FORWARD ELECTRON-TRANSFER FROM PHYLLOQUINONE-A(1) TO IRON-SULFUR CENTERS IN SPINACH PHOTOSYSTEM-I

Forward electron transfer at room temperature from the secondary accep tor A1 (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 t he kinetics of A1- reoxidation during forward electron transfer [half times of 15 ns [Mathis, P., & Setif, P. (1988) FEBS Lett. 237, 65-68] and 200 ns [Brettel, K. (1988) FEBS Lett. 239, 93-98] were reported fo r PSI particles from spinach and Synechococcus sp., respectively]. The monophasic kinetics with t1/2 almost-equal-to 200 ns could be reprodu ced with PSI particles from another cyanobacterium (Synechocystis sp. PCC 6803). In so-called PSI-beta particles from spinach, containing al l membrane-bound electron carriers and approximately 65 antenna chloro phylls per reaction center, the flash-induced absorbance increase arou nd 370 nm, which is indicative of the formation of A1-, decays biphasi cally with t1/2 almost-equal-to 25 and 150 ns and relative amplitudes of approximately 65 and 35%, respectively. The difference spectra of t hese two phases were determined between 330 and 500 nm; they agree wel l below 380 nm but deviate significantly at higher wavelengths. The sp ectrum 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 ab sorbance changes remaining after its completion decay with t1/2 almost -equal-to 250 mus. It is concluded that the 25-ns phase reflects elect ron transfer from A1- to F(X) in approximately 65% of the centers, whe reas the remaining 35% of A1- is reoxidized with t1/2 almost-equal-to 150 ns under moderate redox conditions. The deviations between the spe ctra of the two phases can be explained with the assumption that elect ron transfer from F(X-) to (F(A), F(B)) also proceeds with t1/2 almost -equal-to 150 ns and contributes significantly to the total spectrum o f the 150-ns phase, implying that the F(X-)/F(X) difference spectrum d eviates from the (F(A),F(B))-/(F(A),F(B)) Spectrum. Possible kinetic s chemes for forward electron transfer in PSI-beta particles are discuss ed; assuming that the 25-ns phase reflects the establishment of a redo x equilibrium between reduced A1 and F(X), the redox potentials of A1 and F(X) are found to be very close. Different types of PSI particles from spinach, which were subjected to less harsh preparation procedure s, also exhibit a biphasic reoxidation of A1- but smaller relative amp litudes of the 25-ns phase, down to only 30% for a sample prepared wit hout detergent. It is suggested that PSI in native spinach membranes c ould behave similarly to the cyanobacterial PSI particles.