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



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



Journal of Molecular Biology 272(5): 741-769



An improved structural model of the photosystem I complex from the thermophilic cyanobacterium Synechococcus elongatus is described at 4 ANG 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 apprx -1 ANG . 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, eC-2 and eC'2, presumably functionally link P700 to the corresponding Chla 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 Chla, 83 of which constitute the core antenna system, are presented. The maximal centre-to-centre distance between antenna Chla is ltoreq 16 ANG ; 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 alpha-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") alpha-helices of each of the large core subunits PsaA and PsaB is described. The assignment of the amino acid sequence to the transmembrane alpha-helices is proposed and likely residues involved in coordinating the Chla of the electron transfer system discussed.

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Accession: 009192179

Download citation: RISBibTeXText

PMID: 9368655

DOI: 10.1006/jmbi.1997.1269


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