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首页> 外文期刊>Biochemistry >Carotenoid oxidation in photosystem II.
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Carotenoid oxidation in photosystem II.

机译：光系统II中的类胡萝卜素氧化。

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The oxidation of carotenoid upon illumination at low temperature has been studied in Mn-depleted photosystem II (PSII) using EPR and electronic absorption spectroscopy. Illumination of PSII at 20 K results in carotenoid cation radical (Car+*) formation in essentially all of the centers. When a sample which was preilluminated at 20 K was warmed in darkness to 120 K, Car+* was replaced by a chlorophyll cation radical. This suggests that carotenoid functions as an electron carrier between P680, the photooxidizable chlorophyll in PSII, and ChlZ, the monomeric chlorophyll which acts as a secondary electron donor under some conditions. By correlating with the absorption spectra at different temperatures, specific EPR signals from Car+* and ChlZ+* are distinguished in terms of their g-values and widths. When cytochrome b559 (Cyt b559) is prereduced, illumination at 20 K results in the oxidation of Cyt b559 without the prior formation of a stable Car+*. Although these results can be reconciled with a linear pathway, they are more straightforwardly explained in terms of a branched electron-transfer pathway, where Car is a direct electron donor to P680(+), while Cyt b559 and ChlZ are both capable of donating electrons to Car+*, and where the ChlZ donates electrons when Cyt b559 is oxidized prior to illumination. These results have significant repercussions on the current thinking concerning the protective role of the Cyt b559/ChlZ electron-transfer pathways and on structural models of PSII.
机译：使用EPR和电子吸收光谱法，在贫锰光系统II（PSII）中研究了低温照射下类胡萝卜素的氧化。在20 K下照亮PSII会导致基本上所有中心形成类胡萝卜素阳离子自由基（Car + *）。当将在20 K下预照明的样品在黑暗中加热到120 K时，Car + *被叶绿素阳离子自由基所取代。这表明类胡萝卜素在P680（PSII中的光氧化性叶绿素）和ChlZ（在某些条件下充当二次电子供体的单体叶绿素）之间起电子载体的作用。通过与不同温度下的吸收光谱相关联，可以根据Car值*和ChlZ + *的g值和宽度区分特定的EPR信号。当预先还原细胞色素b559（Cyt b559）时，在20 K下照射会导致Cyt b559氧化，而不会事先形成稳定的Car + *。尽管这些结果可以与线性途径相吻合，但可以用支化电子转移途径更直接地解释它们，其中Car是P680（+）的直接电子供体，而Cyt b559和ChlZ都能够提供电子到Car + *，以及在照明前Cyt b559氧化时ChlZ贡献电子的位置。这些结果对当前关于Cyt b559 / ChlZ电子转移途径的保护作用的思考以及PSII的结构模型具有重大影响。

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