, Volume 221, Issue 1, pp 5-8
Date: 22 Mar 2005

Photoinhibition of photosystem I

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Photosystem I (PSI) is a large pigment-protein complex consisting of about 18 different subunits in plants. Recently, the structure of PSI isolated from pea was solved by X-ray crystallography at a resolution of 4.4 Å (Ben-Shem et al. 2003). This work has highlighted the structural similarities and differences between plant PSI and cyanobacterial PSI, where a 2.5 Å structure was published earlier (Jordan et al. 2001). The subunits of PSI are traditionally divided into 14 core subunits—most of which are also found in cyanobacteria—and four homologous light harvesting complex I (LHCI) subunits, which are specific to plants (Fig. 1). Under certain conditions, other LHCI subunits can be seen—at least in Arabidopsis (Klimmek et al. 2005). The PSI complex binds about 167 chlorophyll molecules (Ben-shem et al. 2003). A special pair of chlorophyll a molecules constitutes the P700 reaction center and another chlorophyll a molecule functions as the primary acceptor A0. The secondary acceptor, A1