Department of Energy, Environmental, and Chemical Engineering, Washington University in St. Louis, St. Louis, MO 63130, USA.
Department of Biology, Washington University in St. Louis, St. Louis, MO 63130, USA.
Biochim Biophys Acta Bioenerg. 2017 Mar;1858(3):249-258. doi: 10.1016/j.bbabio.2017.01.001. Epub 2017 Jan 8.
This paper presents spectroscopic investigations of IsiA, a chlorophyll a-binding membrane protein produced by cyanobacteria grown in iron-deficient environments. IsiA, if associated with photosystem I, supports photosystem I in light harvesting by efficiently transferring excitation energy. However, if separated from photosystem I, IsiA exhibits considerable excitation quenching observed as a substantial reduction of protein-bound chlorophyll a fluorescence lifetime. Previous spectroscopic studies suggested that carotenoids are involved in excitation energy dissipation and in addition play a second role in this antenna complex by supporting chlorophyll a in light harvesting by absorbing in the spectral range inaccessible for chlorophyll a and transferring excitation to chlorophylls. However, this investigation does not support these proposed roles of carotenoids in this light harvesting protein. This study shows that carotenoids do not transfer excitation energy to chlorophyll a. In addition, our investigations do not support the hypothesis that carotenoids are quenchers of the excited state of chlorophyll a in this protein complex. We propose that quenching of chlorophyll a fluorescence in IsiA is maintained by pigment-protein interaction via electron transfer from an excited chlorophyll a to a cysteine residue, an excitation quenching mechanism that was recently proposed to regulate the light harvesting capabilities of the bacteriochlorophyll a-containing Fenna-Mathews-Olson protein from green sulfur bacteria.
本文对在缺铁环境中生长的蓝藻产生的叶绿素 a 结合膜蛋白 IsiA 进行了光谱研究。IsiA 如果与光系统 I 相关联，则通过有效地转移激发能量来支持光系统 I 的光捕获。然而，如果与光系统 I 分离，IsiA 会表现出相当大的激发猝灭，表现为蛋白质结合的叶绿素 a 荧光寿命的显著降低。先前的光谱研究表明，类胡萝卜素参与激发能量耗散，并且通过在叶绿素 a 无法吸收的光谱范围内吸收并将激发转移到叶绿素上来支持光捕获中的叶绿素 a，从而在这个天线复合物中发挥第二个作用。然而，这项研究不支持类胡萝卜素在这种光捕获蛋白中发挥这些提议的作用。本研究表明，类胡萝卜素不会将激发能量转移到叶绿素 a 上。此外，我们的研究不支持类胡萝卜素是该蛋白复合物中叶绿素 a 激发态猝灭剂的假设。我们提出，通过从激发态叶绿素 a 到半胱氨酸残基的电子转移，通过色素-蛋白相互作用来维持 IsiA 中叶绿素 a 荧光的猝灭，这种激发猝灭机制最近被提出用于调节来自绿硫细菌的含细菌叶绿素 a 的 Fenna-Mathews-Olson 蛋白的光捕获能力。
