Department of Physics, Concordia University, Montreal, H4B 1R6, Canada.
School of Molecular Sciences and Biodesign Center for Applied Structural Discovery, Arizona State University, Tempe, Arizona 85287, United States.
J Phys Chem B. 2022 Sep 15;126(36):6891-6910. doi: 10.1021/acs.jpcb.2c04829. Epub 2022 Sep 5.
The PSI-IsiA supercomplex is one of the largest and most complicated assemblies in photosynthesis. The IsiA ring, composed of 18 IsiA monomers (IsiA) surrounding the PSI trimer (PSI), forms under iron-deficient conditions in cyanobacteria and acts as a peripheral antenna. Based on the supercomplex structure recently determined via cryo-EM imaging, we model various optical spectra of the IsiA monomers and IsiA ring. Comparison of the absorption and emission spectra of the isolated IsiA monomers and the full ring reveals that about 2.7 chlorophylls (Chls) are lost in the isolated IsiA monomers. The best fits for isolated monomers spectra are obtained assuming the absence of Chl 508 and Chl 517 and 70% loss of Chl 511. The best model describing all three hexamers and the entire ring suggests that the lowest energy pigments are Chls 511, 514, and 517. Based on the modeling results presented in this work, we conclude that there are most likely three entry points for EET from the IsiA hexamer to the PSI core monomer, with two of these entry points likely being located next to each other (i.e., nine entry points from IsiA to the PSI trimer). Finally, we show that excitation energy transfer inside individual monomers is fast (<2 ps at = 5 K) and at least 20 times faster than intermonomer energy transfer.
PSI-IsiA 超复合体是光合作用中最大、最复杂的组装体之一。IsiA 环由 18 个 IsiA 单体（IsiA）组成，围绕 PSI 三聚体（PSI）排列，在蓝细菌中铁缺乏条件下形成，并作为一个外围天线。基于最近通过冷冻电镜成像确定的超复合体结构，我们对 IsiA 单体和 IsiA 环的各种光学光谱进行建模。比较分离的 IsiA 单体和完整环的吸收和发射光谱表明，分离的 IsiA 单体中丢失了约 2.7 个叶绿素（Chls）。假设不存在 Chl 508 和 Chl 517 且 Chl 511 损失 70%，则可以对分离单体光谱进行最佳拟合。描述所有三个六聚体和整个环的最佳模型表明，最低能量的色素是 Chls 511、514 和 517。基于本工作中提出的建模结果，我们得出结论，很可能有三个从 IsiA 六聚体到 PSI 核心单体的 EET 入口点，其中两个入口点可能彼此相邻（即，从 IsiA 到 PSI 三聚体有九个入口点）。最后，我们表明单个单体内部的激发能量转移很快（在 = 5 K 时<2 ps），并且至少比单体间能量转移快 20 倍。
