Photosystem I of Synechococcus elongatus at 4 Å resolution: Comprehensive structure analysis

An improved structural model of the photosystem I complex from the thermophilic cyanobacterium Synechococcus elongatus is described at 4 Å resolution. This represents the most complete model of a photosystem presently available, uniting both a photosynthetic reaction centre domain and a core antenna system. Most constituent elements of the electron transfer system have been located and their relative centre-to-centre distances determined at an accuracy of ~ 1 Å. These include three pseudosymmetric pairs of Chla and three iron-sulphur centres, F(X), F(A) and F(B). The first pair, a Chla dimer, has been assigned to the primary electron donor P700. One or both Chla of the second pair, eC₂ and eC'₂, presumably functionally link P700 to the corresponding Chla of the third pair, eC₃ and eC'₃, which is assumed to constitute the spectroscopically-identified primary electron acceptor(s), A₀, of PSI. A likely location of the subsequent phylloquinone electron acceptor, Q(K), in relation to the properties of the spectroscopically identified electron acceptor A₁ is discussed. The positions of a total of 89 Chla, 83 of which constitute the core antenna system, are presented. The maximal centre-to-centre distance between antenna Chla is ≤ 16 Å; 81 Chla are grouped into four clusters comprising 21, 23, 17 and 20 Chla, respectively. Two 'connecting' Chla are positioned to structurally (and possibly functionally) link the Chla of the core antenna to those of the electron transfer system. Thus the second and third Chla pairs of the electron transfer system may have a dual function both in energy transfer and electron transport. A total of 34 transmembrane and nine surface α-helices have been identified and assigned to the 11 subunits of the PSI complex. The connectivity of the nine C-terminal (seven transmembrane, two 'surface') α-helices of each of the large core subunits PsaA and PsaB is described. The assignment of the amino acid sequence to the transmembrane α-helices is proposed and likely residues involved in co-ordinating the Chla of the electron transfer system discussed.