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.

Ämnesord

NATURVETENSKAP  -- Biologi (hsv//swe)

NATURAL SCIENCES  -- Biological Sciences (hsv//eng)

Nyckelord

EPR

Photosystem II

Manganese

Water oxidation

Cytochrome b559

Tyrosine radical

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