Cytochrome b6f complex: Difference between revisions

The cytochrome b₆f complex is a dimer, with each [[monomer]] composed of eight subunits.<ref name="Whitelegge-2002">{{Citecite journal | last1vauthors = Whitelegge | first1 = JP. | last2 =, Zhang | first2 = H. | last3 =, Aguilera | first3 = R. | last4 =, Taylor | first4 = RM. | last5 =, Cramer | first5 = 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 = MolMolecular & CellCellular Proteomics | volume = 1 | issue = 10 | pages = 816–27 | date = Oct 2002 | doipmid = 12438564 | doi = 10.1074/mcp.m200045-mcp200 | PMIDdoi-access = 12438564free }}</ref> These consist of four large subunits: a 32 kDa [[cytochrome f]] with a c-type cytochrome, a 25 kDa [[cytochrome b6|cytochrome b₆]] 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">{{Citecite book | last1 = Voet | first1 = Donald J. | titlelast2 = BiochemistryVoet /| Donaldfirst2 J. Voet ;= Judith G. Voet| name-list-style = vanc | title = Biochemistry | year = 2011 | publisher = Wiley, J | location = New York, NY | isbn = 978-0-470-57095-1 | pages = }}</ref> The total molecular weight is 217 kDa.

The crystal structurestructures of cytochrome b₆f complexes from ''Chlamydomonas reinhardtii'', ''Mastigocladus laminosus'', and ''Nostoc'' sp. PCC 7120 have been determined.<ref name="Hasan-2013a"/><ref name="Stroebel-2003">{{Citecite journal | last1vauthors = Stroebel | first1 = D. | last2 =, Choquet | first2 = Y. | last3 =, Popot | first3 = JL. | last4 =, Picot | first4 = 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 | PMIDs2cid = 14647374130033 }}</ref><ref name="Yamashita-2007">{{Citecite journal | last1vauthors = Yamashita | first1 = E. | last2 =, Zhang | first2 = H. | last3 =, Cramer | first3 = WA. | title = Structure of the cytochrome b6f complex: quinone analogue inhibitors as ligands of heme cn. | journal = JJournal Molof BiolMolecular Biology | volume = 370 | issue = 1 | pages = 39–52 | date = Jun 2007 | pmid = 17498743 | pmc = 1993820 | doi = 10.1016/j.jmb.2007.04.011 | PMID = 17498743 | pmc=1993820}}</ref><ref name="Baniulis-2009">{{Citecite journal | last1vauthors = Baniulis | first1 = D. | last2 =, Yamashita | first2 = E. | last3 =, Whitelegge | first3 = JP. | last4 =, Zatsman | first4 = AI. | last5 =, Hendrich | first5 = MP. | last6 =, Hasan | first6 = SS. | last7 =, Ryan | first7 = CM. | last8 =, Cramer | first8 = WA. | title = Structure-Function, Stability, and Chemical Modification of the Cyanobacterial Cytochrome b6f Complex from Nostoc sp. PCC 7120. | journal = JThe BiolJournal Chemof Biological Chemistry | volume = 284 | issue = 15 | pages = 9861–9 | date = Apr 2009 | pmid = 19189962 | pmc = 2665108 | doi = 10.1074/jbc.M809196200 | PMIDdoi-access = 19189962free | pmc=2665108}}</ref><ref name="Hasan-2013b">{{Citecite journal | last1vauthors = Hasan | first1 = SS. | last2 =, Stofleth | first2 = JT. | last3 =, Yamashita | first3 = E. | last4 =, Cramer | first4 = 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 | doipmid = 10.1021/bi301638h23514009 | PMIDpmc = 235140094034689 | pmcdoi =4034689 10.1021/bi301638h }}</ref><ref name="Hasan-2014a">{{Citecite journal | last1vauthors = Hasan | first1 = SS. | last2 =, Cramer | first2 = WA. | title = Internal lipid architecture of the hetero-oligomeric cytochrome b6f complex. | journal = Structure | volume = 22 | issue = 7 | pages = 1008–15 | date =Jun Jul 2014 | pmid = 24931468 | pmc = 4105968 | doi = 10.1016/j.str.2014.05.004 | PMID = 24931468 | pmc=4105968}}</ref>

