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Structure and Function of the Cytochrome c 2:Reaction Center Complex from Rhodobacter sphaeroides

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The Purple Phototrophic Bacteria

Part of the book series: Advances in Photosynthesis and Respiration ((AIPH,volume 28))

Summary

In purple non-sulfur photosynthetic bacteria, a c-type heme protein, cytochrome (Cyt) c 2, serves as the electron donor to the reaction center (RC) which is the site of the initial photochemical electron transfer. The second order rate of electron transfer from Cyt c 2 to the RC is diffusion limited and optimized to facilitate electron transfer through the photosynthetic apparatus. This review summarizes the X-ray crystal structure of the Cyt c 2:RC complex from Rhodobacter (Rba.) sphaeroides and studies based on the structure that elucidate the molecular basis for the role of the complex in electron transfer. The structure of the complex shows the heme cofactor in van der Waals contact with the reaction center and in close proximity to the bacteriochlorophyll dimer, the primary electron donor. The binding interface region includes: a) a solvent-separated region with long-range electrostatic interactions between complementary charged residues on Cyt c 2 and the RC which play a role in protein docking and binding, and b) a small central region with short-range interactions, including hydrophobic, hydrogen bonding, and a cation-π interaction, which play a role in binding the Cyt in close contact to the RC surface to facilitate strong electronic coupling between cofactors for rapid inter-protein electron transfer. Both types of interactions contribute to the binding. However, the two types of interactions have markedly different effects on the electron transfer kinetics. The long-range electrostatic interactions change the second order rate constant by changing the association rate but not the electron transfer rate in the bound state. The short-range interactions do not affect the association rate but change the dissociation rate as well as the electron transfer rate in the bound state. The strength of the binding interactions is optimized to allow sufficiently fast dissociation of the oxidized Cyt c 2 that does not limit the rate of cyclic electron transfer.

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Abbreviations

BChl:

bacteriochlorophyll

BChl2 :

bacteriochlorophyll dimer

Blc. :

Blastochloris

Cyt c 2 :

cytochrome c 2

Rba. :

Rhodobacter

RC:

reaction center

Rps. :

Rhodopseudomonas

Rsp. :

Rhodospirillum

Tch. :

Thermochromatium

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Authors and Affiliations

  1. Stanford Synchrotron Radiation Laboratory, 2575 Sand Hill Rd., Menlo Park, CA, 94025, USA

    Herbert Axelrod

  2. Department of Biochemistry and Molecular Biophysics, University of Arizona, 1041 East Lowell Street, Tucson, AZ, 85721, USA

    Osamu Miyashita

  3. Department of Physics, University of California, San Diego, 9500 Gilman Dr., La Jolla, CA, 92093-0354, USA

    Melvin Okamura

Authors
  1. Herbert Axelrod
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  2. Osamu Miyashita
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  3. Melvin Okamura
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Corresponding author

Correspondence to Melvin Okamura .

Editor information

Editors and Affiliations

  1. University of Sheffield, UK

    C. Neil Hunter

  2. University of Pennsylvania, USA

    Fevzi Daldal

  3. Argonne National Laboratory, USA

    Marion C. Thurnauer

  4. University of British Columbia, Canada

    J. Thomas Beatty

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Axelrod, H., Miyashita, O., Okamura, M. (2009). Structure and Function of the Cytochrome c 2:Reaction Center Complex from Rhodobacter sphaeroides . In: Hunter, C.N., Daldal, F., Thurnauer, M.C., Beatty, J.T. (eds) The Purple Phototrophic Bacteria. Advances in Photosynthesis and Respiration, vol 28. Springer, Dordrecht. https://doi.org/10.1007/978-1-4020-8815-5_17

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