Review Structural and functional organization of the peripheral light-harvesting system in Photosystem I Alexander N. Melkozernov* & Robert E. Blankenship Department of Chemistry and Biochemistry and Center for the Study of Early Events in Photosynthesis, Tempe, AZ 85287-1604, USA; * Author for correspondence (e-mail: alexander.melkozernov@asu.edu; fax: +1-480-965-2747) Received 1 August 2004; accepted in revised form 19 November 2004 Key words: chlorophyll, excitation energy transfer, green algae, higher plants, homology modeling, peripheral light-harvesting antenna, photosynthesis, Photosystem I core antenna, red pigments, three- dimensional structure Abstract This review centers on the structural and functional organization of the light-harvesting system in the peripheral antenna of Photosystem I (LHC I) and its energy coupling to the Photosystem I (PS I) core antenna network in view of recently available structural models of the eukaryotic Photosystem I–LHC I complex, eukaryotic LHC II complexes and the cyanobacterial Photosystem I core. A structural model based on the 3D homology of Lhca4 with LHC II is used for analysis of the principles of pigment arrangement in the LHC I peripheral antenna, for prediction of the protein ligands for the pigments that are unique for LHC I and for estimates of the excitonic coupling in strongly interacting pigment dimers. The presence of chlorophyll clusters with strong pigment–pigment interactions is a structural feature of PS I, resulting in the characteristic red-shifted fluorescence. Analysis of the interactions between the PS I core antenna and the peripheral antenna leads to the suggestion that the specific function of the red pigments is likely to be determined by their localization with respect to the reaction center. In the PS I core antenna, the Chl clusters with a different magnitude of low energy shift contribute to better spectral overlap of Chls in the reaction center and the Chls of the antenna network, concentrate the excitation around the reaction center and participate in downhill enhancement of energy transfer from LHC II to the PS I core. Chlo- rophyll clusters forming terminal emitters in LHC I are likely to be involved in photoprotection against excess energy. Abbreviations: 3D – three-dimensional; Chl – chlorophyll; LHC I – light-harvesting complex I; LHC I-730 – subpopulation of LHC I; Lhca1 and Lhca4 – subunits of the LHC I-730 heterodimer; PS I – Photosystem I; P 700 – primary electron donor in Photosystem I Introduction Photosystem I (PS I) is an important part of the photosynthetic machinery that catalyzes trans- membrane electron transfer via plastocyanin/fer- redoxin oxido-reductase activity and produces NADPH for CO 2 assimilation (Blankenship 2002). In PS I, redox active chlorophylls of the reaction center sensitize solar energy conversion, while the integral core and the peripheral light-harvesting antennas supply the reaction centers with the excitation energy via efficient transfer in the Chl antenna network (Melkozernov and Blankenship in press). PS I complexes from cyanobacteria, green algae and higher plants possess a red-shifted chlorophyll fluorescence, which is widely used as a Photosynthesis Research (2005) 85: 33–50 Ó Springer 2005