The core of the complex is structurally similar to the cytochrome bc₁ core. Cytochrome b₆ and subunit IV are homologous to [[cytochrome b]],<ref name="Widger-1984">{{Citecite journal | last1vauthors = Widger | first1 = WR. | last2 =, Cramer | first2 = WA. | last3 =, Herrmann | first3 = RG. | last4 =, Trebst | first4 = 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 = ProcProceedings Natlof Acadthe SciNational UAcademy Sof ASciences of the United States of America | volume = 81 | issue = 3 | pages = 674–8 | date = Feb 1984 | doipmid = 6322162 | pmc = 344897 | doi = 10.1073/pnas.81.3.674 | pmiddoi-access = 6322162free | pmc=344897}}</ref> and the Rieske iron-sulfur proteins of the two complexes are homologous.<ref name="Carrell-1997">{{Citecite journal | last1vauthors = Carrell | first1 = CJ. | last2 =, Zhang | first2 = H. | last3 =, Cramer | first3 = WA. | last4 =, Smith | first4 = 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 | doipmid = 9438861 | doi = 10.1016/s0969-2126(97)00309-2 | PMIDdoi-access = 9438861free }}</ref> However, cytochrome f and [[cytochrome C1|cytochrome c₁]] are not homologous.<ref name="Martinez-1994">{{Citecite journal | last1vauthors = Martinez | first1 = SE. | last2 =, Huang | first2 = D. | last3 =, Szczepaniak | first3 = A. | last4 =, Cramer | first4 = WA. | last5 =, Smith | first5 = 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 | doipmid = 8081747 | doi = 10.1016/s0969-2126(00)00012-5 | PMIDdoi-access = 8081747free }}</ref>

Cytochrome b₆f contains seven [[prosthetic groups]].<ref name="Baniulis-">{{Citecite journal | last1vauthors = Baniulis | first1 = D. | last2 =, Yamashita | first2 = E. | last3 =, Zhang | first3 = H. | last4 =, Hasan | first4 = SS. | last5 =, Cramer | first5 = WA. | title = Structure-function of the cytochrome b6f complex. | journal = PhotochemPhotochemistry Photobioland Photobiology | volume = 84 | issue = 6 | pages = 1349–58 | monthyear = 2008 | yearpmid = 19067956 2008| doi = 10.1111/j.1751-1097.2008.00444.x | PMIDs2cid = 1906795644992397 }}</ref><ref name="Cramer-2004">{{Citecite journal | last1vauthors = Cramer | first1 = WA. | last2 =, Zhang | first2 = H. | last3 =, Yan | first3 = J. | last4 =, Kurisu | first4 = G. | last5 =, Smith | first5 = 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 | PMID = 15147175 }}</ref> 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).<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">{{Citecite journal | last1vauthors = Hasan | first1 = SS. | last2 =, Zakharov | first2 = SD. | last3 =, Chauvet | first3 = A., | last4 = Stadnytski | first4 =Stadnytskyi V. | last5 =, Savikhin | first5 = S. | last6 =, Cramer | first6 = WA. | title = A map of dielectric heterogeneity in a membrane protein: the hetero-oligomeric cytochrome b6f complex. | journal = JThe PhysJournal Chemof Physical Chemistry B | volume = 118 | issue = 24 | pages = 6614–25 | date = Jun 2014 | doipmid = 10.1021/jp501165k24867491 | PMIDpmc = 248674914067154 | pmcdoi =4067154 10.1021/jp501165k }}</ref>

