | doi = 10.1074/jbc.M809196200 | doi-access = free }}{{cite journal | vauthors = Hasan SS, Stofleth JT, Yamashita E, Cramer WA | title = Lipid-induced conformational changes within the cytochrome b6f complex of oxygenic photosynthesis | journal = Biochemistry | volume = 52 | issue = 15 | pages = 2649–54 | date = Apr 2013 | pmid = 23514009 | pmc = 4034689 | doi = 10.1021/bi301638h }}{{cite journal | vauthors = Hasan SS, Cramer WA | title = Internal lipid architecture of the hetero-oligomeric cytochrome b6f complex | journal = Structure | volume = 22 | issue = 7 | pages = 1008–15 | date = Jul 2014 | pmid = 24931468 | pmc = 4105968 | doi = 10.1016/j.str.2014.05.004 }}

The core of the complex is structurally similar to the cytochrome bc₁ core. Cytochrome b₆ and subunit IV are homologous to cytochrome b,{{cite journal | vauthors = Widger WR, Cramer WA, Herrmann RG, Trebst A | title = Sequence homology and structural similarity between cytochrome b of mitochondrial complex III and the chloroplast b6-f complex: position of the cytochrome b hemes in the membrane | journal = Proceedings of the National Academy of Sciences of the United States of America | volume = 81 | issue = 3 | pages = 674–8 | date = Feb 1984 | pmid = 6322162 | pmc = 344897 | doi = 10.1073/pnas.81.3.674 | doi-access = free | bibcode = 1984PNAS...81..674W }} and the Rieske iron-sulfur proteins of the two complexes are homologous.{{cite journal | vauthors = Carrell CJ, Zhang H, Cramer WA, Smith JL | title = Biological identity and diversity in photosynthesis and respiration: structure of the lumen-side domain of the chloroplast Rieske protein | journal = Structure | volume = 5 | issue = 12 | pages = 1613–25 | date = Dec 1997 | pmid = 9438861 | doi = 10.1016/s0969-2126(97)00309-2 | doi-access = free }} However, cytochrome f and cytochrome c₁ are not homologous.{{cite journal | vauthors = Martinez SE, Huang D, Szczepaniak A, Cramer WA, Smith JL | title = Crystal structure of chloroplast cytochrome f reveals a novel cytochrome fold and unexpected heme ligation | journal = Structure | volume = 2 | issue = 2 | pages = 95–105 | date = Feb 1994 | pmid = 8081747 | doi = 10.1016/s0969-2126(00)00012-5 | doi-access = free }}

Cytochrome b₆f contains seven prosthetic groups.{{cite journal | vauthors = Baniulis D, Yamashita E, Zhang H, Hasan SS, Cramer WA | title = Structure-function of the cytochrome b6f complex | journal = Photochemistry and Photobiology | volume = 84 | issue = 6 | pages = 1349–58 | year = 2008 | pmid = 19067956 | doi = 10.1111/j.1751-1097.2008.00444.x | s2cid = 44992397 }}{{cite journal | vauthors = Cramer WA, Zhang H, Yan J, Kurisu G, Smith JL | title = Evolution of photosynthesis: time-independent structure of the cytochrome b6f complex | journal = Biochemistry | volume = 43 | issue = 20 | pages = 5921–9 | date = May 2004 | pmid = 15147175 | doi = 10.1021/bi049444o }} 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.{{cite journal | vauthors = Hasan SS, Zakharov SD, Chauvet A, Stadnytskyi V, Savikhin S, Cramer WA | title = A map of dielectric heterogeneity in a membrane protein: the hetero-oligomeric cytochrome b6f complex | journal = The Journal of Physical Chemistry B | volume = 118 | issue = 24 | pages = 6614–25 | date = Jun 2014 | pmid = 24867491 | pmc = 4067154 | doi = 10.1021/jp501165k }}

File:Tobacco (Nicotiana tabacum) cyt6bf mutant.jpg) cytochrome b₆f mutant (right) next to normal plant. Plants are used in photosynthesis research to investigate the cyclic photophosphorylation.]]

In photosynthesis, the cytochrome b₆f complex functions to mediate the transfer of electrons and of energy between the two photosynthetic reaction center complexes, Photosystem II and 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.{{cite book | last1 = Berg | first1 = Jeremy M. | last2 = Tymoczko | first2 = John L. | last3 = Stryer | first3 = Lubert | last4 = Stryer | first4 = Lubert | name-list-style = vanc | title = Biochemistry | url = https://archive.org/details/biochemistry0006berg | url-access = registration | year = 2007 | publisher = W.H. Freeman | location = New York | isbn = 978-0-7167-8724-2 }} This cycle, driven by the energy of P700⁺, 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,{{cite journal | vauthors = Munekage Y, Hashimoto M, Miyake C, Tomizawa K, Endo T, Tasaka M, Shikanai T | title = Cyclic electron flow around photosystem I is essential for photosynthesis | journal = Nature | volume = 429 | issue = 6991 | pages = 579–82 | date = Jun 2004 | pmid = 15175756 | doi = 10.1038/nature02598 | bibcode = 2004Natur.429..579M | s2cid = 4421776 }} helping to maintain the proper ratio of ATP/NADPH production for carbon fixation.{{cite book|author1-link=Robert E. Blankenship | last1 = Blankenship | first1 = Robert E. | name-list-style = vanc | title = Molecular mechanisms of photosynthesis | year = 2002 | publisher = Blackwell Science | location = Oxford; Malden, MA | isbn = 978-0-632-04321-7 }}{{cite journal | title = Cyclic photophosphorylation and electron transport | journal = Biochimica et Biophysica Acta (BBA) - Bioenergetics | first = Derek | last = Bendall| year = 1995 | name-list-style = vanc | doi=10.1016/0005-2728(94)00195-B | volume=1229 | pages=23–38| doi-access = free }}

