Misses during Water Oxidation in Photosystem II Are S State-dependent

• The period of four oscillation of the S state intermediates of the water oxidizing complex in Photosystem II (PSII) is commonly analyzed by the Kok parameters. The important miss factor determines the efficiency for each S transition. Commonly, an equal miss factor has been used in the analysis. We have used EPR signals which probe all S states in the same sample during S cycle advancement. This allows, for the first time, to measure directly the miss parameter for each S state transition. Experiments were performed in PSII membrane preparations from spinach in the presence of electron acceptor at 1 degrees C and 20 degrees C. The data show that the miss parameter is different in different transitions and shows different temperature dependence. We found no misses at 1 degrees C and 10% misses at 20 degrees C during the S-1 -> S-2 transition. The highest miss factor was found in the S-2 -> S-3 transition which decreased from 23% to 16% with increasing temperature. For the S-3 -> S-0 transition the miss parameter was found to be 7% at 1 degrees C and decreased to 3% at 20 degrees C. For the S-0 -> S-1 transition the miss parameter was found to be approximately 10% at both temperatures. The contribution from the acceptor side in the form of recombination reactions as well as from the donor side of PSII to the uneven misses is discussed. It is suggested that the different transition efficiency in each S transition partly reflects the chemistry at the CaMn4O5 cluster. That consequently contributes to the uneven misses during S cycle turnover in PSII.

Subject headings

NATURVETENSKAP  -- Biologi -- Biokemi och molekylärbiologi (hsv//swe)

NATURAL SCIENCES  -- Biological Sciences -- Biochemistry and Molecular Biology (hsv//eng)

NATURVETENSKAP  -- Kemi (hsv//swe)

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

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ref (subject category)

art (subject category)