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

The active site for water oxidation in photosystem II goes through five sequential oxidation states (S-0 to S-4) before O-2 is evolved. It consists of a Mn4Ca cluster close to a redox-active tyrosine residue (Tyr(Z)). 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 O-2 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 < Ca/Br < Sr/Cl < Sr/Br. In all cases, the rate of O-2 release was similar to that of the S(3)Tyr(Z)(center dot) to S(0)Tyr(Z) transition, with the slowest kinetics (i.e. the Sr/Br enzyme) being approximate to 6-7 slower than in the native Ca/Cl enzyme. This slowdown in the kinetics was reflected in a decrease in the free energy level of the S-3 state as manifest by thermoluminescence. These observations indicate that Cl- is involved in the water oxidation mechanism. The possibility that Cl- is close to the active site is discussed in terms of recent structural models.