Biology:Cytochrome b6f complex

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Short description: Enzyme
Cytochrome b6f complex
1q90 opm.png
Crystal structure of the cytochrome b6f complex from C. reinhardtii (1q90). Hydrocarbon boundaries of the lipid bilayer are shown by red and blue lines (thylakoid space side and stroma side, respectively).
Identifiers
SymbolB6F
TCDB3.D.3
OPM superfamily92
OPM protein4pv1
Membranome258
Cytochrome b6f complex
Identifiers
EC number7.1.1.6
CAS number79079-13-3
Alt. namesPlastoquinol/plastocyanin reductase
Databases
IntEnzIntEnz view
BRENDABRENDA entry
ExPASyNiceZyme view
KEGGKEGG entry
MetaCycmetabolic pathway
PRIAMprofile
PDB structuresRCSB PDB PDBe PDBsum

The cytochrome b6f complex (plastoquinol/plastocyanin reductase or plastoquinol/plastocyanin oxidoreductase; EC 7.1.1.6) is an enzyme found in the thylakoid membrane in chloroplasts of plants, cyanobacteria, and green algae, that catalyzes the transfer of electrons from plastoquinol to plastocyanin:

plastoquinol + 2 oxidized plastocyanin + 2 H+ [side 1] [math]\displaystyle{ \rightleftharpoons }[/math] plastoquinone + 2 reduced plastocyanin + 4 H+ [side 2].[1]

The reaction is analogous to the reaction catalyzed by cytochrome bc1 (Complex III) of the mitochondrial electron transport chain. During photosynthesis, the cytochrome b6f 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[2] that is later used to synthesize ATP from ADP.

Enzyme structure

The cytochrome b6f complex is a dimer, with each monomer composed of eight subunits.[3] 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.[3][4] 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.[2][5][6][7][8][9]

The core of the complex is structurally similar to the cytochrome bc1 core. Cytochrome b6 and subunit IV are homologous to cytochrome b,[10] and the Rieske iron-sulfur proteins of the two complexes are homologous.[11] However, cytochrome f and cytochrome c1 are not homologous.[12]

Cytochrome b6f contains seven prosthetic groups.[13][14] 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).[5]

The inter-monomer space within the core of the cytochrome b6f complex dimer is occupied by lipids,[9] which provides directionality to heme-heme electron transfer through modulation of the intra-protein dielectric environment.[15]

Cytochrome b6-f complex subunit 6 (PetL)
Identifiers
Symbol?
PfamPF05115
InterProIPR007802

Biological function

Tobacco (Nicotiana tabacum) 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.[2] Electron transport via cytochrome b6f is responsible for creating the proton gradient that drives the synthesis of ATP in chloroplasts.[4]

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.[16] 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,[17] helping to maintain the proper ratio of ATP/NADPH production for carbon fixation.[18][19]

The p-side quinol deprotonation-oxidation reactions within the cytochrome b6f complex have been implicated in the generation of reactive oxygen species.[20] 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.[21]

Reaction mechanism

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):

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)[16]

This reaction occurs through the Q cycle as in Complex III.[22] 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.[23] 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.[24]

Q cycle

Q cycle of cytochrome b6f

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].
  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 bp of cytochrome b6.
  4. Heme bp then transfers the electron to heme bn.
  5. Heme bn 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 bn 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.

Cyclic electron transfer

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.[25] 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.[25] 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:[23][26]

