Caffarri S, Croce R, Breton J, Bassi R
Dipartimento Scientifico e Tecnologico, Università di Verona, Strada Le Grazie 15, Verona 37134, Italy.
J Biol Chem. 2001 Sep 21;276(38):35924-33. doi: 10.1074/jbc.M105199200. Epub 2001 Jul 13.
We have characterized a xanthophyll binding site, called V1, in the major light harvesting complex of photosystem II, distinct from the three tightly binding sites previously described as L1, L2, and N1. Xanthophyll binding to the V1 site can be preserved upon solubilization of the chloroplast membranes with the mild detergent dodecyl-alpha-d-maltoside, while an IEF purification step completely removes the ligand. Surprisingly, spectroscopic analysis showed that when bound in this site, xanthophylls are unable to transfer absorbed light energy to chlorophyll a. Pigments bound to sites L1, L2, and N1, in contrast, readily transfer energy to chlorophyll a. This result suggests that this binding site is not directly involved in light harvesting function. When violaxanthin, which in normal conditions is the main carotenoid in this site, is depleted by the de-epoxidation in strong light, the site binds other xanthophyll species, including newly synthesized zeaxanthin, which does not induce detectable changes in the properties of the complex. It is proposed that this xanthophyll binding site represents a reservoir of readily available violaxanthin for the operation of the xanthophyll cycle in excess light conditions.
我们在光系统II的主要捕光复合物中鉴定出一个叶黄素结合位点，称为V1，它与之前描述的三个紧密结合位点L1、L2和N1不同。在用温和去污剂十二烷基-α-D-麦芽糖苷溶解叶绿体膜后，叶黄素与V1位点的结合可以保留，而IEF纯化步骤会完全去除配体。令人惊讶的是，光谱分析表明，当叶黄素结合在这个位点时，它们无法将吸收的光能传递给叶绿素a。相比之下，结合在L1、L2和N1位点的色素很容易将能量传递给叶绿素a。这一结果表明，这个结合位点不直接参与捕光功能。在强光下通过脱环氧化作用使紫黄质（在正常条件下是该位点的主要类胡萝卜素）耗尽时，该位点会结合其他叶黄素种类，包括新合成的玉米黄质，这不会引起复合物性质的可检测变化。有人提出，这个叶黄素结合位点代表了在过量光照条件下用于叶黄素循环运作的现成紫黄质储备。
