Structure of Photosystem I at 4.5 Å resolution: a short review including evolutionary aspects

Abstract

In photosynthesis of higher plants and cyanobacteria two photosystems are responsible for light induced charge separation. Photosystem II catalyses the electron transfer from water at the lumenal side to quinone at the stromal side, during this process oxygen is evolved. Photosystem I catalyses the electron transport from the soluble electron carrier plastocyanin or cytochrome c 6 at the lumenal side to ferredoxin at the stromal side of the membrane. Photosystem II shows homology to the reaction centre of purple bacteria in sequence as well as in content of electron carriers, yet purple bacteria are not able to use water as electron donor. Photosystem I has nearly no sequence homology to the reaction centre of purple bacteria, there are furthermore differences in the content of electron carriers and in the fact that Photosystem I carries its own antenna system of 90 chlorophyll a molecules. Green sulfur bacteria and heliobacteria are related to Photosystem I (for review on evolutionary relationship see Ref. [1 ]). The major part of Photosystem I is constituted by the two large subunits PsaA and PsaB ( = 83 kDa, each), carrying most of the electron transport chain: P700 (a Chl a dimer), A0 (a monomer of Chl a ), AI (phylloquinone/Vit K l ) and the first of the three [4Fe-4S] clusters F x. In addition, the 90 Chl a molecules of the antenna are bound by these subunits. Three subunits are