Photoinhibition affects the non-heme iron center in photosystem II.
Effects on the PS II acceptor side caused by exposure to strong white light (180 W/m2) of PS II membrane fragments (spinach) at pH 6.5 and 0 degrees C were analyzed by measuring low temperature EPR signals and flash-induced transient changes of the fluorescence quantum yield. The following results were obtained: (a) the extent of the light induced g = 1.9 EPR signal as a measure of photochemical Fe2+QA- formation declines with progressing photoinhibition. The half-life of this effect is independent of the absence or presence of an exogenous electron acceptor during the photoinhibitory treatment; (b) in samples photoinhibited in the absence of an electron acceptor and subsequently incubated with K3[Fe(CN)6] in the dark, the extent of the g = 8 EPR signal (reflecting the oxidized Fe3+ form of the endogenous non-heme iron center) and of the flash-induced change of the fluorescence yield (as a measure of fast electron transfer from QA- to Fe3+ after the first flash; [see (1992) Photosynth. Res. 31, 113-126] exhibits the same dependence on photoinhibition time as the g = 1.9 EPR signal; (c) in samples photoinhibited in the presence of an exogenous electron acceptor, the signals reflecting Fe(3+)-formation and fast electron transfer from QA- to Fe3+ decline faster than the g = 1.9 EPR signal. These results provide for the first time direct evidence that the endogenous non-heme iron center located between QA and QB is susceptible to modifications by light stress. The implications of this finding will be discussed.
Gleiter
H M
HM
Max-Volmer-Institute for Biophysical Chemistry, Technical University Berlin, Germany.
Nugent
J H
JH
Haag
E
E
Renger
G
G
D002412:Cations; D003080:Cold Temperature; D004578:Electron Spin Resonance Spectroscopy; D006420:Hemeproteins / Q000737:chemistry; D007501:Iron / Q000737:chemistry*; D008027:Light; D010777:Photochemistry; D045322:Photosynthetic Reaction Center Complex Proteins / Q000737:chemistry / Q000528:radiation effects*; D045332:Photosystem II Protein Complex; D010944:Plants
D002412:Cations; D006420:Hemeproteins; D045322:Photosynthetic Reaction Center Complex Proteins; D045332:Photosystem II Protein Complex; D007501:Iron