Characterization of the tyrosine-Z radical and its environment in the spin-coupled S2TyrZ* state of photosystem II from Thermosynechococcus elongatus.
The Mn4Ca cluster of photosystem II (PSII) goes through five sequential oxidation states (S0-S4) in the water oxidation process that also involves a tyrosine radical intermediate (TyrZ*). An S2TyrZ* state in which the Mn4Ca cluster and TyrZ* are magnetically coupled to each other and which is characterized by a distinct "split-signal" EPR spectrum can be generated in acetate-treated PSII. This state was examined by high-field EPR (HFEPR) in PSII from Thermosynechococcus elongatus isolated from a D2-Tyr160Phe mutant to avoid spectral contributions from TyrD*. In contrast to the same state in plants, both antiferromagnetic and ferromagnetic spin-spin couplings were observed. The intrinsic g values of TyrZ* in the coupled state were directly measured from the microwave frequency dependence of the HFEPR spectrum. The TyrZ* gx value in the antiferromagnetic centers was 2.0083, indicating that the coupled radical was in a less electropositive environment than in Mn-depleted PSII. Two gx values were found in the ferromagnetically coupled centers, 2.0069 and 2.0079. To put these values in perspective, the second redox-active tyrosine, TyrD*, was examined in various electrostatic environments. The TyrD* gx value changed from 2.0076 in the wild type to 2.0095 when the hydrogen bond from histidine 189 to TyrD* was removed using the D2-His189Leu mutant, indicating a change to a significantly less electropositive environment. BLY3P/6-31+G** density functional calculations on the hydrogen-bonded p-ethylphenoxy radical-imidazole supermolecular model complex showed that the entire range of Tyr* gx values, from 2.0065 to 2.0095, could be explained by the combined effects of hydrogen bonding and the dielectric constant of the local protein environment.
Un
Sun
S
Service de Bioénergétique, URA CNRS 2096, DBJC, CEA Saclay, 91191 Gif sur Yvette, France. sun@herode.saclay.cea.fr
Boussac
Alain
A
Sugiura
Miwa
M
D000458:Cyanobacteria / Q000737:chemistry / Q000235:genetics; D004578:Electron Spin Resonance Spectroscopy; D005609:Free Radicals / Q000737:chemistry*; D006860:Hydrogen Bonding; D045332:Photosystem II Protein Complex / Q000737:chemistry* / Q000235:genetics; D011789:Quantum Theory; D014443:Tyrosine / Q000737:chemistry*
D005609:Free Radicals; D045332:Photosystem II Protein Complex; D014443:Tyrosine