ETC Components

Components of the electron transport system in order are plastoquinone, cytochrome b₆f complex, plastocyanin, and ferredoxin. Each of the components of the ETC has the ability to transfer an electron from a donor to an acceptor, though plastoquinone also transfers a proton. Each of these components undergoes successive rounds of oxidation and reduction, receiving an electron from the PSII and donating the electron to PSI.

Plastoquinone refers to a family of lipid-soluble benzoquinone derivatives with an isoprenoid side chain. In chloroplasts, the common form of plastoquinone contains nine repeating isoprenoid units. Plastoquinone possesses varied redox states, which together with its ability to bind protons and its small size enables it to act as a mobile electron carrier shuttling hydrogen atoms from PSII to the cytochrome b₆f complex.

Plastoquinone is present in the thylakoid membrane as a pool of 6–8 molecules per PSII. Plastoquinone exists as quinone A (Q_A) and quinone B (Q_B); Q_A 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. Q_B is a loosely bound molecule, which accepts two electrons and then takes on two protons before it detaches and becomes Q_BH₂, the mobile reduced form of plastoquinone (plastoquinol). Q_BH₂ is mobile within the thylakoid membrane, allowing a single PSII reaction center to interact with a number of cytochrome b₆f complexes.

Plastoquinone plays an additional role in the cytochrome b₆f complex, operating in a complicated reaction sequence known as a Q-cycle. When Q_B is reduced in PSII, it not only receives two electrons from Q_A but it also picks up two protons from the stroma matrix and becomes Q_BH₂. It is able to carry both electrons and protons (e^- and H⁺ carrier). At the cytochrome b₆f complex level it is then oxidized, but FeS and cytochrome b6 can accept only electrons and not protons. So the two protons are released into the lumen. The Q-cycle of the cytochrome b₆f complex is great because it provides extra protons into the lumen. Here two electrons travel through the two hemes of cytochrome b6 and then reduce Q_B on the stroma side of the membrane. The reduced Q_B takes on two protons from the stroma, becoming Q_BH₂, which migrates to the lumen side of the cytochrome b₆f complex where it is again oxidized, releasing two more protons into the lumen. Thus he Q-cycle allows the formation of more ATP. This Q-cycle links the oxidation of plastoquinol (Q_BH₂) at one site on the cytochrome b₆f complex to the reduction of plastoquinone at a second site on the complex in a process that contributes additional free energy to the electrochemical proton potential.

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.