Photosystem II: Thermodynamics and Kinetics of Electron Transport from Q_A ^- to Q_B(Q_B ^- ) and Deleterious Effects of Copper(II)

Abstract

Studies on thermodynamics and kinetics of electron transfer from QA- to QB(QB-) were performed by monitoring laser flash induced changes of the relative fluorescence emission as a function of temperature (220 K < T < 310 K) in isolated thylakoids and PS II membrane fragments.In addition, effects of bivalent metal ions on PS II were investigated by measuring conventional fluorescence induction curves, oxygen evolution, manganese content and atrazine binding mostly in PS II membrane fragments. It was found: a) the normalized level of the fluorescence remaining 10 s after the actinic flash (Ft/F0) steeply increases at temperatures below -10 to - 20 °C, b) the fast phase of the transient fluorescence change becomes markedly retarded with decreasing temperatures, c) among different cations (Cu2+, Zn2+, Cd2+, Ni2+, Co2+) only Cu2+ exhibits marked effects in the concentration range below 100 μᴍ and d) Cu2+ decreases the normalized variable fluorescence, inhibits oxygen evolution and diminishes the affinity to atrazine binding without affecting the number of binding sites. The content of about four manganeses per functionally competent oxygen evolving complex is not changed by [Cu2+] < 70 μᴍ.Based on these findings it is concluded: i) a temperature dependent equilibrium between an inactive (I) and active (A) state of QA- reoxidation by QB(QB- ) is characterized by standard enthalpies ΔH° of 95 kJ mol-1 and 60 kJ mol-1 and standard entropies ΔS° of 370 kJ K-1 mol-1 and 240 kJ K-1 mol-1 in isolated thylakoids and PS II membrane fragments, respectively, ii) the activation energies of QA- reoxidation by plastoquinone bound to the QB site are about 30 kJ mol-1 (thylakoids) and 40 kJ mol-1 (PS II membrane fragments) in 220 K < T < 300 K, and iii) Cu2+ causes at least a two-fold effect on PS II by modifying the atrazine binding affinity at lower concentrations ( ~ 5 μᴍ) and interference with the redox active tyrosine Yz at slightly higher concentration ( ~ 10 μᴍ) leading to blockage of oxygen evolution.

The functional role of protein dynamics in photosynthetic reaction centers investigated by elastic and quasielastic neutron scattering

“…The data reveal a further hydration-sensitive transition between above 310 K that was attributed to the detachment of the oxygen evolving complex. Figure 4 also shows the temperature dependence of the Q A → Q B electron transfer efficiency in the PS II RC gathered from fluorescence quantum yield experiments [6]. A comparison with the QENS data of PS II membrane fragments suggests that there is a close correlation between the thermal activation of molecular dynamics and the electron transfer efficiency from Q A to Q B [10,13].…”

“…The three temperature ranges A, B, and C are discussed in the text. The electron transfer efficiency according to [6] is shown by a full red line. This figure is reprinted from [10] with permission.…”

“…For example, the light-induced primary charge separation remains fully functional even at very low temperatures, while the Q A → Q B electron transfer is completely inhibited below about 200 K [6] and below a hydration of 45% relative humidity (r.h.) [7]. Likewise, individual reaction steps of the water oxidizing complex possess a Corresponding author: pieper@ut.ee a characteristic dependence on temperature (see [1] and references therein).…”

Mössbauer Studies of Cu(II) Ions Interaction with the Non-Heme Iron and Cytochrome b559in a Chlamydomonas reinhardtii PSI Minus Mutant

“…Depending on the material used and experimental methods applied, various binding sites of Cu 2+ within the quinone-Fe complex have been suggested. Copper has been found to affect the binding affinity of atrazine to PSII [40] and to modify the Q B pocket [16]. Flash--induced absorption spectroscopy studies have shown that the primary charge sep-…”

Copper effect on the protein composition of photosystem II

“…A possible direct interaction between copper and calcium at the oxidising side of PSII was also shown both in vitro (Sabat, 1996) and in vivo (Maksymiec and Baszynski, 1999). Additional effects of Cu(II) toxicity on both the donor side, affecting the Mn-cluster and the extrinsic proteins of the oxygen-evolving complex, and the acceptor side, interacting with the nonheme Fe 2+ have been reported (Renger et al 1993, Jegerschóld et al 1995, Sersen et al 1997. The interaction of Cu(II) toxicity with photoinhibitory and recovery proceses on PSII has been also investigated (Yruela et al 1996b, Pätsikkä et al 1998 It is also needs to be established which physiological effects of copper on photosynthesis in vivo result from inhibition of the electron transport on PSII.…”