Protein Control of the Redox Potential of the Primary Quinone Acceptor in Reaction Centers fromRhodobacter sphaeroides†

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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Stabilization of the electron in the quinone acceptor part of the Rhodobacter sphaeroides reaction centers

BIOPHYSICS ◽

10.1134/s0006350908040106 ◽

2008 ◽

Vol 53 (4) ◽

pp. 291-295

Author(s):

P. P. Knox ◽

P. M. Krasilnikov ◽

P. A. Mamonov ◽

N. Kh. Seifullina ◽

A. F. Uchoa ◽

...

Keyword(s):

Rhodobacter Sphaeroides ◽

Reaction Centers ◽

Quinone Acceptor

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Photoreduction of NADP+ in Photosystem II of Higher Plants: Requirement for Manganese

Zeitschrift für Naturforschung C ◽

10.1515/znc-1992-1-211 ◽

1992 ◽

Vol 47 (1-2) ◽

pp. 57-62 ◽

Cited By ~ 14

Author(s):

Suleyman I. Allakhverdiev ◽

Vyacheslav V. Klimov

Keyword(s):

Photosystem Ii ◽

Higher Plants ◽

Complete Removal ◽

Reaction Centers ◽

Higher Plant ◽

Ps Ii ◽

Alternate Pathway ◽

Manganese Extraction ◽

Quinone Acceptor ◽

Reduced State

Abstract The effects of reversible manganese extraction on NADP+ photoreduction were studied with higher plant subchloroplast preparations of photosystem II (PS II). Under anaerobic conditions, when the reaction centers (RCs) of PS II are “closed” (i.e. in the state [P680 Pheo] QA), and in the presence of ferredoxin-ferredoxin-NADP+ reductase, NADP+ reduction is observed at a rate of 0.8 -1.1 nmol/mg × chlorophyll × h. After complete removal of manganese from PS II, the rate of NADP+ reduction is reduced 40 - 50-fold. Upon the addition of Mn at a concentration of approx. 4 Mn atoms per reaction center, the NADP+ reduction is restored up to 85 -90% of the initial value. When half of this amount of Mn is combined with about 40 times of the equivalent concentration of other divalent ions (Ca2+, Sr2+, Mg2+ etc.) the reaction is also reactivated. Dinoseb (10-6 m) an inhibitor of electron transfer in PS II prevents NADP+ photoreduction. It is concluded that under conditions when the first quinone acceptor, QA, is in its reduced state (QA-) electrons are transferred from reduced pheophytin (Pheo·̅) to NADP+, indicating that PS II can reduce NADP+ without the participation of PS I. On the basis of these and literature data, an alternate pathway for electron phototransfer in PS II reaction centers of higher plants is suggested. Some problems concerning the Z-scheme are discussed.

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The Effects of Bicarbonate Depletion and Formate Incubation on the Kinetics of Oxidation-Reduction Reactions of the Photosystem II Quinone Acceptor Complex

Zeitschrift für Naturforschung C ◽

10.1515/znc-1984-0514 ◽

1984 ◽

Vol 39 (5) ◽

pp. 382-385 ◽

Cited By ~ 42

Author(s):

Howard H. Robinson ◽

Julian J. Eaton-Rye ◽

Jack J. S. van Rensen ◽

Keyword(s):

Photosystem Ii ◽

Redox State ◽

Oxygen Electrode ◽

Thylakoid Membranes ◽

Acceptor Site ◽

Kinetics Of Oxidation ◽

Oxidation Reduction ◽

Primary Quinone Acceptor ◽

Quinone Acceptor ◽

Kinetics Of

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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