Photosystem I Electron Transfer Reactions–Components and Kinetics

All oxygenic photosynthetic organisms contain a membrane protein complex, known as Photosystem I, that catalyzes a light-induced transfer of electrons from reduced plastocyanin to ferredoxin. This complex contains several bound electron carriers that are involved in the initial charge separation and stabilization. These carriers include the reaction center chlorophyll, P700, which undergoes oxidation in the light, and a number of compounds that serve as electron carriers in what is now believed to be a sequence of electron transfer events: a monomeric chlorophyll a molecule is the initial electron acceptor, a phylloquinone molecule is the second electron acceptor and a series of bound iron-sulfur clusters then function as terminal electron acceptors. The properties of these electron carriers and recent evidence supporting their proposed roles are described and an attempt is made to present a unified picture of our understanding of the role of these individual components. The kinetic sequence of the electron transfer events is then considered in detail. After the discussion of these bound electron carriers, the reduction of ferredoxin by the Photosystem I complex is described and the role of this soluble electron carrier in catalyzing a cyclic transfer of electrons around Photosystem I is considered.