CHARGE EQUILIBRIUM BETWEEN THE WATER-OXIDIZING COMPLEX AND THE ELECTRON-DONOR TYROSINE-D IN PHOTOSYSTEM-II

The decay kinetics for the S2 and S3 states of the water-oxidizing complex have been measured with an unmodulated Joliot-type oxygen electrode in isolated spinach thylakoids. The S2 and S3 states decay biphasically (Vermaas, W.F.J., Renger, G. and Dohnt, G. (1985) Biochim. Biophys. Acta 764, 194-202) with half-decay times of 1-1.5 s and 30-35 s at room temperature. The proportion of the fast phase is negligible in preilluminated thylakoids but increases during dark adaptation to 22-24% for both S2 and S3. This process, t 1 2 ≈ 10 min, is accompanied with the conversion of the S0 state to S1 in about 25% of the centers. Chemical reduction of tyrosine-D+, which gives rise to the EPR Signals IIslow, by dichlorphenolindophenol/ascorbate increases the proportion of the fast decaying phase of S2 and S3 to about 70-80%. The decay of S2 is accompanied by the accumulation of S1 and the decay of S3 results in a transient increase of S2. These data led us to conclude that the fast phase in the S2 and S3 decay is correlated with one-electron donation from tyrosine-D to the water-oxidizing complex located within the same center. This process results in the S3D → S2D+ (→ S1D+) and S2D → S1D+ univalent sequences of deactivating reactions. The electron transfer from tyrosine-D to the S2 and S3 states is strongly temperature-dependent and shows 0.46 and 0.49 eV activation energy, respectively, over the +8 to +37°C temperature range. The deactivation process which is reflected by the slower phase of S2 and S3 decay has an activation energy of 0.65 and 0.76 eV, respectively. An extension of the Kok model of oxygen evolution is also presented taking into account the effect of fast electron donation from tyrosine-D to the water-oxidizing complex. © 1990.