Zernike Institute for Advanced Materials , University of Groningen , Nijenborgh 4 , 9747 AG Groningen , The Netherlands.
Department of Biochemistry and Molecular Biology , The Pennsylvania State University , University Park , State College , Pennsylvania 16802 , United States.
J Phys Chem B. 2018 Jul 5;122(26):6712-6723. doi: 10.1021/acs.jpcb.8b02875. Epub 2018 Jun 21.
Green sulfur bacteria can grow photosynthetically by absorbing only a few photons per bacteriochlorophyll molecule per day. They contain chlorosomes, perhaps the most efficient light-harvesting antenna system found in photosynthetic organisms. Chlorosomes contain supramolecular structures comprising hundreds of thousands of bacteriochlorophyll molecules, which are properly positioned with respect to one another solely by self-assembly and not by using a protein scaffold as a template for directing the mutual arrangement of the monomers. These two features-high efficiency and self-assembly-have attracted considerable attention for developing light-harvesting systems for artificial photosynthesis. However, reflecting the heterogeneity of the natural system, detailed structural information at atomic resolution of the molecular aggregates is not yet available. Here, we compare the results for chlorosomes from the wild type and two mutants of Chlorobaculum tepidum obtained by polarization-resolved, single-particle fluorescence-excitation spectroscopy and theoretical modeling with results previously obtained from nuclear-magnetic resonance spectroscopy and cryo-electron microscopy. Only the combination of information obtained from all of these techniques allows for an unambiguous description of the molecular packing of bacteriochlorophylls within chlorosomes. In contrast to some suggestions in the literature, we find that, for the chlorosomes from the wild type as well as for those from mutants, the dominant secondary structural element features tubular symmetry following a very similar construction principle. Moreover, the results suggest that the various options for methylation of the bacteriochlorophyll molecules, which are a primary source of the structural (and spectral) heterogeneity of wild-type chlorosome samples, are exploited by nature to achieve improved spectral coverage at the level of individual chlorosomes.
绿硫细菌每天仅通过吸收每个菌绿素分子的几个光子就能进行光合作用。它们含有菌光体，这可能是在光合生物中发现的最高效的光收集天线系统。菌光体包含由数十万细菌叶绿素分子组成的超分子结构，这些分子彼此之间的正确定位仅通过自组装来实现，而不使用蛋白质支架作为指导单体相互排列的模板。这两个特点——高效率和自组装——引起了人们对开发用于人工光合作用的光收集系统的极大关注。然而，反映出自然系统的异质性，目前还没有关于分子聚集体的原子分辨率详细结构信息。在这里，我们通过偏振分辨的单颗粒荧光激发光谱学和理论建模，比较了来自嗜热绿杆菌的野生型和两种突变体的菌光体的结果，与以前通过核磁共振光谱学和冷冻电子显微镜获得的结果进行了比较。只有结合所有这些技术获得的信息，才能对菌光体中细菌叶绿素的分子堆积情况进行明确的描述。与文献中的一些建议相反，我们发现，对于野生型和突变型的菌光体，占主导地位的二级结构元件特征是管状对称，遵循非常相似的构建原理。此外，研究结果表明，各种细菌叶绿素分子的甲基化选择，是野生型菌光体样品结构（和光谱）异质性的主要来源，被自然界利用来实现单个菌光体水平的光谱覆盖的改善。
