Biosynthetic exchange of bromide for chloride and strontium for calcium in the photosystem II oxygen-evolving enzymes

Abstract

The active site for water oxidation in photosystem II goes through five sequential oxidation states (S0 to S4) before O 2 is evolved. It consists of a Mn4Ca cluster close to a redox-active tyrosine residue (TyrZ). Cl- is also required for enzyme activity. To study the role of Ca2+ and Cl- in PSII, these ions were biosynthetically substituted by Sr2+ and Br-, respectively, in the thermophilic cyanobacterium Thermosynechococcus elongatus. Irrespective of the combination of the non-native ions used (Ca/Br, Sr/Cl, Sr/Br), the enzyme could be isolated in a state that was fully intact but kinetically limited. The electron transfer steps affected by the exchanges were identified and then investigated by using time-resolved UV-visible absorption spectroscopy, time-resolved O2 polarography, and thermoluminescence spectroscopy. The effect of the Ca2+/Sr2+ and Cl -/Br- exchanges was additive, and the magnitude of the effect varied in the following order: Ca/Cl