View source for Cytochrome b6f complex

{{Short description|Enzyme}}
{{Infobox protein family
| Symbol = B6F
| Name = Cytochrome b6f complex
| image = 1q90_opm.png
| width =
| caption = Crystal structure of the cytochrome b6f complex from ''C. reinhardtii'' ({{PDB3|1q90}}). Hydrocarbon boundaries of the [[lipid bilayer]] are shown by red and blue lines (thylakoid space side and stroma side, respectively).

| SMART =
| PROSITE =
| MEROPS =
| SCOP = 
| TCDB = 3.D.3
| OPM family = 92
| OPM protein = 4pv1
| CAZy =
| CDD =
| Membranome superfamily = 258
}}

{{enzyme
| Name = Cytochrome b<sub>6</sub>f complex
| AltNames = Plastoquinol/plastocyanin reductase
| CAS_number = 79079-13-3
| EC_number = 7.1.1.6
| image =
| width = 250
| caption = 
}}

The '''cytochrome ''b'''''<sub>6</sub>'''''f'' complex''' (plastoquinol/plastocyanin reductase or plastoquinol/plastocyanin oxidoreductase;  {{EC number|7.1.1.6}}) is an enzyme found in the [[thylakoid]] membrane in [[chloroplast]]s of plants, [[cyanobacteria]], and [[green algae]], that catalyzes the transfer of electrons from [[plastoquinol]] to [[plastocyanin]]:
: [[plastoquinol]] + 2 oxidized [[plastocyanin]] + 2 H<sup>+</sup> [side 1] <math>\rightleftharpoons</math> [[plastoquinone]] + 2 reduced [[plastocyanin]] + 4 H<sup>+</sup> [side 2].<ref>[https://www.enzyme-database.org/query.php?ec=7.1.1.6 ExplorEnz: EC 7.1.1.6]</ref>
The reaction is analogous to the reaction catalyzed by [[cytochrome bc1 complex|cytochrome bc<sub>1</sub>]] (Complex III) of the [[mitochondria]]l [[electron transport chain]].  During [[photosynthesis]], the cytochrome b<sub>6</sub>f 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, contributing to the generation of an electrochemical (energy) gradient<ref name="Hasan-2013a">{{cite journal | vauthors = Hasan SS, Yamashita E, Baniulis D, Cramer WA | title = Quinone-dependent proton transfer pathways in the photosynthetic cytochrome b6f complex | journal = Proceedings of the National Academy of Sciences of the United States of America | volume = 110 | issue = 11 | pages = 4297–302 | date = Mar 2013 | pmid = 23440205 | pmc = 3600468 | doi = 10.1073/pnas.1222248110 | doi-access = free }}</ref> that is later used to synthesize [[adenosine triphosphate|ATP]] from [[adenosine diphosphate|ADP]].

== Enzyme structure ==
The cytochrome b<sub>6</sub>f complex is a dimer, with each [[monomer]] composed of eight subunits.<ref name="Whitelegge-2002">{{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 }}</ref> These consist of four large subunits: a 32 kDa [[cytochrome f]] with a c-type cytochrome, a 25 kDa [[cytochrome b6|cytochrome b<sub>6</sub>]] with a low- and high-potential heme group, a 19 kDa [[Rieske protein|Rieske iron-sulfur protein]] containing a [[2Fe-2S cluster|[2Fe-2S] cluster]], and a 17 kDa subunit IV; along with four small subunits (3-4 kDa): PetG, PetL, PetM, and PetN.<ref name="Whitelegge-2002"/><ref name="Voet">{{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 }}</ref> The total molecular weight is 217 kDa.

