5.B.4.1.1
The plant photosystem I (PSI) supercomplex at 3.4 Å resolution (Amunts et al., 2007). It contains 4 light harvesting chlorophyll a/b binding proteins as well as 13 additional constituents. One helix (TMS) proteins, OHP1 (O81208) amd OHP2 (Q9FEC1) play an essential role in the assembly or stabilization of photosynthetic pigment-protein complexes, especially photosystem reaction centers, in the thylakoid membrane (Beck et al. 2017). PSI consists of two complexes, a reaction center and light-harvesting complex (LHC), which together form the PSI-LHC supercomplex. The crystal structure of plant PSI has been solved with two distinct crystal forms. The first, crystallized at pH 6.5, exhibited P21 symmetry; the second, crystallized at pH 8.5, exhibited P212121 symmetry. The surfaces involved in binding plastocyanin and ferredoxin were identical in both forms. The crystal structure at 2.6 Å resolution revealed 16 subunits, 45 transmembrane helices, and 232 prosthetic groups, including 143 chlorophyll a, 13 chlorophyll b, 27 beta-carotene, 7 lutein, 2 xanthophyll, 1 zeaxanthin, 20 monogalactosyl diglyceride, 7 phosphatidyl diglyceride, 5 digalactosyl diglyceride, 2 calcium ions, 2 phylloquinone, and 3 iron sulfur clusters (Caspy and Nelson 2018). The model revealed detailed interactions, providing mechanisms for excitation energy transfer and its modulation in one of nature's most efficient photochemical machine. The photoexcitation response of cyanobacterial Photosystem I has been studied following reconstitution in proteoliposomes (Niroomand et al. 2017).  The structure of the red algal phycobilisome-PSII-PSI-LHC megacomplex has been determined (You et al. 2023). In oxygenic photosynthetic organisms, light energy is captured by antenna systems and transferred to photosystem II (PSII) and photosystem I (PSI) to drive photosynthesis. The antenna systems of red algae consist of soluble phycobilisomes (PBSs) and transmembrane light-harvesting complexes (LHCs). Excitation energy transfer pathways from PBS to photosystems remain unclear owing to the lack of structural information. You et al. 2023 presented in situ structures of PBS-PSII-PSI-LHC megacomplexes from the red alga Porphyridium purpureum at near-atomic resolution using cryoEM and in situ single-particle analysis, providing interaction details between PBS, PSII and PSI. The structures reveal several unidentified and incomplete proteins and their roles in the assembly of the megacomplex, as well as a huge and sophisticated pigment network, providing a solid structural basis for unravelling the mechanisms of PBS-PSII-PSI-LHC megacomplex assembly, efficient energy transfer from PBS to the two photosystems, and regulation of energy distribution between PSII and PSI (You et al. 2023).

---