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The earth's primary energy conversion of sunlight into biomass is oxygeneic photosynthesis, driven by large protein-cofactor complexes in the plasma membrane of photosynthetic bacteria and in thylakoid membranes within chloroplasts of plants. These complexes, photosystem II and photosystem I, capture light energy and act sequentially to raise the energy of electrons. These electrons are utilized in electron transport chains to generate a proton gradient across the membrane as well as NADPH. The electromotive force of the proton gradient is used by ATP Synthase to synthesize ATP. Together, this ATP and the NADPH provide energy to drive the light-independent Calvin Cycle, which fixes carbon from CO2 in organic compounds. See Figure 1 for a schematic of this process.
At left are shown the protein subunits of the integral membrane cytochrome b6f complex of the cyanobacterium, Mastigocladus laminosus (Kurisu, et al., 2003), oriented with the stroma at top and lumen at bottom. In the light dependent reactions of oxygenic photosynthesis, this complex functions as an electronic connection between photosystem II (PSII) and photosystem I (PSI), and in the process serves to generate a proton gradient across the plasma (bacteria) or thylakoid membrane (plants) that will drive ATP Synthase-mediated ATP production. Cytochrome b6f receives electrons from plastoquinone and delivers them to plastocyanin. See Figure 1 for a schematic of this process.
The cytochrome b6f complex exists as a dimer, with each monomer possessing four small hydrophobic subunits and four large subunits:
iron sulfur protein [ISP].
II. Overall Structure and the Electron/Proton Pathways
The dimer has 26 transmembrane helices. Dimer stability is facilitated by domain swapping of the ISP, as its lumenal (bottom) domain is included in the monomer opposite from its stromal-side loops and transmembrane helix, which spans the membrane obliquely.
The proteins of each monomer harbor the cofactors centrally involved in the electron transfer from plastoquinone to plastocyanin, four hemes and one 2Fe-2S cluster. The 2Fe-2S cluster is embedded in the ISP. The hemes are: Heme x, Heme bn, and Heme bp, all bound by cytochrome b6; Heme f is sequestered by cytochrome f.
Focusing on the electron transfer cofactors of one monomer, and representing reduced plastoquinone (PQH2) and plastocyanin (PC) schematically, the paths of electrons and protons through the complex may be visualized in two parts of a cycle, each part consisting of a high-potential electron transfer center (HETC - magenta arrows) and a low-potential ETC (LETC - yellow arrows). The contribution of 2 electrons by PQH2, each of which is differentially transferred through the complex, is called electron bifurcation. The oxidation of 2 PQH2s sequentially to semiquinones (SQs) and then oxidized plastoquinones (Qs), with the regeneration of PQH2 at the stromal surface, is termed the "Q cycle."
First part of the cycle:
- PQH2 binds to the complex. It is oxidized to a semiquinone (SQ) by the iron-sulfur center (HETC) and releases two protons (H+s)to the thylakoid lumen
- The reduced 2Fe-2S center transfers its electron through Heme f to PC, which can carry electrons to photosystem I (PSI).
- In the LETC, SQ becomes oxidized to Q as it transfers its electron to Heme bp of cytochrome b6.
- Heme bp then transfers the electron to Heme bn.
bn reduces Heme
x which then reduces a stromal plastoquinone (Q) with one electron to form SQ.
part of the cycle:
- A second PQH2 binds to the complex, which now has a semi-reduced, stromal quinone (SQ).
- As before, one electron reduces another oxidized PC through the HETC, releasing two H+s to the thylakoid lumen in the process.
- As in the first part of the cycle, transfer of an electron through the LETC occurs. In this part of the cycle, the stromal SQ is completely reduced and takes up two H+s from the stroma to form PQH2 to complete a Q cycle.
- The oxidized Q and the reduced PQH2 that has been regenerated diffuse into the membrane.
- Note that
the net results of these two stages of cytochrome b6f
activity are: 1) the "pumping" of 4 H+s from the stroma
to the lumen; 2) The generation of PQH2 via
the LETC with
concommitant uptake of 2 H+s from the stroma; 3) The oxidation of 2 PQH2s that donate an electron to each of
2 plastocyanins (PCs)
via the HETC.
The distances, in Angstroms, between the electron transferring cofactors is indicated.
Kurisu, G., Zhang, H., Smith, J.L., and Cramer, W.. (2003). Strucuture of the Cytochrome b6f Complex of Oxygenic Photosynthesis: Tuning the Cavity. Science 302: 1009-1014.