Photosystem I at 4 angstrom resolution represents the first structural model of a joint photosynthetic reaction centre and core antenna, system

The 4 Angstrom X-ray structure model of trimeric photosystem I of the cyanobacterium Synechococcus elongatus reveals 31 transmembrane, nine surface and three stromal alpha-helices per monomer, assigned to the 11 protein subunits: PsaA and PsaB are related by a pseudo two-fold axis normal to the membrane plane, along which the electron transfer pigments are arranged, 65 antenna chlorophyll a (Chi a) molecules separated by less than or equal to 16 Angstrom form an oval, clustered net, continuous with the electron transfer chain through the second and third Chi a pairs of the electron transfer system, This suggests a dual role for these Chi a both in excitation energy and electron transfer, The architecture of the protein core indicates quinone and iron-sulphur type reaction centres to have a common ancestor.