  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

References

  1. ↑ ExplorEnz: EC 7.1.1.6
  2. ↑ 2.0 2.1 2.2 "Quinone-dependent proton transfer pathways in the photosynthetic cytochrome b6f complex". Proceedings of the National Academy of Sciences of the United States of America 110 (11): 4297–302. Mar 2013. doi:10.1073/pnas.1222248110. PMID 23440205. 
  3. ↑ 3.0 3.1 "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". Molecular & Cellular Proteomics 1 (10): 816–27. Oct 2002. doi:10.1074/mcp.m200045-mcp200. PMID 12438564. 
  4. ↑ 4.0 4.1 Voet, Donald J.; Voet, Judith G. (2011). Biochemistry. New York, NY: Wiley, J. ISBN 978-0-470-57095-1. 
  5. ↑ 5.0 5.1 "An atypical haem in the cytochrome b(6)f complex". Nature 426 (6965): 413–8. Nov 2003. doi:10.1038/nature02155. PMID 14647374. 
  6. ↑ "Structure of the cytochrome b6f complex: quinone analogue inhibitors as ligands of heme cn". Journal of Molecular Biology 370 (1): 39–52. Jun 2007. doi:10.1016/j.jmb.2007.04.011. PMID 17498743. 
  7. ↑ "Structure-Function, Stability, and Chemical Modification of the Cyanobacterial Cytochrome b6f Complex from Nostoc sp. PCC 7120". The Journal of Biological Chemistry 284 (15): 9861–9. Apr 2009. doi:10.1074/jbc.M809196200. PMID 19189962. 
  8. ↑ "Lipid-induced conformational changes within the cytochrome b6f complex of oxygenic photosynthesis". Biochemistry 52 (15): 2649–54. Apr 2013. doi:10.1021/bi301638h. PMID 23514009. 
  9. ↑ 9.0 9.1 "Internal lipid architecture of the hetero-oligomeric cytochrome b6f complex". Structure 22 (7): 1008–15. Jul 2014. doi:10.1016/j.str.2014.05.004. PMID 24931468. 
  10. ↑ "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". Proceedings of the National Academy of Sciences of the United States of America 81 (3): 674–8. Feb 1984. doi:10.1073/pnas.81.3.674. PMID 6322162. 
  11. ↑ "Biological identity and diversity in photosynthesis and respiration: structure of the lumen-side domain of the chloroplast Rieske protein". Structure 5 (12): 1613–25. Dec 1997. doi:10.1016/s0969-2126(97)00309-2. PMID 9438861. 
  12. ↑ "Crystal structure of chloroplast cytochrome f reveals a novel cytochrome fold and unexpected heme ligation". Structure 2 (2): 95–105. Feb 1994. doi:10.1016/s0969-2126(00)00012-5. PMID 8081747. 
  13. ↑ "Structure-function of the cytochrome b6f complex". Photochemistry and Photobiology 84 (6): 1349–58. 2008. doi:10.1111/j.1751-1097.2008.00444.x. PMID 19067956. 
  14. ↑ "Evolution of photosynthesis: time-independent structure of the cytochrome b6f complex". Biochemistry 43 (20): 5921–9. May 2004. doi:10.1021/bi049444o. PMID 15147175. 
  15. ↑ "A map of dielectric heterogeneity in a membrane protein: the hetero-oligomeric cytochrome b6f complex". The Journal of Physical Chemistry B 118 (24): 6614–25. Jun 2014. doi:10.1021/jp501165k. PMID 24867491. 
  16. ↑ 16.0 16.1 Berg, Jeremy M.; Tymoczko, John L.; Stryer, Lubert; Stryer, Lubert (2007). Biochemistry. New York: W.H. Freeman. ISBN 978-0-7167-8724-2. https://archive.org/details/biochemistry0006berg. 
  17. ↑ "Cyclic electron flow around photosystem I is essential for photosynthesis". Nature 429 (6991): 579–82. Jun 2004. doi:10.1038/nature02598. PMID 15175756. Bibcode: 2004Natur.429..579M. 
  18. ↑ Blankenship, Robert E. (2002). Molecular mechanisms of photosynthesis. Oxford ; Malden, MA: Blackwell Science. ISBN 978-0-632-04321-7. 
  19. ↑ Bendall, Derek (1995). "Cyclic photophosphorylation and electron transport". Biochimica et Biophysica Acta (BBA) - Bioenergetics 1229: 23–38. doi:10.1016/0005-2728(94)00195-B. 
  20. ↑ "Mechanism of enhanced superoxide production in the cytochrome b(6)f complex of oxygenic photosynthesis". Biochemistry 52 (50): 8975–83. Dec 2013. doi:10.1021/bi4013534. PMID 24298890. 
  21. ↑ "Traffic within the cytochrome b6f lipoprotein complex: gating of the quinone portal". Biophysical Journal 107 (7): 1620–8. Oct 2014. doi:10.1016/j.bpj.2014.08.003. PMID 25296314. Bibcode: 2014BpJ...107.1620H. 
  22. ↑ "Some New Structural Aspects and Old Controversies Concerning the Cytochrome b6f Complex of Oxygenic Photosynthesis". Annual Review of Plant Physiology and Plant Molecular Biology 47: 477–508. Jun 1996. doi:10.1146/annurev.arplant.47.1.477. PMID 15012298. 
  23. ↑ 23.0 23.1 "Transmembrane traffic in the cytochrome b6f complex". Annual Review of Biochemistry 75: 769–90. 2006. doi:10.1146/annurev.biochem.75.103004.142756. PMID 16756511. 
  24. ↑ "Cryo-EM Structure of the Spinach Cytochrome B 6 F Complex at 3.6 Å Resolution". Nature 575 (7783): 535–539. November 2019. doi:10.1038/s41586-019-1746-6. PMID 31723268. http://eprints.whiterose.ac.uk/154030/1/Malone_et_al_Nature.pdf. 
  25. ↑ 25.0 25.1 "Cyclic electron transfer in plant leaf". Proceedings of the National Academy of Sciences of the United States of America 99 (15): 10209–14. Jul 2002. doi:10.1073/pnas.102306999. PMID 12119384. Bibcode: 2002PNAS...9910209J. 
  26. ↑ "Structure of the cytochrome b6f complex: new prosthetic groups, Q-space, and the 'hors d'oeuvres hypothesis' for assembly of the complex". Photosynthesis Research 85 (1): 133–43. 2005. doi:10.1007/s11120-004-2149-5. PMID 15977064. Bibcode: 2005PhoRe..85..133C. 

Further reading

External links



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