Abstract:
:By exposing photosystem II (PSII) samples to an incrementing number of excitation flashes at room temperature, followed by freezing, we could compare the Mn-derived multiline EPR signal from the S2 oxidation state as prepared by 1, 5, 10, and 25 flashes of light. While the S2 multiline signals exhibited by these samples differed very little in spectral shape, a significant increase of the relaxation rate of the signal was detected in the multiflash samples as compared to the S2-state produced by a single oxidation. A similar relaxation rate increase was observed for the EPR signal from Y(D*). The temperature dependence of the multiline spin-lattice relaxation rate is similar after 1 and 5 flashes. These data are discussed together with previously reported phenomena in terms of a light-adaptation process of PSII, which commences on the third flash after dark-adaptation and is completed after 10 flashes. At room temperature, the fast-relaxing, light-adapted state falls back to the slow-relaxing, dark-adapted state with t(1/2) = 80 s. We speculate that light-adaptation involves changes necessary for efficient continuous water splitting. This would parallel activation processes found in many other large redox enzymes, such as Cytochrome c oxidase and Ni-Fe hydrogenase. Several mechanisms of light-adaptation are discussed, and we find that the data may be accounted for by a change of the PSII protein matrix or by the light-induced appearance of a paramagnetic center on the PSII donor side. At this time, no EPR signal has been detected that correlates with the increase of the relaxation rates, and the nature of such a new paramagnet remains unclear. However, the relaxation enhancement data could be used, in conjunction with the known Mn-Y(D) distance, to estimate the position of such an unknown relaxer. If positioned between Y(D) and the Mn cluster, it would be located 7-8 A from the spin center of the S2 multiline signal.
journal_name
Biochemistryjournal_title
Biochemistryauthors
Peterson S,Ahrling KA,Högblom JE,Styring Sdoi
10.1021/bi026848csubject
Has Abstractpub_date
2003-03-11 00:00:00pages
2748-58issue
9eissn
0006-2960issn
1520-4995journal_volume
42pub_type
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