Two different ways of stimulating photosynthetic oxygen evolution by copper(II) ions and trivalent lanthanides.

Abstract

We have studied the influence of lanthanides, which are suitable probes for Ca2+ binding sites, and copper cations on the water oxidation process within photosystem II prepared from Nicotiana tabacum. The oxygen evolution measurements under short saturating flashes were performed using "a three electrode system". The content of Cu, Mn, Dy and Eu ions in PSII was determined by AAS or ICP. The inhibitory action of copper and lanthanides on photosystem II is well known. However, we have observed that copper II ions at about equimolar Cu2+/PSII RC proportions stimulate oxygen evolution two-fold. The same effect we have observed for Eu3+ and Dy3+. The molecular mechanism of Cu2+ action is different than for lanthanides. We have found that there are two Eu (Dy) ¿ binding sites in photosystem II: one with high affinity in the core of PSII and the other one with lower affinity in the 33 kDa extrinsic protein. The tightly bound Eu (Dy) ions are responsible for the inhibition of oxygen evolution whereas the other binding site would be responsible for the stimulatory effect. The stimulation of O2 yield by Cu2+ in the presence of high excess of Ca2+ together with the analysis of copper content in PSII preparations form the tobacco wild-type and tobacco mutant deficient in light harvesting complex, as well as the inhibitory effect of azide on the oxygen evolution in thylakoids allowed us to postulate that Cu2+ is probably a native component of PSII and takes part in the oxygen evolution process.