Abstract
We have found that short chain plastoquinones effectively stimulated photoreduction of the low potential form of cytochrome b(559) and were also active in dark oxidation of this cytochrome under anaerobic conditions in Triton X-100-solubilized photosystem II (PSII) particles. It is also shown that molecular oxygen competes considerably with the prenylquinones in cytochrome b(559) oxidation under aerobic conditions, indicating that both molecular oxygen and plastoquinones could be electron acceptors from cytochrome b(559) in PSII preparations. alpha-Tocopherol quinone was not active in the stimulation of cytochrome photoreduction but efficiently oxidized it in the dark. Both the observed photoreduction and dark oxidation of the cytochrome were not sensitive to 3-(3,4-dichlorophenyl)-1, 1-dimethylurea. It was concluded that both quinone-binding sites responsible for the redox changes of cytochrome b(559) are different from either the Q(A) or Q(B) site in PSII and represent new quinone-binding sites in PSII.
Highlights
Under the impairment of the oxygen-evolving complex, when electron donation from the oxygen-evolving complex to P680 is inhibited, the electrons from the high potential (HP) form of cyt b559 are transferred to P680 via the redox active chlorophyll Z (Chlz) [1]
Absorption Spectra of Different cyt b559 Forms—To observe only redox changes of the low potential (LP) form of cyt b559 in photosystem II (PSII) particles and to avoid possible absorption changes due to the HP form, we have performed our measurements in the presence of Triton X-100, which is known to convert the HP form into the LP form of cyt b559 [19]
The difference absorption spectra in the ␣-band region of cyt b559 absorption show (Fig. 1) that our PSII preparation solubilized in 0.2% Triton X-100 does not contain any detectable cyt b559 HP redox form, whereas the ascorbate reduces the cytochrome to practically the same level as that produced by MD, which is a stronger reductant than ascorbate
Summary
Under the impairment of the oxygen-evolving complex, when electron donation from the oxygen-evolving complex to P680 is inhibited (donor side inhibition), the electrons from the HP form of cyt b559 are transferred to P680 via the redox active chlorophyll Z (Chlz) [1]. It was suggested [1] that under such conditions the HP form is converted by an unknown molecular switch to the LP cyt b559 form, which after its autoxidation cannot donate electrons to P680. We have included for these studies ␣-tocopherol quinone (␣-TQ), a natural thylakoid prenylquinone of unknown function in plants [12] that is an effective quencher of PSII fluorescence [13] and affects the cyclic electron flow around PSII [14]
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