In [[photosynthesis]], the cytochrome b₆f complex functions to mediate the transfer of electrons and of energy between the two photosynthetic reaction center complexes, from [[Photosystem II]] toand [[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 b₆f 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 b₆f complex plays a central role in [[cyclic photophosphorylation]], when [[NADP+|NADP⁺]] is not available to accept electrons from reduced [[ferredoxin]].<ref name=Berg/>{{cite book This| cyclelast1 results= inBerg the| creationfirst1 of= aJeremy protonM. gradient| bylast2 cytochrome= b₆f,Tymoczko which| canfirst2 be= usedJohn toL. drive| ATPlast3 synthesis.= Stryer It| hasfirst3 also= beenLubert shown| thatlast4 this= cycleStryer is| essentialfirst4 for= photosynthesis,<refLubert | name="Munekage-2004">{{Citelist-style journal= vanc | last1title = MunekageBiochemistry | first1url = Yhttps://archive.org/details/biochemistry0006berg | last2url-access = Hashimotoregistration | first2year = M.2007 | last3publisher = MiyakeW.H. Freeman | first3location = C.New York | last4isbn = Tomizawa978-0-7167-8724-2 |}}</ref> first4This =cycle, K.driven |by last5the =energy Endoof |[[P700]]⁺, first5contributes =to Tthe creation of a proton gradient that can be used to drive ATP synthesis. |It last6has been shown that this cycle is essential for photosynthesis,<ref name="Munekage-2004">{{cite Tasakajournal | first6vauthors = Munekage Y, Hashimoto M., |Miyake last7C, =Tomizawa ShikanaiK, |Endo first7T, =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 | PMIDbibcode = 151757562004Natur.429..579M | s2cid = 4421776 }}</ref> in which it is proposedhelping to help maintain the proper ratio of ATP/NADPH production for [[carbon fixation]].<ref>{{Citecite 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 | pages = }}</ref><ref>{{cite journal | title = Cyclic photophosphorylation and electron transport | journal = Biochimica et Biophysica Acta (BBA) - Bioenergetics | first = Derek | last = Bendall| idyear = 1995 | accessdatename-list-style = 2011-05-17vanc | 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">{{Citecite journal | last1vauthors = Baniulis* | first1 = D. | last2 =, Hasan* | first2 = SS. | last3 =, Stofleth | first3 = JT. | last4 = Yamashita | first4 = E. | last5 =, Cramer | first5 = WA. | title = Mechanism of enhanced superoxide production in the cytochrome b(6)f complex of oxygenic photosynthesis (*equal first authorship). | journal = Biochemistry | volume = 52 | issue = 50 | pages = 8975–83 | date = Dec 2013 | doipmid = 10.1021/bi401353424298890 | PMIDpmc = 242988904037229 | pmcdoi =4037229 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">{{Citecite journal | last1vauthors = Hasan | first1 = SS. | last2 =, Proctor | first2 = EA. | last3 = Dokholyan | first3 = NV. | last4 =, Yamashita | first4 = E. | last5 =, Dokholyan | first5 = NV. | last6 =, Cramer | first6 = WA. | title = Traffic within the cytochrome b6f lipoprotein complex: gating of the quinone portal. | journal = Biophysical JJournal | volume = 107 | issue = 7 | pages = 1620–8 | date = Oct 2014 | pmid = 25296314 | pmc = 4190601 | doi = 10.1016/j.bpj.2014.08.003 | PMIDbibcode = 25296314 |2014BpJ...107.1620H pmc=4190601}}</ref>

The cytochrome ''b''₆''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|plastoquinone]] (QH₂) and [[plastocyanin]] (Pc):

# 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}}.

# In the low-potential ETC, the electron from heme b_n is transferred to SQ, and the completely reduced Q^2-2− takes up two protons from the stroma to form QH₂.

In contrast toUnlike 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.<ref name="Joliot-2002">{{cite journal | vauthors = Joliot P, Joliot A | title = cyclicCyclic electron transfer in plant leaf | journal = PNASProceedings of the National Academy of Sciences of the United States of America | idvolume = 99 | accessdateissue = 2011-05-1715 | 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 forof howthe plastoquinonereduction isof reducedplastoquinone 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">{{Citecite journal | last1vauthors = Cramer | first1 = WA. | last2 =, Yan | first2 = J. | last3 =, Zhang | first3 = H. | last4 =, Kurisu | first4 = G. | last5 =, Smith | first5 = 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 = PhotosynthPhotosynthesis ResResearch | volume = 85 | issue = 1 | pages = 133–43 | monthyear = 2005 | yearpmid = 200515977064 | doi = 10.1007/s11120-004-2149-5 | PMIDbibcode = 159770642005PhoRe..85..133C | s2cid = 20731696 }}</ref>

== References ==

{{Reflist|233em}}

[[Category:Iron-sulfurIron–sulfur proteins]]