PHOTOSYSTEM-II - THERMODYNAMICS AND KINETICS OF ELECTRON-TRANSPORT FROM Q(A)(-) TO Q(B)(Q(B)(-)) AND DELETERIOUS EFFECTS OF COPPER(II)

Studies on thermodynamics and kinetics of electron transfer from Q(A)- to Q(B)(Q(B)-) 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 (F(t)/F0) steeply increases at temperatures below -10 to -20-degrees-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 muM 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 muM. Based on these findings it is concluded: i) a temperature dependent equilibrium between an inactive (I) and active (A) state of Q(A)- reoxidation by Q(B)(Q(B)-) is characterized by standard enthalpies DELTAH-degrees of 95 kJ mol-1 and 60 kJ mol-1 and standard entropies DELTAS-degrees 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 Q(A)- 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 (approximately 5 muM) and interference with the redox active tyrosine Y(Z) at slightly higher concentration (approximately 10 muM) leading to blockage of oxygen evolution.