Environment of TyrZ in photosystem II from Thermosynechococcus elongatus in which PsbA2 is the D1 protein

The main cofactors that determine the photosystem II (PSII) oxygen evolutionactivity are borne by the D1 and D2 subunits. In the cyanobacteriumThermosynechococcus elongatus, there are threepsbA genes coding for D1. Among the 344 residuesconstituting D1, there are 21 substitutions between PsbA1 and PsbA3, 31 betweenPsbA1 and PsbA2, and 27 between PsbA2 and PsbA3. Here, we present the firststudy of PsbA2-PSII. Using EPR and UV-visible time-resolved absorptionspectroscopy, we show that: (i) the time-resolved EPR spectrum of Tyr_Z^• in the(S₃Tyr_Z^•)′ is slightly modified; (ii) the split EPR signalarising from Tyr_Z^• in the (S₂Tyr_Z^•)′ state induced by near-infraredillumination at 4.2 K of the S₃Tyr_Z state is significantlymodified; and (iii) the slow phases of P₆₈₀^+⋅ reduction by Tyr_Z areslowed down from the hundreds of μs time range to the ms time range,whereas both the S₁Tyr_Z^• → S₂Tyr_Z andthe S₃Tyr_Z^• → S₀Tyr_Z + O₂transition kinetics remained similar to those in PsbA(1/3)-PSII. These resultsshow that the geometry of the Tyr_Z phenol and its environment, likelythe Tyr-O···H···Nϵ-His bonding,are modified in PsbA2-PSII when compared with PsbA(1/3)-PSII. They also point tothe dynamics of the proton-coupled electron transfer processes associated withthe oxidation of Tyr_Z being affected. From sequence comparison, wepropose that the C144P and P173M substitutions in PsbA2-PSIIversus PsbA(1/3)-PSII, respectively located upstream of theα-helix bearing Tyr_Z and between the two α-helicesbearing Tyr_Z and its hydrogen-bonded partner, His-190, areresponsible for these changes.