Photosystem I (PSI)

Photosystem I is a highly complex, multi-subunit protein–pigment supercomplex embedded in the membranes of photosynthetic organisms, including cyanobacteria, algae, and plants[5][1][2][3][7]. It consists of a core reaction center made primarily of the PsaA and PsaB subunits and is surrounded by several auxiliary subunits for structure and regulation. In higher plants, PSI integrates with a light-harvesting complex (LHCI) to increase the efficiency of solar energy capture and adaptation to diverse light environments[1][3][2]. The complex contains more than 100 cofactors, including chlorophylls, carotenoids, phylloquinones, and iron-sulfur clusters which are essential for its function in driving electron transport and participating in downstream photochemical reactions[1][3][7]. The electrons energized by light are transferred from plastocyanin via PSI to ferredoxin, and eventually used to reduce NADP+ to NADPH, fueling essential biosynthetic pathways and the final steps of the photosynthetic light reactions[5][1][2][7]. PSI is fundamental for sustaining solar-powered metabolism and carbon fixation in oxygenic phototrophs[5][1][3][6][7]. Key recent structural insights have revealed the arrangement of subunits and cofactors, unveiling mechanisms of energy transfer and adaptation to various stressors such as extreme light conditions[4][1][3]. PSI evolution has been studied in both thylakoid-membrane and thylakoid-free cyanobacteria, illuminating pathways of photosynthetic diversification[6]. It has no direct relevance as a “target” in human medicine, but it is an essential molecular machine for life on Earth and the study of biochemistry and bioenergetics.

Other names

Plastocyanin–ferredoxin oxidoreductasePSI