Cytochrome b6/f complex as an indigenous photodynamic generator of singlet oxygen in thylakoid membranes

Photochem Photobiol. 2000 Jan;71(1):103-9. doi: 10.1562/0031-8655(2000)071<0103:CBFCAA>2.0.CO;2.

Abstract

Possible association of photodynamic sensitization by cytochrome b6/f complex (cyt b6/f) via singlet oxygen (1O2) mechanism with photoinhibition damage to photosystem II (PS II) was studied using such subthylakoid preparations as photosystem I (PS I) particles, PS II core complex and cyt b6/f from spinach leaves. Upon exposure to bright light, PS II core complex lost photosynthetic electron transport activity to a certain extent, whose-spectral dependence implied that pheophytin a is likely involved in photoinactivation of PS II core complex in itself. The presence of PS I particles exerted virtually no effect on PS II core photoinactivation. However, the inclusion of cyt b6/f in samples resulted in a marked exacerbation of the photoinactivation, particularly in UV-A and blue light. Such effect of cyt b6/f was suppressed by azide and enhanced by the medium deuteration. Photogeneration of 1O2 from cyt b6/f was confirmed by ESR and spectrophotometry, chemically trapping 1O2. Action spectra for both 1O2 photoproduction and PS II core photoinactivation by cyt b6/f bore a close resemblance to each other, seemingly carrying the absorption characteristics of the Rieske Fe-S protein. A complex deficient in the Rieske protein prepared from intact cyt b6/f showed virtually no generation of 1O2 in light, whereas an efficient photoformation of 1O2 was seen in the Rieske protein preparation. The results suggest that cyt b6/f, rather specifically the Rieske center, may play a prominent role in photoinhibition processes through type II photosensitization in thylakoids.

Publication types

  • Research Support, Non-U.S. Gov't

MeSH terms

  • Cytochrome b Group / metabolism*
  • Cytochrome b6f Complex
  • Oxygen*
  • Photosynthetic Reaction Center Complex Proteins / metabolism
  • Photosystem I Protein Complex
  • Photosystem II Protein Complex
  • Singlet Oxygen
  • Spinacia oleracea
  • Thylakoids / metabolism*

Substances

  • Cytochrome b Group
  • Photosynthetic Reaction Center Complex Proteins
  • Photosystem I Protein Complex
  • Photosystem II Protein Complex
  • Singlet Oxygen
  • Cytochrome b6f Complex
  • Oxygen