The crystal structures of cytochrome b<sub>6</sub>f complexes from ''Chlamydomonas reinhardtii'', ''Mastigocladus laminosus'', and ''Nostoc'' sp. PCC 7120 have been determined.<ref name="Hasan-2013a"/><ref name="Stroebel-2003">{{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 }}</ref><ref name="Yamashita-2007">{{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 }}</ref><ref name="Baniulis-2009">{{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 }}</ref><ref name="Hasan-2013b">{{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 }}</ref><ref name="Hasan-2014a">{{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 }}</ref>

The core of the complex is structurally similar to the cytochrome bc<sub>1</sub> core. Cytochrome b<sub>6</sub> and subunit IV are homologous to [[cytochrome b]],<ref name="Widger-1984">{{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 }}</ref> and the Rieske iron-sulfur proteins of the two complexes are homologous.<ref name="Carrell-1997">{{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 }}</ref> However, cytochrome f and [[cytochrome C1|cytochrome c<sub>1</sub>]] are not homologous.<ref name="Martinez-1994">{{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 }}</ref>

Cytochrome b<sub>6</sub>f contains seven [[prosthetic groups]].<ref name="Baniulis-">{{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 }}</ref><ref name="Cramer-2004">{{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 }}</ref> Four are found in both cytochrome b<sub>6</sub>f and bc<sub>1</sub>: the c-type heme of cytochrome c<sub>1</sub> and f, the two b-type hemes (b<sub>p</sub> and b<sub>n</sub>) in bc<sub>1</sub> and b<sub>6</sub>f, and the [2Fe-2S] cluster of the Rieske protein. Three unique prosthetic groups are found in cytochrome b<sub>6</sub>f: [[chlorophyll a]], [[β-carotene]], and heme c<sub>n</sub> (also known as heme x).<ref name="Stroebel-2003"/>

The inter-monomer space within the core of the cytochrome b6f complex dimer is occupied by lipids,<ref name="Hasan-2014a"/> which provides directionality to heme-heme electron transfer through modulation of the intra-protein dielectric environment.<ref name="Hasan-2014b">{{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 }}</ref>
{|class=wikitable
|-
|{{Pfam box
|Name=Cytochrome b6-f complex subunit 6 (PetL)
|symbol=Cyt_b6/f_cplx_su6
| Pfam = PF05115
| Pfam_clan =
| InterPro =  IPR007802
}}
|}

== Biological function ==
[[File:Tobacco (Nicotiana tabacum) cyt6bf mutant.jpg|thumb|left|Tobacco (''[[Nicotiana tabacum]]'') cytochrome b<sub>6</sub>f mutant (right) next to normal plant. Plants are used in photosynthesis research to investigate the cyclic photophosphorylation.]]
In [[photosynthesis]], the cytochrome b<sub>6</sub>f 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 [[Thylakoid lumen|lumen]].<ref name="Hasan-2013a"/> [[Electron transport]] via cytochrome b<sub>6</sub>f is responsible for creating the [[proton gradient]] that drives the synthesis of [[Adenosine triphosphate|ATP]] in chloroplasts.<ref name=Voet/>

In a separate reaction, the cytochrome b<sub>6</sub>f complex plays a central role in [[cyclic photophosphorylation]], when [[NADP+|NADP<sup>+</sup>]] is not available to accept electrons from reduced [[ferredoxin]].<ref name=Berg>{{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 }}</ref> This cycle, driven by the energy of [[P700]]<sup>+</sup>, 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,<ref name="Munekage-2004">{{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 }}</ref> helping to maintain the proper ratio of ATP/NADPH production for [[carbon fixation]].<ref>{{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 }}</ref><ref>{{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 }}</ref>

The p-side quinol deprotonation-oxidation reactions within the cytochrome b6f complex have been implicated in the generation of reactive oxygen species.<ref name="Baniulis-2014">{{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 }}</ref> 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.<ref name="Hasan-2014c">{{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 }}</ref>

==Reaction mechanism==
The cytochrome ''b''<sub>6</sub>''f'' complex is responsible for "[[Light-dependent reaction#Noncyclic photophosphorylation|non-cyclic]]" '''(1)''' and "[[Light-dependent reaction#Cyclic photophosphorylation|cyclic]]" '''(2)''' electron transfer between two mobile redox carriers, [[plastoquinol]] (QH<sub>2</sub>) and [[plastocyanin]] (Pc):

