Decoupling of the processes of molecular oxygen synthesis and electron transport in Ca2+-depleted PSII membranes

Abstract

Extraction of Ca(2+) from the O(2)-evolving complex (OEC) of photosystem II (PSII) membranes with 2 M NaCl in the light (PSII(-Ca/NaCl)) results in 90% inhibition of the O(2)-evolution reaction. However, electron transfer from the donor to acceptor side of PSII, measured as the reduction of the exogenous acceptor 2,6-dichlorophenolindophenol (DCIP) under continuous light, is inhibited by only 30%. Thus, calcium extraction from the OEC inhibits the synthesis of molecular O(2) but not the oxidation of a substrate we term X, the source of electrons for DCIP reduction. The presence of electron transfer across PSII(-Ca/NaCl) membranes was demonstrated using fluorescence induction kinetics, a method that does not require an artificial acceptor. The calcium chelator, EGTA (5 mM), when added to PSII(-Ca/NaCl) membranes, does not affect the inhibition of O(2) evolution by NaCl but does inhibit DCIP reduction up to 92% (the reason why electron transport in Ca(2+)-depleted materials has not been noticed before). Another chelator, sodium citrate (citrate/low pH method of calcium extraction), also inhibits both O(2) evolution and DCIP reduction. The role of all buffer components (including bicarbonate and sucrose) as possible sources of electrons for PSII(-Ca/NaCl) membranes was investigated, but only the absence of chloride anions strongly inhibited the rate of DCIP reduction. Substitution of other anions for chloride indicates that Cl(-) serves its well-known role as an OEC cofactor, but it is not substrate X. Multiple turnover flash experiments have shown a period of four oscillations of the fluorescence yield (both the maximum level, F(max), and the fluorescence level measured 50 s after an actinic flash in the presence of DCMU) in native PSII membranes, reflecting the normal function of the OEC, but the absence of oscillations in PSII(-Ca/NaCl) samples. Thus, PSII(-Ca/NaCl) samples do not evolve O(2) but do transfer electrons from the donor to acceptor sides and exhibit a disrupted S-state cycle. We explain these results as follows. In Ca(2+)-depleted PSII membranes, obtained without chelators, the oxidation of the OEC stops after the absorption of three quanta of light (from the S1 state), which should convert the native OEC to the S4 state. An one-electron oxidation of the water molecule bound to the Mn cluster then occurs (the second substrate water molecule is absent due to the absence of calcium), and the OEC returns to the S3 state. The appearance of a sub-cycle within the S-state cycle between S3-like and S4-like states supplies electrons (substrate X is postulated to be OH(-)), explains the absence of O(2) production, and results in the absence of a period of four oscillation of the normal functional parameters, such as the fluorescence yield or the EPR signal from S2. Chloride anions probably keep the redox potential of the Mn cluster low enough for its oxidation by Y(Z)(*).

Oxygen-Evolving Porous Glass Plates Containing the Photosynthetic Photosystem II Pigment–Protein Complex

“…Subsequent charge recombination in the dark reoxidizes Q A – (Figure d), and decreases the fluorescence yield to the low F 0 level again . It has been reported that the quantum yield of charge separation was decreased, and the kinetics of Q A – reoxidation were slowed by denaturing the oxygen-evolving complex of PSII . The yield of charge-separation of PSII for PSII/PGP20 was calculated as 0.89 ± 0.04, which is comparable to that for PSII in the solution system (0.91 ± 0.03).…”

“…22 It has been reported that the quantum yield of charge separation was decreased, and the kinetics of Q A − reoxidation were slowed by denaturing the oxygenevolving complex of PSII. 25 The yield of charge-separation of PSII for PSII/PGP20 was calculated as 0.89 ± 0.04, which is comparable to that for PSII in the solution system (0.91 ± 0.03). The kinetics of Q A − reoxidation in PSII/PGP20 were similar to those of PSII in solution.…”

Light increases resistance of thylakoid membranes to thermal inactivation

“…It should be noted that in this work we used PSII particles without calcium, in which manganese catalytic center cannot oxidize water molecules to molecular oxygen. However, Ca-depleted PSII particles have a high electron transport speed (about 70% of the control (Semin et al 2008)). The reduction of the DCPIP arti cial acceptor is determined by electrons formed as a result of incomplete oxidation of water to hydrogen peroxide (Semin et al 2013).…”

Toxic Effect of Terbium on the High Plant Photosystem Ii Function

“…В других случаях наблюдается «антиизоморфизм», при котором ион РЗЭ блокирует функцию Ca 2+ [Золин, Коренева, 1980]. Известно, что водоокисляющий комплекс ФС2 содержит ион Ca 2+ , удаление которого приводит к ингибированию кислород-выделяющей функции [Ghanotakis et al, 1984b;Semin et al, 2008]. Можно предположить, что ингибирование активности ФС2 ионами Tb 3+ связано с нефункциональным замещением иона Ca 2+ в водоокисляющем комплексе.…”