Three-Dimensional Structure of Photosystem I

Abstract

In water oxidising photosynthesis, the primary processes responsible for the conversion of the energy of visible light into chemical energy take place at two multi-subunit protein pigment complexes, the photosystems I and II (PSI and PSH), which are embedded in the thylakoid membrane. PSI belongs to the larger group of type-I photosynthetic reaction centres (RC), characterised by iron sulphur clusters as intrinsic terminal electron acceptors, whereas PSII is a representative of the type-II RC, which possess quinone cofactors as their terminal acceptors [1]. The structures of the type-II RC from the purple bacteria Rhodopseudomonas viridis [2,3] and Rhodobacter sphaeroides [4,5,6,7,8] are well investigated at resolutions of 3 A or better. Because of the known similarities between these reaction centres (PbRC) and PSII their structures were supposed to be of relevance for the PSII structure [9]. In contrast, the crystallographic analysis of PSI isolated from the cyanobacterium Synechococcus elongatus at 4 A resolution [10,11] provides the most detailed structural information about a type-I reaction centre up to now.