Molecular aspects of photosystem I

Photosystem I(PSI) in higher plants consists of 17 polypeptide subunits. Cofactors are chlorophyll a and b, beta-carotene, phylloquinone and iron-sulfur clusters. Eight subunits are specific for higher plants while the remaining ones are also present in cyanobacteria, Two 80-kDa subunits (PSI-A and -B) constitute the major part of PSI and bind most of the pigments and electron donors and accepters. The 9-kDa PSI-C carries the remaining electron accepters which are [4Fe-4S] iron sulfur clusters. PSI-D, -E and -H have importance for integrity and function at the stromal face of PSI while PSI-F has importance for function at the lumenal face. PSI-N is localized at the lumenal side, but its function is unknown. Four subunits are light-harvesting chlorophyll alb-binding proteins. The remaining subunits are integral membrane proteins with poorly understood function. Subunit interactions have been studied in reconstitution experiments and by cross-linking studies. Based on these data, it is concluded that iron-sulfur cluster F-B is proximal to F-X and that F-A is the terminal acceptor in PSI. Similarities between PSI and the reaction center from green sulfur bacteria are discussed.