InterPro domain: IPR036357

Oxygenic photosynthesis uses two multi-subunit photosystems (I and II) located in the cell membranes of cyanobacteria and in the thylakoid membranes of chloroplasts in plants and algae. Photosystem II (PSII) has a P680 reaction centre containing chlorophyll 'a' that uses light energy to carry out the oxidation (splitting) of water molecules, and to produce ATP via a proton pump. Photosystem I (PSI) has a P700 reaction centre containing chlorophyll that takes the electron and associated hydrogen donated from PSII to reduce NADP+ to NADPH. Both ATP and NADPH are subsequently used in the light-independent reactions to convert carbon dioxide to glucose using the hydrogen atom extracted from water by PSII, releasing oxygen as a by-product.

This entry represents subunit VIII (PsaI) of the photosystem I (PSI) reaction centre. PSI is located, along with photosystem II (PSII), in the thylakoid photosynthetic membranes of plants, green algae and cyanobacteria. The crystal structure of PSI from the thermophilic cyanobacterium Synechococcus elongatus (Thermosynechococcus elongatus) has 12 protein subunits and 127 cofactors comprising 96 chlorophylls, 2 phylloquinones, 3 4Fe4S clusters, 22 carotenoids, 4 lipids, and a putative calcium ion [ 1 ]. PsaI consists of a single transmembrane helix, and has a crucial role in aiding normal structural organisation of PsaL within the PSI complex and the absence of PsaI alters PsaL organisation, leading to a small, but physiologically significant, defect in PSI function [ 2 ]. PsaL encodes a subunit of PSI and is necessary for trimerisation of PSI. PsaL may constitute the trimer-forming domain in the structure of PSI [ 3 ].

1. Three-dimensional structure of cyanobacterial photosystem I at 2.5 A resolution. Nature 411, 909-17
2. Mutational analysis of photosystem I polypeptides in the cyanobacterium Synechocystis sp. PCC 6803. Targeted inactivation of psaI reveals the function of psaI in the structural organization of psaL. J. Biol. Chem. 270, 16243-50
3. PsaL subunit is required for the formation of photosystem I trimers in the cyanobacterium Synechocystis sp. PCC 6803. FEBS Lett. 336, 330-4