Photosystem I reaction centre subunit VIII (IPR001302)

Short name: PSI_PsaI

Overlapping homologous superfamilies

Family relationships



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 [PMID: 11418848]. 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 [PMID: 7608190]. 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 [PMID: 8262256].

GO terms

Biological Process

GO:0015979 photosynthesis

Molecular Function

No terms assigned in this category.

Cellular Component

GO:0009522 photosystem I

Contributing signatures

Signatures from InterPro member databases are used to construct an entry.