Destabilization of the Oxygen Evolving Complex of Photosystem II by Al³⁺

Abstract

The inhibitory effect of Al(3+) on photosynthetic electron transport was investigated in isolated thylakoid membranes of spinach. A combination of oxygen evolution, chlorophyll fluorescence induction (FI) and decay and thermoluminescence measurements have been used to characterize photosystem II (PSII) electron transport in the presence of this toxic metal cation. Our results show that below 3 mm, Al(3+) already caused a destabilization of the Mn4 O5 Ca cluster of the oxygen evolving complex (OEC). At these concentrations, an increase in the relative amplitude of the first phase (OJ) of FI curve and retardation of the fluorescence decay kinetics following excitation with a single turnover flash were also observed. A transmembrane structural modification of PSII polypeptides due to the interaction of Al(3+) at the OEC is proposed to retard electron transfer between the quinones QA and QB . Above 3 mm, Al(3+) strongly retarded fluorescence induction and significantly reduced Fv /Fm together with the maximal amplitude of chlorophyll fluorescence induced by a single turnover flash. This chlorophyll fluorescence quenching was attributed to the formation of P680(+) due to inhibition of electron transfer between tyrosine 161 of D1 subunit and P680.

Characterization of the structural changes and photochemical activity of photosystem I under Al3+ effect

“…It has been reported in previous studies that Al 3+ exerts an inhibitory effect on the photochemical activity of PSII complex in isolated thylakoid membranes and in PSII submembrane fractions [32,34,37,38]. However, there is a lack of information about the site(s), and the mode(s) of the Al 3+ interaction with PSI complex.…”

“…However, the amplitude of the second phase of P700 photooxidation increased. Under higher Al 3+ concentrations, above 3 mM, PSII activity was inhibited [37,38] and therefore the inhibition of electrons generated by PSII to PSI led to a suppression of the biphasic patern. At the same range of Al 3+ concentration, the amplitude of P700 photooxidation phase increased markedly with increasing Al 3+ concentrations.…”

“…After that, the second slower phase of the oxidation of P700 was established with a stable steady state level. Even though 1 mM Al 3+ is known to alter a fraction of PSII complexes [37,38], addition of 1 mM Al 3+ keep the biphasic pattern of P700 photooxidation owing to the remaining functional PSII centers that still could donate electrons to reduce a proportion of P700 + . However, the amplitude of the second phase of P700 photooxidation increased.…”

“…Under its trivalent cationic form, Al 3+ acts as a highly toxic non-essential element for plants growth causing a number of adverse effects on physiological and biochemical processes, especially the inhibition of photosynthetic activity. At this level, it was shown that Al 3+ inhibits photosynthetic electron transport in photosystem II (PSII), affects the PSII RC, destabilizes the oxygen evolution complex (OEC), impairs light harvesting complex (LHCII) antenna of PSII and induces a conformational changes of the secondary structure polypeptides of PSII [31][32][33][34][35][36][37][38].…”

Mechanism of Interaction of Al3+ with the Proteins Composition of Photosystem II

“…PSII complex has been considered to be the major target of several toxic metal cations [ 37 – 42 ]. Among these, Al 3+ , the solubilized toxic form of aluminum in acid soils, represents one of the major environmental stresses [ 43 – 47 ].…”