The Cytochrome b6f Complex of Oxygenic Photosynthesis

Paolo DaSilva and David Marcey
© 2017, David Marcey

I. Introduction
II.
Structure and the Electron/Proton Pathways
III. References


Directions

This exhibit displays molecules in the left part of the screen, and text that addresses structure-function relationships of the molecules in the right part (below). Use the scroll bar to the right to scroll through the text of this exhibit.

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I. Introduction

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:


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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."

The distances, in Angstroms, between the electron transferring cofactors is indicated.


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III. References

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.


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