The p-side quinol deprotonation-oxidation reactions within the cytochrome b6f complex have been implicated in the generation of reactive oxygen species.{{cite journal | vauthors = Baniulis D, Hasan SS, Stofleth JT, Cramer WA | title = Mechanism of enhanced superoxide production in the cytochrome b(6)f complex of oxygenic photosynthesis | journal = Biochemistry | volume = 52 | issue = 50 | pages = 8975–83 | date = Dec 2013 | pmid = 24298890 | pmc = 4037229 | doi = 10.1021/bi4013534 }} 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.{{cite journal | vauthors = Hasan SS, Proctor EA, Yamashita E, Dokholyan NV, Cramer WA | title = Traffic within the cytochrome b6f lipoprotein complex: gating of the quinone portal | journal = Biophysical Journal | volume = 107 | issue = 7 | pages = 1620–8 | date = Oct 2014 | pmid = 25296314 | pmc = 4190601 | doi = 10.1016/j.bpj.2014.08.003 | bibcode = 2014BpJ...107.1620H }}

The cytochrome b₆f complex is responsible for "non-cyclic" (1) and "cyclic" (2) electron transfer between two mobile redox carriers, plastoquinol (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.{{cite journal | vauthors = Cramer WA, Soriano GM, Ponomarev M, Huang D, Zhang H, Martinez SE, Smith JL | title = Some New Structural Aspects and Old Controversies Concerning the Cytochrome b6f Complex of Oxygenic Photosynthesis | journal = Annual Review of Plant Physiology and Plant Molecular Biology | volume = 47 | pages = 477–508 | date = Jun 1996 | pmid = 15012298 | doi = 10.1146/annurev.arplant.47.1.477 }} 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.{{cite journal | vauthors = Cramer WA, Zhang H, Yan J, Kurisu G, Smith JL | title = Transmembrane traffic in the cytochrome b6f complex | journal = Annual Review of Biochemistry | volume = 75 | pages = 769–90 | year = 2006 | pmid = 16756511 | doi = 10.1146/annurev.biochem.75.103004.142756 }} 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.{{cite journal | vauthors = Malone LA, Qian P, Mayneord GE, Hitchcock A, Farmer DA, Thompson RF, Swainsbury DJ, Ranson NA, Hunter CN, Johnson MP | display-authors = 6 | title = Cryo-EM Structure of the Spinach Cytochrome B 6 F Complex at 3.6 Å Resolution | journal = Nature | volume = 575 | issue = 7783 | pages = 535–539 | date = November 2019 | pmid = 31723268 | doi = 10.1038/s41586-019-1746-6 | s2cid = 207987984 | url = http://eprints.whiterose.ac.uk/154030/1/Malone_et_al_Nature.pdf }}

File:Q-cycle cytochrome b6f.png

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|reason=Semiquinones have lost just a single proton and single electron. Why does this say that two protons are released?|date=February 2017}}.
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.

Unlike 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.{{cite journal | vauthors = Joliot P, Joliot A | title = Cyclic electron transfer in plant leaf | journal = Proceedings of the National Academy of Sciences of the United States of America | volume = 99 | issue = 15 | pages = 10209–14 | date = Jul 2002 | pmid = 12119384 | doi = 10.1073/pnas.102306999 | pmc=126649| doi-access = free | bibcode = 2002PNAS...9910209J }} 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:{{cite journal | vauthors = Cramer WA, Yan J, Zhang H, Kurisu G, Smith JL | title = Structure of the cytochrome b6f complex: new prosthetic groups, Q-space, and the 'hors d'oeuvres hypothesis' for assembly of the complex | journal = Photosynthesis Research | volume = 85 | issue = 1 | pages = 133–43 | year = 2005 | pmid = 15977064 | doi = 10.1007/s11120-004-2149-5 | bibcode = 2005PhoRe..85..133C | s2cid = 20731696 }}

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

{{Reflist|33em}}

• {{cite journal |last1=Sarewicz |first1=M |last2=Pintscher |first2=S |last3=Pietras |first3=R |last4=Borek |first4=A |last5=Bujnowicz |first5=Ł |last6=Hanke |first6=G |last7=Cramer |first7=WA |last8=Finazzi |first8=G |last9=Osyczka |first9=A |title=Catalytic Reactions and Energy Conservation in the Cytochrome bc(1) and b(6)f Complexes of Energy-Transducing Membranes. |journal=Chemical Reviews |date=24 February 2021 |volume=121 |issue=4 |pages=2020–2108 |doi=10.1021/acs.chemrev.0c00712 |pmid=33464892 |pmc=7908018 |doi-access=free}}

• [http://bio.purdue.edu/people/faculty/cramer/cramerlab/cytbf.html Structure-Function Studies of the Cytochrome b₆f Complex] - Current research on cytochrome b₆f in William Cramer's Lab at Purdue University, USA
• {{UMichOPM|families|superfamily|3}} - Calculated positions of b6f and related complexes in membranes
• {{MeshName|Cytochrome+b6f+Complex}}
• {{MeshName|Plastoquinol-plastocyanin+reductase}}

{{Multienzyme complexes}}

{{Proton pumps}}

{{Diphenol family oxidoreductases}}

{{Enzymes}}

{{Portal bar|Biology|border=no}}

{{DISPLAYTITLE:Cytochrome b₆f complex}}

Category:Hemoproteins

Category:Iron–sulfur proteins

Category:Light reactions

Category:Integral membrane proteins

Category:EC 1.10.99