Photosystem I

Photosystem I (PS I) is one of three major photo- synthetic energy transducing complexes found in thylakoids of higher plants, cyano- bacteria and green bacteria.  The complex traps light and converts it to reducing power to drive the ender- gonic reduction of NADP⁺ to NADPH.

Transport of Electrons by PS I

    At the core of PS I is a pair of chlorophyll a molecules ("the special pair") known as P700.  Upon absorption of a photon, P700 is excited to P700*, which is sufficiently reducing to transfer the electron to A₀, a chlorophyll monomer, in about 14 ps, resulting in the formation of P700⁺A₀^- .phylloquinone, known as A₁, then accepts the electron from the chlorophyll in 35 ps. A nearby iron-sulfur cluster, F_X, is reduced by the phylloquinone in about 200 ps . Iron-sulfur clusters, F_A and F_B, from an adjoining subunit accept the electrons in 170 ns. The electrons are then donated to a soluble Ferredoxin (Fd) which shuttles to FNR (ferredoxin:NADP+ reductase), which catalyzes the reduction of NADP⁺. The remaining P700⁺ is rereduced via plastocyanin with complex kinetics.

Structure of PSI

    PS I contains 11-13 polypeptides, depending on the plant species, and a variety of redox-active prosthetic groups. At the enzymatic heart of the complex are 3 subunits known as PsaA, PsaB, and PsaC.   The prosthetic groups, P700, A₀, A₁, and F_X, are liganded within the PsaA and PsaB proteins, while the F_A and F_B iron-sulfur clusters are found in the PsaC protein (). The placement of these groups is based on the low resolution (4.0 Å) structure of PSI by pulsed EPR experiments by analogy with the bacterial reaction center of purple photosynthetic bacteria and by pulsed EPR and by kinetics experiments. By cross-linking analysis, it was possible to determine the tentative positions of PsaA, PsaB, and other subunits of PS I . Further examination of the prosthetic groups and/or higher resolution structure will provide new insights into the dynamics of electron transfer and the structure of PS I. This proposal hopes to fill some gaps in the knowledge of PS I structure, particularly, the specific site of the PC docking.

PS I Sites of Interest