Photosystem I protein complex (PSI)

Photosystem I protein complex is a large multisubunit membrane-bound complex present in the thylakoid membranes of photosynthetic organisms[3][6][7][1]. It typically consists of 12–17 core protein subunits (including PsaA, PsaB, PsaC, PsaD, among others) and is associated with numerous non-covalently bound cofactors: chlorophylls, carotenoids, iron–sulfur clusters, phylloquinones, and lipids[3][4][6][5][8]. PSI functions as a sunlight-driven electron conductor, transferring electrons from plastocyanin (or cytochrome c6) on the luminal side to ferredoxin or flavodoxin at the stromal side through a chain of electron carriers[2][4][6]. This electron flow provides the reducing power needed for carbon fixation in the Calvin cycle, making PSI central to energy conversion in photosynthesis[7][2][4][9]. The complex also helps generate a proton-motive force across the membrane, contributing to ATP synthesis[7]. PSI is extremely efficient at harvesting solar energy, approaching nearly 100% quantum efficiency in energy conversion[1][7]. It is not a therapeutic target but essential for the survival of plants, algae, and cyanobacteria. Disruption of PSI leads to decreased photosynthetic efficiency and impaired growth in these organisms.

Other names

Photosystem IPSIplastocyanin–ferredoxin oxidoreductase