Period-four Modulation of Photosystem II Primary Quinone Acceptor (QA) Reduction/Oxidation Kinetics in Thylakoid Membranes

Chloroplast thylakoid membranes depleted of bicarbonate exhibit a slowed oxidation of the primary quinone acceptor (Qᴀ) by the secondary quinone acceptor (Qв) of photosystem II. The kinetics of these slowed reactions have been followed by using short xenon flashes of light both to excite photosystem II and to probe the redox state of Qᴀ. Thylakoids incubated with formate but not depleted of bicarbonate showed the same pattern of slowed reaction kinetics of the quinone acceptors as seen in bicarbonate-depleted| thylakoids. This led us to conclude that there was a simple competition between bicarbonate and formate at this site; however, steady-state electron transfer measured with an oxygen electrode showed that although the bicarbonate- depleted thylakoids were indeed inhibited, rates in the formate-incubated thylakoids were only slightly slowed. We suggest that the inhibition seen at the quinone acceptor site of photosystem II depends in a subtle way upon the rate of exchange of bicarbonate and formate at this site.

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Light-Induced Oxidation-Reduction Reactions of Photosystem II in Dichlorophenyl-dimethyl Urea (DCM U) Inhibited Thylakoids

Zeitschrift für Naturforschung C ◽

10.1515/znc-1990-3-420 ◽

1990 ◽

Vol 45 (3-4) ◽

pp. 258-264

Author(s):

Jeff A. Nemson ◽

Anastasios Melis

Keyword(s):

Difference Spectrum ◽

Thylakoid Membranes ◽

Donor Side ◽

Difference Spectra ◽

Ps Ii ◽

Oxidation Reduction ◽

Primary Quinone Acceptor ◽

Dimethyl Urea ◽

Absorbance Difference ◽

Quinone Acceptor

Abstract Illumination of thylakoid membranes in the presence of 3-(3′,4′-dichlorophenyl)-1,1-dimethyl urea (DCMU) causes the reduction of the primary quinone acceptor QA of photosystem II (PS II) and the storage of a positive charge on the donor side of the photochemical reaction center. These oxidation-reduction reactions are accompanied by characteristic changes of absorbance in the ultra-violet region of the spectrum. The PS II-related absorbance difference spectra (250 -350 nm) were compared in control and hydroxylamine-treated thylakoid membranes, and in thylakoids suspended in the presence of carbonyl cyanide-p-(trifluoromethoxy)- phenylhydrazone (FCCP). The light minus dark difference spectra were dominated by the Q-A minus QA difference spectrum. Qualitatively, the three spectra were identical in the 300 - 350 nm region, however, they showed distinct differences in the 250 - 300 nm region. The latter arose because of different contributions from the donor side of PS II in the thylakoid membrane of the three samples. The result suggested that FCCP acts as the ultimate electron donor in DCMU - poisoned chloroplasts. Therefore, the absorbance difference spectrum in the presence of FCCP reflected a contribution from the Q-A minus QA component only. Deconvolution of the absorbance difference spectra of control and hydroxylamine-treated thylakoids yielded difference spectra attributed to the oxidation of a component on the donor side of PS II. This component did not conform with the known Mn(III) → Mn(IV) transition. Rather, it indicated the oxidation of a modified form of Mn in the presence of DCMU , probably a Mn(II) → Mn(III) transition. The results are discussed in terms of the use of DCMU - poisoned thylakoid membranes in the quantitation of the primary quinone acceptor QA by spectrophotometric approaches.

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