Photoelectric Characterization of Forward Electron Transfer to Iron-Sulfur Centers in Photosystem I

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Abstract
The photoelectric response of oriented PS I membranes from the cyanobacterium Synechocystis 6803 has been investigated in the nanosecond time range. Besides an unresolved rapidly rising phase, there is a further positive electrogenic phase with a rise time constant of 220 +/- 20 ns. The amplitude of the 220-ns phase is 66 +/- 10% that of the subnanosecond phase. The fast phase contains two kinetic components faster than 100 ps, which have recently been resolved and attributed to primary charge separation (P+Ao-formation) and subsequent electron transfer to A1, respectively (Hecks, B., Wulf, K., Breton, J., Leibl, W., & Trissl, H.-W. (1994) Biochemistry 33, 8619-8624). The 220-ns phase is lost under conditions where iron-sulfur centers FA, FB, and Fx are prereduced, and its kinetics match the reoxidation kinetics of A1- as verified by absorbance change measurements at 380 nm. Therefore, this electrogenic phase is attributed to electron transfer to the iron-sulfur centers that function as further electron acceptors in the PS I reaction center. Gradual removal of FA and FB by urea treatment reveals that the amplitude of the 220-ns phase is linearly correlated with the fraction of FA,B present. However, complete removal of FA,B does not lead to a complete loss of the nanosecond phase but reduces its amplitude by more than a factor of 2 to yield an amplitude of 25-30% relative to the initial picosecond rise, with only a slight change in kinetics. The residual amplitude is further reduced when a large fraction of Fx is removed.(ABSTRACT TRUNCATED AT 250 WORDS)

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