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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).

Photosystem I of Arabidopsis thaliana
(PsaA-L; Lhca 1-4)
PsaA (chlorophyll a apoprotein; 750 aas; 7 TMSs) (P56766)
PsaB (chlorophyll a apoprotein; 734 aas; 11-12 TMSs) (P56767)
PsaC (iron sulfur center protein) (P62090)
PsaD (Reaction Center Subunit II) (Q9SA56)
PsaE (Reaction Center Subunit IV) (Q9S714)
PsaF (Reaction Center Subunit III) (Q9SUI8)
PsaG (Reaction Center Subunit V) (Q9S7N7)
PsaH (Reaction Center Subunit VI) (Q9SUI6)
PsaI (Reaction Center Subunit VIII) (P56768)
PsaJ (Reaction Center Subunit IX) (P56769)
PsaK (Reaction Center Subunit X) (Q9SUI5)
PsaL (Reaction Center Subunit XI) (Q9SUI4)
Lhca1 (225 aas; 1-2 TMSs) (ABD37878)
Lhca2 (257 aas; 1-2 TMSs) (Q9SYW8)
Lhca3 (273 aas; 1-2 TMSs) (Q43381)
Lhca4 (244 aas; 2-4 TMSs) (Q6YWJ7)

Photosystem I reaction center from a chlorophyll d-containing cyanobacterium, Acaryochloris marina (Xu et al. 2021). Photosystem I (PSI) is a large protein supercomplex that catalyzes the light-dependent oxidation of plastocyanin (or cytochrome c6 ) and the reduction of ferredoxin. This catalytic reaction is realized by a transmembrane electron transfer chain consisting of a primary electron donor (a special chlorophyll (Chl) pair) and electron acceptors A0 , A1 , and three Fe4 S4 clusters, FX , FA , and FB. Xu et al. 2021 reported the PSI structure from a Chl d-dominated cyanobacterium Acaryochloris marina at 3.3 Å resolution obtained by single-particle cryo-electron microscopy. The A. marina PSI exists as a trimer with three identical monomers. The structure reveals a unique composition of electron transfer chain proteins in which the primary electron acceptor, A0, is composed of two pheophytins a rather than Chl a found in other well-known PSI structures. A novel subunit Psa27 is observed in the A. marina PSI structure. In addition, 77 Chls, 13 alpha-carotenes, two phylloquinones, three Fe-S clusters, two phosphatidyl glycerols, and one monogalactosyl-diglyceride were identified in each PSI monomer. This provides a structural basis for deciphering the mechanism of photosynthesis in a PSI complex with Chl d as the dominating pigment, absorbing far-red light (Xu et al. 2021).

Terrabacteria group
PSI of Acaryochloris marina

PsaA, 753 aas and 11 TMSs, B0C474
PsaB, 736 aas and 11 TMSs, B0C475
PsaC, 81 aas and probably 0 TMSs, B0CB42
PsaD, 139 aas and 0 TMSs, B0C8F1
PsaE, 89 aas and 0 TMSs, B0C5D5
PsaF, 167 aas and 3 TMSs in a 1 + 2 TMS arrangement, B0C7S7
PsaJ, 51 aas and 1 TMS, B0C7S6
PsaK, 86 aas and 1 TMS, B0CA71
PsaL, 153 aas and 2 C-terminal TMSs
PsaM, 31 aas and 1 TMS