Protective role of CaCl₂ against Pb²⁺ inhibition in photosystem II

Abstract

The Pb2+ treatment of Photosystem II (PS II) membrane fragments, either intact or depleted in 17 and 23 kDa extrinsic polypeptides, inhibits PS II activity. When CaC1, was present in the assay, the Pbz+ inhibition was significantly reduced in both types of PS II membranes, suggesting a protective role of CaCI, against Pb2+ inhibition. However, in either case, the degree of PS II inhibition by Pbz+ was higher in the protein depleted than in intact PS II. It showed that the loss of endogenous Ca*+ induced by p 01 yp p e tide depletion causes the PS II to be more susceptible to Pbz+ The interaction of Pb2+ with CaCl, in protein-depleted PS II was competitive.Our results suggest that Pbz+ competes for binding to the CaZ+-and Cl-active sites in the water-splitting complex. Since Pb2+ inhibition of PS II activity cannot be reversed by CaCl, but can be reversed by diphenylcarbazide, we conclude that Pb2+ induced inhibition of PS II activity was mediated via the water-splitting system.

Effect of Pb2+ ions on photosynthetic apparatus

“…The mechanism of inhibitory action of Pb(NO 3 ) 2 and Pb(CH 3 CHOO) 2 consists in the formation of complexes between Pb 2+ and amino acid residues in photosynthetic proteins as was confirmed by fluorescence spectroscopy. Our results are in a good accordance with former findings of Miles et al (1972), Rashid and Popovic (1990), Prasad et al (1991) and Romanowska et al (2012). …”

“…Similar inhibition of DCPIP photoreduction caused by Pb 2+ was also observed by Miles at al. (1972), Rashid and Popovic (1990), Prasad et al (1991) and Romanowska et al (2012 . For Pb(CH 3 COO) 2 , k 1 = 141 and k 2 = 3162 .…”

“…Qufei and Fashui (2009) studied the effects of Pb 2+ ions on the secondary structure and function of PS2 of Spirodela polyrrhiza and found that Pb 2+ ions accumulated in PS2 and damaged its secondary structure, decreased the absorbance of visible light, inhibited energy transfer among amino acids within the PS2 protein-pigment complex, and reduced energy transport from tyrosine residue to Chla. Rashid and Popovic (1990) (Rashid et al 1994). According to Ahmed and Tajmir-Riahi (1993) conformational changes in light-harvesting chlorophyll (LHCII) subunits induced by Pb treatment may lead to incomplete assembly followed by degradation.…”

Tolerance and prospection of phytoremediator woody species of Cd, Pb, Cu and Cr

“…Macronutrient deficiencies in plants are often a manifestation of toxic effects due to heavy metals (Siedleska, 1995). Some macroelements, including Ca, Mg and P play a protective role against the toxic effects of heavy metals (Rashid and Popovic, 1990). Lead competes with Ca for the same coupling site in the cell (Godbold and Kettner, 1991).…”

Multigene engineering: dawn of an exciting new era in biotechnology

“…In photosynthetic organisms, mercury inhibits the oxygen-evolving enzyme (OEE) complex, binds to thylakoid membranes [42] and removes EP33 (one of the proteins of the OEE complex [43]). Mercury reduces the variable fluorescence (which provides a measure of photosynthetic efficiency) owing to additional inhibitory sites on the donor side of photosystem II, causing damage to the light-harvesting complexes and structural changes in the antenna pigments that affect the primary photochemistry; mercury also inhibits plastocyanin [44]. Nuclear codon optimized merA (mercury ion reductase) and merB (organomercurial lyase) genes were used to obtain transgenic plants that are resistant to mercury and organomercurials, respectively (up to 10 μM) [45 • ].…”