In oxygenic photosynthesis, photosystem I (PSI) drives electron transfer from plastocyanin or cytochrome c to ferredoxin. PSI forms a core complex that consists of 11–14 subunits and associates more than one hundred cofactors such as chlorophyll a, carotenes, naphthoquinones, lipids, and iron-sulfur (Fe-S) clusters. PSI complex is embedded in the thylakoid membranes and harvests light using antenna pigments, transfers excitons from antenna to reaction center, where photochemical reaction occurs, and stabilizes the charge separation to allow for oxidation of plastocyanin or cytochrome c and reduction of ferredoxin. PSI core complex forms a trimer in cyanobacteria but associates light-harvesting complexes I (LHCI) to form a PSI-LHCI supercomplex in plants and algae. The crystal structures of PSI core trimer and PSI-LHCI supercomplex have revealed that the subunits and cofactors of PSI are organized to optimize the efficiency of light harvesting and photochemical reaction. However it remains elusive how these components of PSI are integrated into a functional structure in photosynthetic organisms.
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