Cytochrome b6f complex

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Cytochrome b 6 f complex from Chlamydomanas reinhardtii ( 1q90 ). The borders of the lipid double membrane are drawn in red (luminal side) and blue (stromal side).

The cytochrome b 6 f complex (also plastoquinol plastocyanin oxidoreductase) is a protein complex that is embedded in the thylakoid membrane of plants, algae and cyanobacteria . In addition to photosystem II and photosystem I , it is one of the central protein complexes of oxygenic photosynthesis . Its task is to transfer electrons from membrane-bound plastoquinone to soluble plastocyanin or cytochrome  c 6 and to “pump” protons from the stroma into the lumen. Its function is thus similar to that of the cytochrome bc1 complex .

structure

The functional complex is a homo-dimer of about 220  kDa , the two monomers of which are made up of nine subunits each. In plants, the three subunits PetC, PetM, ferredoxin-NADP + reductase / petH are encoded on the nuclear genome and the six subunits PetA, PetB, PetD, PetN, PetG and PetL are encoded on the plastid genome. The monomer contains seven cofactors: a chlorophyll - a molecule, a β-carotene , an iron-sulfur cluster , the heme of cytochrome  f , and the three heme groups of cytochrome  b 6 (heme  b n , b p and x ).

PetC, also called Rieske protein after its discoverer John S. Rieske , is one of the nuclear-coded subunits and contains a 2-iron-2-sulfur cluster . It is important for the stability of the dimer because its transmembrane helix interacts with the helix of the other monomer. The role of the core-coded subunit PetM is not yet known. The third nuclear - encoded subunit is ferredoxin-NADP + reductase (FNR, encoded by the petH gene ), which is located on the stromal side of the complex.

The three plastoma-coded subunits PetA (cytochrome  f ), PetB (cytochrome  b 6 ), PetD (subunit IV) are involved in linear electron transport together with the Rieske protein and are essential for the assembly, stability and function of the complex. Cytochrome  f contains a cytochrome c -type, so the heme is covalently bound via thioether bonds of two cysteine ​​residues. The b 6 subunit of the complex contains two cytochromes of the b type which, according to their relative orientation to the electronegative lumen or electropositive stroma, cytochrome  b n and cytochrome  b p (also corresponding to their redox potential cytochrome  b L from English. Low and b H from high ). The two hemes are non-covalently bound by two histidines. The binding is likely to occur spontaneously. The function of the third heme ("heme x " or "heme c n ") is unknown.

The three small plastoma-coded subunits PetN, PetG and PetL are located on the periphery of the complex and are believed to be involved in an interaction with the other photosynthetic complexes. PetN and PetG appear to be essential as their elimination in Chlamydomonas and tobacco leads to the loss of the complex. In higher plants there is only one non-essential subunit, namely PetL. In Chlamydomonas, on the other hand, switching off PetL leads to a destabilization of the complex, which greatly reduces the complex content. The cyanobacterium Synechocystis meanwhile has a functional cytochrome b 6 complex without PetL.

function

The light reaction of photosynthesis is a sequence of electron transitions that takes place in plants in the thylakoids of the green chloroplasts. During the linear electron transport, Photosystem II uses light energy to extract electrons from water. The electrons are first transferred to the membrane-bound molecule plastoquinone (PQH 2 ), and then via the cytochrome b 6 f complex to plastocyanin (PC) and finally via photosystem I to NADP + . The complex plays a central role in the electron transport chain of oxygenic photosynthesis, since the oxidation of plastoquinol is the slowest and therefore rate-limiting step in linear electron transport.

PQH 2 reduced by photosystem II diffuses in the thylakoid membrane to the Q O center of the cytochrome b 6 f complex. This mediates two successive electron transitions. The first electron is withdrawn from PQH 2 by the Rieske protein, a 2-iron-2-sulfur protein . This creates a semiquinone radical PQH • - . The cytochromes b n and b p of the b subunit transfer the second electron from the PQH • - to an oxidized plastoquinone (PQ), which is then protonated by H + from the stroma and thus contributes significantly to the development of the thylakoid proton gradients ( Q cycle ). The PQH 2 oxidation is relatively slow with a duration of about 5 ms and is thus the rate-limiting step in the electron transport chain. This is probably due to the necessary change in conformation of the Rieske protein and the restricted diffusion of the PQH 2 to the Q O center of the complex, which is located in a deeply sunk pocket.

