X-Ray Crystallographic Structure Analysis of Cyanobacterial Photosystem I at 2.5 A Resolution

Abstract

Photosystem I isolated from the thermophilic cyanobacterium Synechococcus elongatus was crystallized in a form suitable for X-ray structure analysis at 2.5 A resolution, resulting in a structural model at atomic detail which forms the basis for understanding the numerous interactions between 12 protein subunits, 96 chlorophyll, 22 carotenoid, two phylloquinone, four lipid molecules and three [4Fe4S] clusters. Six chlorophylls and two phylloquinones involved in transmembrane charge separation are arranged in two branches of pronounced twofold pseudo-symmetry, most significantly broken at the primary donor P700 consisting of a chlorophylla/chlorophylla' heterodimer with an asymmetric hydrogen-bonding environment. The dominating ligands to the central Mg2+ ions of the chlorophylls are histidine side chains, but other ligands not observed in the structures of chlorophyll-binding proteins so far, among these methionine side chains and the phosphodiester group of a phospholipid, have also been found in the structure. Each of the carotenoids (17 in all-trans- and five in different cis-configurations) is in close contact with at least one chlorophyll, in agreement with their two major functions in photoprotection and light-harvesting.