Part 1 - Introduction

Photosystem I (PSI),1 a membrane-bound multisubunit protein complex with a molecular mass of around 356 kDa, is located in the thylakoids of plants, algae, and cyanobacteria (1). It utilizes light energy to oxidize plastocyanin or cytochrome c and to reduce ferredoxin or flavodoxin. The monomeric PSI reaction center complex is composed of more than 11 different proteins, of which the PsaA and PsaB are functionally the most important since they bind the majority of the chlorophyll (Chl) and redox-active cofactors involved in energy conversion and charge separation. Recently a high resolution structure of PSI isolated from the thermophilic cyanobacterium <i>Synechococcus</i> <i>elongatus</i> has been obtained at 2.5 Å resolution (2). This work revealed the precise organization of the transmembrane helices of PsaA and PsaB proteins together with the redox active cofactors and Chls they bind. Surrounding these two proteins are other, smaller transmembrane PSI subunits of which some, namely PsaJ, PsaK, PsaL, PsaM, and PsaX, were also shown to directly or indirectly bind Chl a in the x-ray structure of cyanobacterial PSI and are likely to aid or regulate excitation energy transfer within the PSI complex. In addition, three stromally bound extrinsic proteins, PsaC, PsaD, and PsaE, optimize the binding of the mobile electron carriers ferredoxin or flavodoxin prior to reduction of NADP. with PsaC containing the iron-sulfur centers FA and FB (1).