Is phylloquinone an obligate electron carrier in photosystem I?

Abstract

Comparative quantitative analysis of phylloquinone content and photochemically competent P-700 has been performed on photosystem I particles subjected to photolysis with ultraviolet irradiation. Nonirradiated control particles exhibit a phylloquinone/P-700 stoichiometry of 1.9 + 0.2. Photolysis of the photosystem I particles induces a progressive depletion of phylloquinone, however, photochemistry as assayed at room temperature by the photooxidation of P-700 is unaffected. These data are not consistent with the assignment of phylloquinone as a functional intermediate at room temperature between P-700 and the iron-sulfur clusters, center A and center B.Electron carrier A1; P-700; Photosystem I; Phylloquinone; Signal 1; (Spinach subchloroplast particle)

Electron transfer reactions in photosystem I following vitamin K1 depletion by ultraviolet irradiation

“…The light source, Blak Ray model B 100A (Ultraviolet Products, San Gabriel, CA) provided wavelengths of 300-400 nm at an intensity of 50 W. m -2. These conditions were similar to those used by Palace et al [9].…”

“…The decay halftime for the optical transient in the control preparation was -30 ms and can be assumed to be due to the recombination between P700 + and the reduced terminal acceptors FA/FB [1]. After vitamin K1 depletion (lower trace), the optical transient still showed a slow recombination in confirmation of [9] but the decay halftime was longer. Fig.2 (upper panel) shows that, during the irradiation treatment, the decay halftime of the transient increased to -60 ms, and upon more detailed analysis, we found that this was due to the introduction of a slower decay component with a halftime of approx.…”

“…Additional controversy regarding the participation of vitamin K1 in PSI electron transfer came from studies of preparations of PSI that were depleted of vitamin K1 by UV irradiation to destroy the phylloquinone in situ. Palace et al [9] reported that the kinetics of the room temperature charge recombination between P700 ÷ and the reduced terminal FeS centers was not affected following vitamin K1 destruction suggesting the non-participation of vitamin K~ in the electron transfer sequence. Zeigler et al [10] also showed that depletion of vitamin K1 by irradiation treatment of PS I did not alter the EPR spectrum of A1 and concluded that the signal ascribed to the acceptor A1 does not originate from vitamin K~.…”

Vitamin K1 (phylloquinone) restores the turnover of FeS centers in the ether‐extracted spinach PS I particles

“…As we have shown [7] the extraction of VK~ did not totally stop the light-induced reduction of FeS centers at 10 K. The flash yield of FeS reduction was found to suppress only 50°7o by the extraction of VK~ at 15 K [171. Another argument comes from the fact that the attempted destruction of VK~ by UV light did not inhibit PSI photochemistry [18,19].…”

New herbicide‐binding site in the photosynthetic electron‐transport chain

“…Some arguments against the function of phylloquinone, however, still remain ~ [16,17] and its binding site has not been well characterized.…”