5.B.4.1.2
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).