Cytochrome c6

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Cytochrome c 6 ( Chlamydomonas reinhardtii )
Cytochrome c6 (Chlamydomonas reinhardtii)
Ribbon model (heme as rods), according to PDB  1cyj (C. reinhardtii)

Existing structural data : 1A2S , 1C6O , 1C6R , 1C6S , 1CED , 1CTJ , 1CYI , 1CYJ , 1F1F , 1GDV , 1KIB , 1LS9 , 2DGE , 2V08 , 3DMI , 3DR0

Mass / length primary structure 85-90 amino acids, approx. 10 kDa
Cofactor Hamm
Identifier
External IDs

Cytochrome  c 6 (also cytochrome  c 552 , cytochrome  c 553 ) is a monomeric, soluble heme protein that is involved in photosynthetic electron transport in many cyanobacteria and some algae . It has here the function of the soluble electron carrier between cytochrome bc 1 complex or cytochrome b 6 f complex and photosystem I . Some cyanobacteria only produce cytochrome  c 6 ; higher plants have lost this ability and instead synthesize the copper-containing plastocyanin . A variant of cytochrome c 6 found in higher plants such as thale cress ( Arabidopsis thaliana ) probably only has a signaling function. Some cyanobacteria and eukaryotic algae both synthesize proteins. Like plastocyanin, the protein can only accept and release one electron. It is a class I cytochrome  c that is encoded by the petJ gene and weighs about 10  kDa .

Function in the photosynthetic electron transport chain

The light reaction of photosynthesis is a sequence of electron transitions that takes place in plants in the thylakoids of the green chloroplasts. The protein complex Photosystem II uses light energy to extract electrons from water. The electrons are first transferred to the small molecule plastoquinone and then via the cytochrome b 6 f complex in order to get from there with the help of plastocyanin or cytochrome  c 6 into photosystem I in order to ultimately reduce NADP + .

Although both cytochrome  c 6 and plastocyanin can accept electrons from the cytochrome b 6 f complex and donate them to photosystem I, they have no similarity in sequence or structure. However, they have a number of similar physical properties: both proteins have similar molecular masses (approx. 10 kDa), diameter approx. 3 nm), redox potentials (approx. +370 mV) and isoelectric points (approx. 4.5). Cytochrome  c 6 does not depend on the PsaF subunit for efficient binding to photosystem I and thus binds to the complex in a different way than plastocyanin.

properties

Cytochrome  c 6 is related to the soluble cytochrome c of the mitochondria and cytochrome  c 2 of the purple bacteria . The heme group is covalently bound via two cysteines by means of thioether bonds . In eukaryotes, the cytochrome c 6 precursor (pre-apocyte  c 6 ) is synthesized in the cytosol , which after translocation into the thylakoid lumen is converted into its mature form by peptidases. A cytochrome c - lyase equips the apoprotein of the heme group.

Expression in cases of copper deficiency in Chlamydomonas reinhardtii

The green alga Chlamydomonas reinhardtii , along with some cyanobacteria and other eukaryotic algae, is one of a number of organisms that can produce both cytochrome  c 6 and plastocyanin. In C. reinhardtii the protein is expressed at copper concentrations below 50 nM and functionally replaces plastocyanin as a luminal electron carrier. Plastocyanin expression is suppressed under these conditions. When sufficient copper is available again, plastocyanin expression is reactivated and cytochrome  c 6 is no longer produced.

Significance as a signaling molecule in higher plants

Furthermore, a modified form of cytochrome c 6 was found in higher plants  , which has an additional, strongly conserved twelve amino acid long motif. This variant is expressed in small amounts in photosynthetically active tissues in Arabidopsis thaliana and may have a signaling function. Arabidopsis cytochrome c 6 cannot functionally replace plastocyanin because it has a redox potential that is too low to oxidize cytochrome f . A donation of electrons to photosystem I would still be possible. In kinetic analyzes with Arabidopsis PS I, binding of Arabidopsis cytochrome  c 6 was measured 100 times less than that of plastocyanin ; In contrast, cytochrome  c 6 from algae shows an efficiency similar to that of Arabidopsis plastocyanin.

Individual evidence

  1. ↑ a b c C. A. Kerfeld, HP Anwar, R. Interrante, S. Merchant, TO Yeates: The structure of chloroplast cytochrome c6 at 1.9 A resolution: evidence for functional oligomerization . In: Journal of Molecular Biology . tape 250 , no. 5 , July 28, 1995, pp. 627-647 , doi : 10.1006 / jmbi.1995.0404 , PMID 7623381 .
  2. ↑ AB Hope: Electron transfers amongst cytochrome f, plastocyanin and photosystem I: kinetics and mechanisms . In: Biochimica et Biophysica Acta . tape 1456 , no. 1 , January 3, 2000, p. 5-26 , PMID 10611452 .
  3. ↑ Manuel Hervás, José A. Navarro, Fernando P. Molina-Heredia, Miguel A. De la Rosa: The reaction mechanism of Photosystem I reduction by plastocyanin and cytochrome c 6 follows two different kinetic models in the cyanobacterium Pseudanabaena sp. PCC 6903 . In: Photosynthesis Research . tape 57 , no. 1 , July 1998, ISSN  0166-8595 , p. 93-100 , doi : 10.1023 / a: 1006036105296 .
  4. ↑ G. Howe, S. Merchant: Maturation of thylakoid lumen proteins proceeds post-translationally through an intermediate in vivo . In: Proceedings of the National Academy of Sciences . tape 90 , no. 5 , March 1, 1993, p. 1862-1866 , PMID 8446600 , PMC 45980 (free full text).
  5. ↑ Z. Xie, S. Merchant: A novel pathway for cytochromes c biogenesis in chloroplasts . In: Biochimica et Biophysica Acta . tape 1365 , no. 1-2 , June 10, 1998, pp. 309-318 , PMID 9693743 .
  6. ↑ a b P. M. Wood: Interchangeable copper and iron proteins in algal photosynthesis. Studies on plastocyanin and cytochrome c-552 in Chlamydomonas . In: European Journal of Biochemistry . tape 87 , no. 1 , June 1, 1978, ISSN  0014-2956 , pp. 9-19 , PMID 208838 .
  7. ↑ S. Merchant, L. Bogorad: Metal ion regulated gene expression: use of a plastocyanin-less mutant of Chlamydomonas reinhardtii to study the Cu (II) -dependent expression of cytochrome c-552 . In: The EMBO Journal . tape 6 , no. 9 , September 1987, pp. 2531-2535 , PMID 2824187 , PMC 553670 (free full text).
  8. ↑ G. Sandmann, P. Böger: Copper-mediated Lipid Peroxidation Processes in Photosynthetic Membranes . In: Plant Physiology . tape 66 , no. 5 , November 1980, ISSN  0032-0889 , pp. 797-800 , PMID 16661528 , PMC 440728 (free full text).
  9. ↑ Christopher J. Howe, Beatrix G. Schlarb-Ridley, Juergen Wastl, Saul Purton, Derek S. Bendall: The novel cytochrome c6 of chloroplasts: a case of evolutionary bricolage? In: Journal of Experimental Botany . tape 57 , no. 1 , 2006, ISSN  0022-0957 , p. 13-22 , doi : 10.1093 / jxb / erj023 , PMID 16317035 .
  10. ↑ a b Fernando P. Molina-Heredia, Jrgen Wastl, José A. Navarro, Derek S. Bendall, Manuel Hervás: Photosynthesis: a new function for an old cytochrome? In: Nature . tape 424 , no. 6944 , July 3, 2003, p. 33-34 , doi : 10.1038 / 424033b , PMID 12840749 .