A redox study of the electron transport pathway responsible for generation of the slow electrochromic phase in chloroplasts

M. E. Girvin, W. A. Cramer

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The amplitude of the slow phase of the electrochromic bandshift and the dark redox state of cytochrome b6, as well as its flash-induced turnover, have been measured as a function of ambient redox potential between +200 and -200 mV. Formation of a quinol-like donor with an Em,7 = +100 ± 10 mV is required for generation of the slow phase. 80-100% of the amplitude of this signal with a t 1 2 = 3-4 ms is observed at -200 mV where cytochrome b6 was almost fully reduced (Em,7 of dark and flash-induced photoreduction was -30 mV and -75 mV, respectively). The change in the photoreduction of cytochrome b6 above 0 mV had an Em,7 of +50 mV, about 50 mV more negative than the midpoint at this pH for the onset of the slow electrochromic change. At potentials below -140mV the amplitude of b6 photoreduction becomes small or negligible. The nature of the cytochrome b6 photoresponse is changed at potentials below -140 mV from a net photoreduction with a t 1 2 = ≲ 1 ms to a photooxidation with a t 1 2 = 15-20 ms that is substantially slower than the electrochromic band-shift with a t 1 2 = 3-4 ms. It is concluded that the slow electrochromic phase probably does not arise from a mechanism involving a turnover of cytochrome b6. From consideration of the possible flash-induced electron-transfer steps and alternative mechanisms for generation of the slow phase, it is suggested that it may arise from a redox-linked H+ pump involving the high potential iron-sulfur protein.

Original languageEnglish (US)
Pages (from-to)29-38
Number of pages10
JournalBBA - Bioenergetics
Issue number1
Publication statusPublished - Oct 26 1984



  • (Spinach chloroplast)
  • Cytochrome b
  • Electrochromic bandshift
  • Electron transport
  • Photosystem I
  • Redox potential

ASJC Scopus subject areas

  • Biophysics
  • Biochemistry
  • Cell Biology

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