Cytochrome b6f complex

Cytochrome b6f complex ( also plastoquinol - plastocyanin oxidoreductase ) is a protein complex, which is embedded in the thylakoid membrane of plants, algae and cyanobacteria. In addition to the photosystem II and photosystem I, he is one of the key protein complexes of oxygenic photosynthesis. He has the task of transferring electrons from membrane-bound plastoquinone to plastocyanin and cytochrome c6 soluble and "pump" protons from the stroma into the lumen. Its function is thus similar to the cytochrome bc1 complex.

Structure

The functional complex is a homo- dimer of about 220 kDa, the two monomers are composed of nine subunits. In plants, the three subunits PETC, PETM, ferredoxin -NADP -Reduktase/petH are encoded on the nuclear genome and the six subunits peta, PETB, PETD, PETN, PETG and PETL on the plastid genome. The monomer contains seven cofactors: a chlorophyll-a molecule, a β carotene, an iron-sulfur cluster that heme cytochrome f, and the three cytochrome b6 heme ( heme BN, BP, and x).

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

The three plastomcodierten subunits PETA ( cytochrome f), PETB ( cytochrome b6), PETD ( subunit IV ) are involved together with the Rieske protein on linear electron transport and is essential for assembly, stability and function of the complex. Cytochrome f contains a cytochrome c- type heme is thus covalently linked via thioether bonds by two cysteine ​​residues. The b6 subunit of the complex contains two cytochromes of the b- type, according to their relative orientation to the electronegative lumen or electro-positive stromal cytochrome bn and cytochrome bp ( also according to their redox potential cytochrome bL of Engl. Low and bH of high) are called. The two hemes are non-covalently bound by two histidines. The binding probably occurs spontaneously. The function of the third heme ( " heme x " or " heme cn " ) is unknown.

The three small plastomcodierten subunits PETN, PETG and PETL are located at the periphery of the complex, and probably involved in an interaction with the other photosynthetic complexes. PETN and PETG seem to be essential, since its off leads in Chlamydomonas and tobacco to the loss of the complex. Thus, there exists only one non-essential subunit, namely PETL In higher plants. In Chlamydomonas, however, leads one off of PETL to a destabilization of the complex, whereby the complex content is greatly reduced. The cyanobacterium Synechocystis has meanwhile a functional cytochrome b6 complex without PETL.

Function

The light response of the photosynthesis is a sequence of electron transitions that take place in the thylakoids in plants of green chloroplasts. While the linear electron transport and photosystem II wins under the use of light energy electrons from water. The electrons are transferred to NADP in the first membrane-bound molecule plastoquinone ( PQH2 ), and then via the cytochrome b6f complex in plastocyanin (PC) and finally the photosystem I. The complex occupies a central role in the electron transport chain of oxygenic photosynthesis, because the oxidation of plastoquinol is the slowest and thus ratenlimitierende step of linear electron transport.

By photosystem II reduced PQH2 diffuses into the thylakoid membrane to the QO - center of the cytochrome b6f complex. This gives two successive electron transitions. The first electron is PQH2 withdrawn by Rieske protein, a two -iron - sulfur protein 2. This produces a semiquinone radical PQH • -. Cytochromes BN and BP of the b subunit transmit the second electron from the PQH • - to an oxidized plastoquinone (PQ), which is then protonated by H from the stroma, and thus significantly to the structure of the thylakoidalen proton contribution (Q- cycle). PQH2 the oxidation is provided with a duration of approximately 5 ms is relatively slow and so the ratenlimitierende step of the electron transport chain. This is due probably to the necessary conformational change of the Rieske protein and the limited diffusion of the PQH2 for QO - center of the complex, which is located in a deep recessed pocket.

The electron is transferred from the Rieske protein of the cytochrome f subunit directly on the luminal soluble protein plastocyanin. In some algae and cyanobacteria, the transmission is sometimes the alternative, soluble electron carrier cytochrome c6.

In Summa PQH2 is reoxidized to PQ, an electron is re-used in the Q- cycle and an electron is eventually transferred to plastocyanin, each of which may accept an electron. This disclosure also a proton from the stroma of the chloroplasts is translocated into the thylakoid lumen per electron.