Young Scientist Forum

A plant Photosystem I (PSI) is a large membrane super-complex that drives photosynthesis. PSI captures sunlight through sophisticated pigment network and uses the energy to perform transmembrane electron transfer. It consists of the reaction center complex (RC), where the charge separation reaction takes place and the light harvesting complex (LHCI), which serves as an additional antenna system. PSI performs a photochemical activity with the unprecedented quantum yield of close to 100%, being the most efficient light capturing and energy conversion machine. We determined the X-ray crystal structure of the intact PSI from plants at 3.4 Å resolution (1,2). The current crystal structure provides a picture at near atomic detail of 17 protein subunits and shows how the biological significance of plant PSI is matched by its structural elements. 3038 amino acids were assigned, as well as 168 chlorophylls, two phyloquinones, three Fe4S4 clusters and five carotenoids. The final model consists of not less than 45 transmembrane helices and represents one of the most complicated membrane complexes for which a near atomic model was determined. The remarkable feature of PSI is the unprecedented high content of non-protein components – approximately one third of the total mass of about 600 kDa consists of different cofactors. The structure reveals intriguing insights regarding unique interactions between the RC and the LHCI complexes and provides a structural basis for the state transitions phenomenon. In addition, putative docking sites of the soluble electron carriers are described for the first time. References: 1. Amunts, Drory and Nelson. Nature 2007. 2. Amunts and Nelson. Structure 2009.