InterPro domain: IPR035982

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.

Photosystem I (PSI) [ 1 ] is an integral membrane protein complex that uses light energy to mediate electron transfer from plastocyanin to ferredoxin. It is found in the chloroplasts of plants and cyanobacteria. PSI is composed of at least 14 different subunits, two of which, PSI-G (gene psaG) and PSI-K (gene psaK), are small hydrophobic proteins of about 7 to 9 Kd and evolutionary related [ 2 ]. Both seem to contain two transmembrane regions. Cyanobacteria contain only PSI-K.

1. Structure, function and organization of the Photosystem I reaction center complex. Biochim. Biophys. Acta 895, 167-204
2. The PSI-K subunit of photosystem I from barley (Hordeum vulgare L.). Evidence for a gene duplication of an ancestral PSI-G/K gene. J. Biol. Chem. 268, 18912-6