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 chang es of the relative fluorescence emission as a function of temperature (220 K < T < 310 K) in isolated thylakoids and PS II membrane fragment s. In addition, effects of bivalent metal ions on PS II were investiga ted by measuring conventional fluorescence induction curves, oxygen ev olution, manganese content and atrazine binding mostly in PS II membra ne fragments. It was found: a) the normalized level of the fluorescenc e remaining 10 s after the actinic flash (F(t)/F0) steeply increases a t temperatures below -10 to -20-degrees-C, b) the fast phase of the tr ansient 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 1 00 muM and d) Cu2+ decreases the normalized variable fluorescence, inh ibits oxygen evolution and diminishes the affinity to atrazine binding without affecting the number of binding sites. The content of about f our manganeses per functionally competent oxygen evolving complex is n ot changed by [Cu2+] < 70 muM. Based on these findings it is concluded : i) a temperature dependent equilibrium between an inactive (I) and a ctive (A) state of Q(A)- reoxidation by Q(B)(Q(B)-) is characterized b y standard enthalpies DELTAH-degrees of 95 kJ mol-1 and 60 kJ mol-1 an d 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, respective ly, 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+ cause s at least a two-fold effect on PS II by modifying the atrazine bindin g affinity at lower concentrations (approximately 5 muM) and interfere nce with the redox active tyrosine Y(Z) at slightly higher concentrati on (approximately 10 muM) leading to blockage of oxygen evolution.