Biosynthetic Exchange of Bromide for Chloride and Strontium for Calcium in the Photosystem Il Oxygen-evolving Enzymes.

The active site for water oxidation in photosystem H goes through five sequential oxidation states (S0 to S4) before O2 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 BC, 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 < Ca/Br < Sr/Cl < Sr/Br. In all cases, the rate of O2 release was similar to that of the S3Tyrz• to S0TyrZ transition, with the slowest kinetics (i.e. the Sr/Br enzyme) being ≈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 S3 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. [ABSTRACT FROM AUTHOR]