{| style="margin:auto; width:45em; text-align:center; white-space:nowrap;"
|-
| H<sub>2</sub>O
| →
| style="background:lightgreen" | photosystem II
| →
| QH<sub>2</sub>
| →
| style="background:pink" | Cyt ''b''<sub>6</sub>''f''
| →
| style="background:lightblue" | Pc
| →
| style="background:yellow" | photosystem I
| →
| style="text-align:left" | NADPH
| style="text-align:right" | '''(1)'''
|-
| colspan=4 |
| QH<sub>2</sub>
| →
| style="background:pink" | Cyt ''b''<sub>6</sub>''f''
| →
| style="background:lightblue" | Pc
| →
| style="background:yellow" | photosystem I
| →
| style="text-align:left" | Q
| style="text-align:right" | '''(2)'''
|}

Cytochrome b<sub>6</sub>f catalyzes the transfer of electrons from plastoquinol to plastocyanin, while pumping two protons from the stroma into the thylakoid lumen:
<br />
:QH<sub>2</sub> + 2Pc(Cu<sup>2+</sup>) + 2H<sup>+</sup> (stroma) → Q + 2Pc(Cu<sup>+</sup>) + 4H<sup>+</sup> (lumen)<ref name=Berg/>

This reaction occurs through the [[Q cycle]] as in Complex III.<ref name="Cramer-1996">{{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 }}</ref> Plastoquinol 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]].<ref name="Cramer-2006">{{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 }}</ref> 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.<ref>{{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 }}</ref>

===Q cycle===
[[File:Q-cycle cytochrome b6f.png|thumb|right|450px|Q cycle of cytochrome b<sub>6</sub>f]]
'''First half of Q cycle'''
# QH<sub>2</sub> 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}}.
# 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<sub>p</sub> of cytochrome b6.
# Heme b<sub>p</sub> then transfers the electron to heme b<sub>n</sub>.
# Heme b<sub>n</sub> reduces Q with one electron to form SQ.
'''Second half of Q cycle'''
# A second QH<sub>2</sub> 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<sub>n</sub> is transferred to SQ, and the completely reduced Q<sup>2−</sup> takes up two protons from the stroma to form QH<sub>2</sub>.
# The oxidized Q and the reduced QH<sub>2</sub> that has been regenerated diffuse into the membrane.

===Cyclic electron transfer===
Unlike Complex III, cytochrome b<sub>6</sub>f catalyzes another electron transfer reaction that is central to [[cyclic photophosphorylation]]. The electron from [[ferredoxin]] (Fd) is transferred to plastoquinone and then the cytochrome b<sub>6</sub>f complex to reduce plastocyanin, which is reoxidized by P700 in Photosystem I.<ref name="Joliot-2002">{{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 }}</ref> 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.<ref name="Joliot-2002"/> 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:<ref name="Cramer-2006"/><ref name="Cramer-2005">{{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 }}</ref>
# Fd (red) + heme x (ox) → Fd (ox) + heme x (red)
# heme x (red) + Fd (red) + Q + 2H<sup>+</sup> → heme x (ox) + Fd (ox) + QH<sub>2</sub>

== References ==
{{Reflist|33em}}

== Further reading ==
*{{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}}

== External links ==
* [http://bio.purdue.edu/people/faculty/cramer/cramerlab/cytbf.html Structure-Function Studies of the Cytochrome ''b''<sub>6</sub>''f'' Complex] - Current research on cytochrome ''b''<sub>6</sub>''f'' 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 b<sub>6</sub>f complex}}

[[Category:Hemoproteins]]
[[Category:Iron–sulfur proteins]]
[[Category:Light reactions]]
[[Category:Integral membrane proteins]]
[[Category:EC 1.10.99]]