The electron is transferred from the Rieske protein via the cytochrome f subunit directly to the luminal, soluble protein plastocyanin. In some algae and cyanobacteria, the transfer sometimes happens to the alternative, soluble electron carrier cytochrome  c 6 .

In sum, PQH 2 is reoxidized to PQ, one electron is reused in the Q cycle and one electron is finally transferred to plastocyanin , which can take up one electron at a time. During this transfer, one proton per electron is translocated from the stroma of the chloroplasts into the thylakoid lumen.

Individual evidence

  1. ^ Whitelegge, JP, et al. (2002): 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. In: Mol Cell Proteomics 1 (10): pp. 816-827; PMID 12438564
  2. ↑ Rieske, JS., Maclennan, DH. and Coleman, R. (1964): Isolation and properties of an iron-protein from the (reduced coenzyme Q) -cytochrome C reductase complex of the respiratory chain . In: Biochemical and Biophysical Research Communications 15 (4); 338-344; doi: 10.1016 / 0006-291X (64) 90171-8
  3. ^ Hauska, G., Schütz, M., Büttner, M. (1996): The Cytochrome b6f Complex - Composition, Structure and Function. In: Oxygenic Photosynthesis: The Light Reaction, ed. ODRaYCF, Kluwer Academic Publisher.
  4. ↑ Maiwald, D., et al. (2003): Knock-out of the genes coding for the Rieske protein and the ATP synthase delta subunit of Arabidopsis. Effects on photosynthesis, thylakoid protein composition, and nuclear chloroplast gene expression. In: Plant Physiol 133 (1); Pp. 191-202; PMID 12970486
  5. ↑ a b c d Baniulis et al (2008): Structure-function of the cytochrome b6f complex . In: Photochem Photobiol 84 (6) pp. 1349-1358, PMID 19067956
  6. ↑ Hager, M., et al. (1999): Targeted inactivation of the smallest plastid genome-encoded open reading frame reveals a novel and essential subunit of the cytochrome b (6) f complex. In: Embo J , 18 (21); Pp. 5834-5842; PMID 10545095
  7. ↑ Berthold, DA, CL Schmidt, and R. Malkin (1995): The deletion of petG in Chlamydomonas reinhardtii disrupts the cytochrome bf complex. In: J Biol Chem , 270 (49); Pp. 29293-29298; PMID 7493961
  8. ↑ Schwenkert, S., et al. (2007): Role of the low-molecular-weight subunits PetL, PetG, and PetN in assembly, stability, and dimerization of the cytochrome b6f complex in tobacco. In: Plant Physiol 144 (4); Pp. 1924-1935; PMID 17556510
  9. ↑ Fiebig, A., S. Stegemann, and R. Bock (2004): Rapid evolution of RNA editing sites in a small non-essential plastid gene. In: Nucleic Acids Research 32 (12); Pp. 3615-3622; PMID 15240834
  10. ↑ Takahashi, Y., et al. (1996): The chloroplast ycf7 (petL) open reading frame of Chlamydomonas reinhardtii encodes a small functionally important subunit of the cytochrome b6f complex. In: Embo J 15 (14); Pp. 3498-3506; PMID 8670852
  11. ↑ Kaneko, T., et al. (1996): Sequence analysis of the genome of the unicellular cyanobacterium Synechocystis sp. strain PCC6803. II. Sequence determination of the entire genome and assignment of potential protein coding regions. In: DNA Res 3 (3); Pp. 109-136; PMID 8905231
  12. ^ Mitchell (1975): Proton motive redox mechanism of Cytochrome-b-c1 complex in respiratory chain - proton motive ubiquinone cycle . In: FEBS Lett 56 (1); 1-6; PMID 239860
  13. ^ Haehnel (1984): Photosynthetic Electron Transport in Higher Plants . In: Annu Rev Plant Biol 35 ; 659-693; doi: 10.1146 / annurev.pp.35.060184.003303
  14. ↑ Hope (2000): Electron transfers amongst cytochrome f, plastocyanin and photosystem I: kinetics and mechanisms . In: Biochim Biophys Acta 1456 (1) pp. 5-26; PMID 10611452