Plastoquinone is present in the thylakoid membrane as a pool of 6–8 molecules per PSII. Plastoquinone exists as quinone A (QA) and quinone B (QB); QA is tightly bound to the reaction center complex of PSII and it is immovable. It is the primary stable electron acceptor of PSII, and it accepts and transfers one electron at time. QB is a loosely bound molecule, which accepts two electrons and then takes on two protons before it detaches and becomes QBH2, the mobile reduced form of plastoquinone (plastoquinol). QBH2 is mobile within the thylakoid membrane, allowing a single PSII reaction center to interact with a number of cytochrome b6f complexes.
The cytochrome b6f complex is the intermediate protein complex in linear photosynthetic electron transport. The cytochrome b6f complex essentially couples PSII and PSI and also provides the means of proton gradient formation by using cytochrome groups as redox centers in the ETC thereby separating the electron/hydrogen equivalent into its electron and proton components. The electrons are transferred to PSI via plastocyanin and the protons are released into the thylakoid lumen of the chloroplast. The electron transport from PSII to PSI via cytochrome b6f complex occurs in about 7 ms, representing the rate limiting step of the photosynthetic process.
The cytochrome b6f exists as a dimer of 217 kDa. The monomeric complex contains four large subunits (18–32 kDa), including cytochrome f, cytochrome b6, the Rieske FeS iron-sulfur protein (ISP), and subunit IV, as well as four small hydrophobic subunits, PetG, PetL, PetM, and PetN. The monomeric unit contains 13 transmembrane helices: four in cytochrome b6 (helices A to D); three in subunit IV (helices E to G); and one each in cytochrome f, the ISP, and the four small hydrophobic subunits PetG, PetL, PetM, and PetN. The monomer includes four hemes, one [2Fe-2S] cluster, one chlorophyll a, one β-carotene, and one plastoquinone. The extrinsic domains of cytochrome f and the ISP are on the luminal side of the membrane and are ordered in the crystal structure. Loops and chain termini on the stromal side are less well ordered. The ISP contributes to dimer stability by domain swapping, its transmembrane helix obliquely spans the membrane in one monomer, and its extrinsic domain is part of the other monomer. The two monomers form a protein-free central cavity on each side of the transmembrane interface.
Cytochrome c6 is a small soluble electron carrier. It is a highly a-helical heme-containing protein. It is located on the luminal side of the thylakoid membrane where it catalyzes the electron transport from the membrane-bound cytochrome b6f complex to PSI. It is the sole electron carrier in some cyanobacteria.
Plastocyanin operates in the inner aqueous phase of the photosynthetic vesicle, transferring electrons from cytochrome f to PSI. It is a small protein (10 kDa) composed of a single polypeptide that is coded for in the nuclear genome. Plastocyanin is a β-sheet protein with copper as the central ion that is ligated to four residues of the polypeptide. The copper ion serves as a one-electron carrier with a midpoint redox potential (0.37 eV) near that of cytochrome f. Plastocyanin shuttles electrons from the cytochrome b6f complex to PSI by diffusion. Plastocyanin is more common in green algae and completely substitutes for cytochrome c6 in the chloroplasts of higher plants. In cyanobacteria and green algae where both cytochrome c6 and plastocyanin are encoded, the alternative expression of the homologous protein is regulated by the availability of copper.
Ferredoxin is a small protein (11 kDa), and has the distinction of being one of the strongest soluble reductants found in cells (midpoint redox potential = -0.42 eV). The amino acid sequence of ferredoxin and the three-dimensional structure are known in different species. Plants contain different forms of ferredoxin, all of which are encoded in the nuclear genome. In some algae and cyanobacteria, ferredoxin can be replaced by a flavoprotein. Ferredoxin operates in the stromal aqueous phase of the chloroplast, transferring electrons from PSI to a membrane associated flavoprotein, known as FNR. A 2Fe2S cluster, ligated by four cysteine residues, serves as one-electron carrier.
Once an electron reaches ferredoxin, however, the electron pathway branches, enabling redox free energy to enter other metabolic pathways in the chloroplast. For example, ferredoxin can transfer electrons to nitrite reductase, glutamate synthase, and thioredoxin reductase.
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