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

Abstract

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 Chl a and three iron-sulphur centres, F X, F A and F B. The first pair, a Chl a dimer, has been assigned to the primary electron donor P700. One or both Chl a of the second pair, eC 2 and eC 2′ , presumably functionally link P700 to the corresponding Chl a of the third pair, eC 3 and eC 3′ , which is assumed to constitute the spectroscopically-identified primary electron acceptor(s), A 0, of PSI. A likely location of the subsequent phylloquinone electron acceptor, Q K, in relation to the properties of the spectroscopically identified electron acceptor A 1 is discussed. The positions of a total of 89 Chl a, 83 of which constitute the core antenna system, are presented. The maximal centre-to-centre distance between antenna Chl a is ⩽16 Å; 81 Chl a are grouped into four clusters comprising 21, 23, 17 and 20 Chl a, respectively. Two “connecting” Chl a are positioned to structurally (and possibly functionally) link the Chl a of the core antenna to those of the electron transfer system. Thus the second and third Chl a 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 Chl a of the electron transfer system discussed.