Consequences of the structure of the cytochrome b6f complex for its charge transfer pathways

Biochim Biophys Acta. May-Jun 2006;1757(5-6):339-45. doi: 10.1016/j.bbabio.2006.04.020. Epub 2006 May 4.

Abstract

At least two features of the crystal structures of the cytochrome b6f complex from the thermophilic cyanobacterium, Mastigocladus laminosus and a green alga, Chlamydomonas reinhardtii, have implications for the pathways and mechanism of charge (electron/proton) transfer in the complex: (i) The narrow 11 x 12 A portal between the p-side of the quinone exchange cavity and p-side plastoquinone/quinol binding niche, through which all Q/QH2 must pass, is smaller in the b6f than in the bc1 complex because of its partial occlusion by the phytyl chain of the one bound chlorophyll a molecule in the b6f complex. Thus, the pathway for trans-membrane passage of the lipophilic quinone is even more labyrinthine in the b6f than in the bc1 complex. (ii) A unique covalently bound heme, heme cn, in close proximity to the n-side b heme, is present in the b6f complex. The b6f structure implies that a Q cycle mechanism must be modified to include heme cn as an intermediate between heme bn and plastoquinone bound at a different site than in the bc1 complex. In addition, it is likely that the heme bn-cn couple participates in photosytem I-linked cyclic electron transport that requires ferredoxin and the ferredoxin: NADP+ reductase. This pathway through the n-side of the b6f complex could overlap with the n-side of the Q cycle pathway. Thus, either regulation is required at the level of the redox state of the hemes that would allow them to be shared by the two pathways, and/or the two different pathways are segregated in the membrane.

Publication types

  • Research Support, N.I.H., Extramural
  • Research Support, Non-U.S. Gov't
  • Review

MeSH terms

  • Amino Acid Sequence
  • Animals
  • Benzoquinones / metabolism
  • Binding Sites
  • Chlamydomonas reinhardtii / enzymology
  • Chloroplasts / enzymology
  • Cyanobacteria / enzymology
  • Cytochrome b6f Complex / chemistry*
  • Cytochrome b6f Complex / physiology
  • Electron Transport
  • Ferredoxin-NADP Reductase / metabolism
  • Ferredoxins / metabolism
  • Heme / metabolism
  • Models, Molecular*
  • Molecular Sequence Data
  • Protein Subunits / chemistry
  • Protein Subunits / physiology
  • Sequence Homology, Amino Acid

Substances

  • Benzoquinones
  • Ferredoxins
  • Protein Subunits
  • quinone
  • Heme
  • Cytochrome b6f Complex
  • Ferredoxin-NADP Reductase