Electron paramagnetic resonance studies of photosynthetic electron transport: Photoreduction of ferredoxin and membrane-bound iron-sulfur centers

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Abstract
EPR spectrometry was used to investigate, at physiological temperatures, light-induced electron transport from membrane-bound Fe-S components (bound ferredoxin) to soluble ferredoxin and NADP+ in membrane fragments (from the blue-green alga, Nostoc muscorum) that had high rates of electron transport from water to NADP+ and from an artificial electron donor, reduced dichlorophenolindophenol (DCIPH2) to NADP+. Illumination at 20.degree. C resulted in the photoreduction of membrane-bound FE-S centers A and B. Photoreduction by water gave EPR signals of both centers A and B; photoreduction by DGIPH2 was found to generate a strong EPR signal of only center B. When water was the reductant, the addition and photoreduction of soluble ferredoxin generated additional signals characteristic of soluble ferredoxin without causing a decrease in the amplitude of the signals due to centers A and B. The further addition of NADP+ (and its photoreduction) greatly diminished signals due to the bound Fe-S centers and to soluble ferredoxin. An outflow of electrons from center B to soluble ferredoxin and NADP+ was particularly pronounced when DCIPH2 was the reductant. These observations provide the first evidence for a light-induced electron transport between membrane-bound Fe-S centers and ferredoxin-NADP+. The relationship of these observations to current concepts of photosynthetic electron transport is discussed.

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