Yashar Feyziyev

Electron transfer from cytochrome b559 and tyrosineD to the S2 and S3 states of the water oxidizing complex in photosystem II

We have investigated the electron transfer from reduced tyrosine YD (YDred) and cytochrome b559 to the S2 and S3 states of the water oxidizing complex (WOC) in Photosystem II. The EPR signal of oxidized cyt b559, the S2 state multiline EPR signal and the EPR signal from YD@? were measured to follow the electron transfer to the S2 and S3 states at 245 and 275 K. The majority of the S2 centers was reduced directly from YDred but at 245 K we observed oxidation of cyt b559 in about 20% of the centers. Incubation of the YDredS3 state resulted in biphasic changes of the S2 multiline signal. The signal first increased due to reduction of the S3 state. Thereafter, the signal decreased due to decay of the S2 state. In contrast, the YD@? signal increased with a monophasic kinetics at both temperatures. Again, we observed oxidation of cyt b559 in about 20% of the PSII centers at 245 K. This oxidation correlated with the decay of the S2 state. The complex changes can be explained by the conversion of YDredS3 centers (present initially) to YD@?S1 centers, via the intermediate YD@?S2 state. The early increase of the S2 state multiline signal involves electron transfer from YDred to the S3 state resulting in formation of YD@?S2. This state is reduced by cyt b559 resulting in a single exponential oxidation of cyt b559. Taken together, these results indicate that the electron donor to S2 is cyt b559 while cyt b559 is unable to compete with YDred in the reduction of the S3 state in the pre-reduced samples. We also followed the decay of the S2 and S3 states and the oxidation of cyt b559 in samples where YD was oxidized from the start. In this case cyt b559 was able to reduce both the S2 and the S3 states suggesting that different pathways exist for the electron transfer from cyt b559 to the WOC. The activation energies for the YDredS2->YD@?S1 and YDredS3->YD@?S2 transformations are 0.57 and 0.67 eV, respectively, and the reason for these large activation energies is discussed. (Less)

Spin conversion of cytochrome b559 in photosystem II induced by exogenous high potential quinone

The spin-state of cytochrome b559 (Cyt b559) was studied in photosystem II (PSII) membrane fragments by low-temperature EPR spectroscopy. Treatment of the membranes with 2,3-dichloro-5,6-dicyano-p-benzoquinone (DDQ) converts the native low-spin (LS) form of Cyt b559 to the high-spin (HS) form characterized with the g= 6.19 and g= 5.95 split signal. The HS Cyt b559 was pH dependent with the amplitude increasing toward more acidic pH values (pH 5.5-8.5). The HS state was not photochemically active upon 77 and 200 K continuous illumination under our conditions and was characterized by a low reduction potential (=<0 V). It was also demonstrated that DDQ treatment damages the oxygen evolving complex, leading to inhibition of oxygen evolution, decrease of the S2-state EPR multiline signal and release of Mn2+. In parallel, studies of model systems containing iron(III) protoporphyrin IX chloride (FeIIIPor), which is a good model compound for the Cyt b559 prosthetic group, were performed by using optical and EPR spectroscopy. The interaction of FeIIIPor with imidazole (Im) in weakly polar solvent results in formation of bis-imidazole coordinated heme iron (FeIIIPor Im2) which mimic the bis-histidine axial ligation of Cyt b559. The reaction of DDQ with the LS FeIIIPor Im2 complex leads to its transformation into the HS state (g@?=5.95, g@?=2.00). It was shown that the spin conversion occurs due to the donor-acceptor interaction of coordinated imidazole with this high-potential quinone causing the displacement of imidazole from the axial position. The similar mechanism of DDQ-induced spin change is assumed to be valid for the native membrane Cyt b559 in PSII centers. (Less)