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[摘要]：The main cofactors that determine the photosystem II (PSII) oxygen evolution activity are borne by the D1 and D2 subunits. In the cyanobacterium Thermosynechococcus elongatus, there are three psbA genes coding for D1. Among the 344 residues constituting D1, there are 21 substitutions between PsbA1 and PsbA3, 31 between PsbA1 and PsbA2, and 27 between PsbA2 and PsbA3. Here, we present the first study of PsbA2-PSII. Using EPR and UV-visible time-resolved absorption spectroscopy, we show that: (i) the time-resolved EPR spectrum of Tyr(Z)(center dot) in the (S(3)Tyr(Z)(center dot))' is slightly modified; (ii) the split EPR signal arising from Tyr(Z)(center dot) in the (S(2)Tyr(Z)(center dot))' state induced by near-infrared illumination at 4.2 K of the S(3)Tyr(Z) state is significantly modified; and (iii) the slow phases of P-680(+). reduction by Tyr(Z) are slowed down from the hundreds of mu s time range to the ms time range, whereas both the S(1)Tyr(Z)(center dot) -> S(2)Tyr(Z) and the S(3)Tyr(Z)(center dot) -> 3 S(0)Tyr(Z) + O-2 transition kinetics remained similar to those in PsbA(1/3)-PSII. These results show that the geometry of the Tyr(Z) phenol and its environment, likely the Tyr-O center dot center dot center dot H center dot center dot center dot N epsilon-His bonding, are modified in PsbA2-PSII when compared with PsbA(1/3)-PSII. They also point to the dynamics of the proton-coupled electron transfer processes associated with the oxidation of Tyr(Z) being affected. From sequence comparison, we propose that the C144P and P173M substitutions in PsbA2-PSII versus PsbA(1/3)-PSII, respectively located upstream of the alpha-helix bearing Tyr(Z) and between the two alpha-helices bearing Tyr(Z) and its hydrogen-bonded partner, His-190, are responsible for these changes. |
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