Kishi Rikako, Imanishi Michie, Kobayashi Masayuki, Takenaka Shinji, Madigan Michael T, Wang-Otomo Zheng-Yu, Kimura Yukihiro
Graduate School of Agricultural Science, Kobe University, Kobe 657-8501, Japan.
Ariake National College of Technology, Omuta, Fukuoka 836-8585, Japan.
Biochim Biophys Acta Bioenerg. 2021 Jan 1;1862(1):148307. doi: 10.1016/j.bbabio.2020.148307. Epub 2020 Sep 11.
Redox-active quinones play essential roles in efficient light energy conversion in type-II reaction centers of purple phototrophic bacteria. In the light-harvesting 1 reaction center (LH1-RC) complex of purple bacteria, Q is converted to QH upon light-induced reduction and QH is transported to the quinone pool in the membrane through the LH1 ring. In the purple bacterium Rhodobacter sphaeroides, the C-shaped LH1 ring contains a gap for quinone transport. In contrast, the thermophilic purple bacterium Thermochromatium (Tch.) tepidum has a closed O-shaped LH1 ring that lacks a gap, and hence the mechanism of photosynthetic quinone transport is unclear. Here we detected light-induced Fourier transform infrared (FTIR) signals responsible for changes of Q and its binding site that accompany photosynthetic quinone reduction in Tch. tepidum and characterized Q and QH marker bands based on their N- and C-isotopic shifts. Quinone exchanges were monitored using reconstituted photosynthetic membranes comprised of solubilized photosynthetic proteins, membrane lipids, and exogenous ubiquinone (UQ) molecules. In combination with C-labeling of the LH1-RC and replacement of native UQ by ubiquinones of different tail lengths, we demonstrated that quinone exchanges occur efficiently within the hydrophobic environment of the lipid membrane and depend on the side chain length of UQ. These results strongly indicate that unlike the process in Rba. sphaeroides, quinone transport in Tch. tepidum occurs through the size-restricted hydrophobic channels in the closed LH1 ring and are consistent with structural studies that have revealed narrow hydrophobic channels in the Tch. tepidum LH1 transmembrane region.
氧化还原活性醌在紫色光合细菌的II型反应中心的高效光能转换中起着至关重要的作用。在紫色细菌的捕光1反应中心（LH1-RC）复合体中，Q在光诱导还原后转化为QH，并且QH通过LH1环被转运到膜中的醌池。在紫色细菌球形红杆菌中，C形的LH1环包含一个用于醌转运的间隙。相比之下，嗜热紫色细菌嗜热栖热菌具有一个没有间隙的封闭O形LH1环，因此光合醌转运的机制尚不清楚。在这里，我们检测到了嗜热栖热菌中光合醌还原过程中负责Q及其结合位点变化的光诱导傅里叶变换红外（FTIR）信号，并根据它们的N和C同位素位移对Q和QH标记带进行了表征。使用由溶解的光合蛋白、膜脂和外源泛醌（UQ）分子组成的重组光合膜监测醌交换。结合LH1-RC的C标记以及用不同尾长的泛醌替代天然UQ，我们证明了醌交换在脂膜的疏水环境中高效发生，并且取决于UQ的侧链长度。这些结果强烈表明，与球形红杆菌中的过程不同，嗜热栖热菌中的醌转运是通过封闭的LH1环中尺寸受限的疏水通道进行的，这与揭示嗜热栖热菌LH1跨膜区域中狭窄疏水通道的结构研究一致。
