Forward electron transfer from phylloquinone A1 to iron-sulfur centers in spinach photosystem I

Sétif, P.; Brettel, K.

Biochemistry 32(31): 7846-7854


ISSN/ISBN: 0006-2960
PMID: 8347589
DOI: 10.1021/bi00082a002
Accession: 008701159

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Forward electron transfer at room temperature from the secondary acceptor A-1 (phylloquinone) to the iron-sulfur centers F-X, F-B, and F-A was studied by flash-absorbance spectroscopy in different photosystem I (PSI) preparations in order to resolve the controversy concerning the kinetics of A-1- reoxidation during forward electron transfer (half times of 15 ns (Mathis, P., and Setif, P. (1988) FEBS Lett. 237, 65-68) and 200 ns (Brettel, K. (1988) FEBS Lett. 239, 93-98) were reported for PSI particles from spinach and Synechococcus sp., respectively). The monophasic kinetics with t-l/2 apprxeq 200 ns could be reproduced with PSI particles from another cyanobacterium (Synechocystis sp. PCC 6803). In so-called PSI-beta particles from spinach, containing all membrane-bound electron carriers and approximately 65 antenna chlorophylls per reaction center, the flash-induced absorbance increase around 370 nm, which is indicative of the formation of A-1-, decays biphasically with t-1/2 apprxeq 25 and 150 ns and relative amplitudes of approximately 65 and 35%, respectively. The difference spectra of these two phases were determined between 330 and 500 nm; they agree well below 380 nm but deviate significantly at higher wavelengths. The spectrum of the sum of the two phases is similar to the spectrum of the 200-ns phase in cyanobacteria. Upon chemical reduction of the terminal acceptors F-A and F-B, only the 25-ns phase is conserved and the absorbance changes remaining after its completion decay with t-l/2 apprxeq 250 mu-S. It is concluded that the 25-ns phase reflects electron transfer from A-1- to F-X in approximately 65% of the centers, whereas the remaining 35% of A-1- is reoxidized with t-1/2 apprxeq 150 ns under moderate redox conditions. The deviations between the spectra of the two phases can be explained with the assumption that electron transfer from F-X- to (F-A,F-B) also proceeds with t-1/2 apprxeq 150 ns and contributes significantly to the total spectrum of the 150-ns phase, implying that the F-X-/F-X difference spectrum deviates from the (F-A,F-B)-/(F-A,F-B) spectrum. Possible kinetic schemes for forward electron transfer in PSI-beta particles are discussed; assuming that the 25-ns phase reflects the establishment of a redox equilibrium between reduced A-1 and F-X, the redox potentials of A-1 and F-X are found to be very close. Different types of PSI particles from spinach, which were subjected to less harsh preparation procedures, also exhibit a biphasic reoxidation of A-1- but smaller relative amplitudes of the 25-ns phase, down to only 30% for a sample prepared without detergent. It is suggested that PSI in native spinach membranes could behave similarly to the cyanobacterial PSI particles.