Introduction to Photosystem I: Reaction Center Function, Composition and Structure

Abstract

The Photosystem I (PS I) complex in cyanobacteria functions most typically as a light-driven, cytochrome c6:ferredoxin oxidoreductase. The adaptability of cyanobacteria to conditions of nutrient availability allows cytochrome c6 to be replaced by plastocyanin when copper is plentiful, and ferredoxin to be replaced by flavodoxin when iron is limiting. These changes, however, do not lead to any known alterations in the polypeptide composition of the membrane-bound PS I complex. This multiprotein complex incorporates all of the biochemical machinery required to produce efficient charge separation across the thylakoid membrane in a process that culminates in the conversion of a red photon to chemical free energy. The membrane-bound components which comprise the complex include an array of ∼ 110 antenna chlorophyll a molecules to provide a large optical cross-section to incoming photons, a series of inorganic and organic cofactors to carry out the acts of charge separation and charge stabilization, and a matrix of eleven polypeptides to provide ligands to the photoactive components. These components are arranged in a motifbelieved to be shared by all photochemical reaction centers: in PS I a chlorophyll (a) dimer serves as the primary electron donor; a chlorophyll (a) monomer serves as the primary electron acceptor; and a quinone (phylloquinone) serves as the intermediate electron acceptor. Other common features include the presence of a protein (hetero)dimer (PsaA and PsaB), which binds the antenna chlorophylls, the electron donor and acceptor chlorophylls, and the two quinone molecules. This shared photochemical motif is broken by the inter-polypeptide iron-sulfur cluster FX, which occupies the same relative position as the non-heme iron in Type-II (quinone-type) reaction centers, but which is redox active in Type I (iron-sulfur type) reaction centers. The addition of two iron-sulfur clusters, FB and FA located on a separate polypeptide, PsaC, provides a path for the electrons out of the membrane phase and to the stromal phase, allowing ferredoxin to be reduced with high quantum efficiency. The other PS I polypeptides serve ancillary roles in stabilizing PsaC and docking ferredoxin or flavodoxin (PsaD), in enhancing ferredoxin reduction and allowing for cyclic electron flow (PsaE), and in forming trimers of the PS I complex in the membrane and facilitating state transitions (PsaL). The functions of the remaining polypeptides, PsaF, Psal, PsaJ, PsaK, and PsaM, are unclear; however it is increasingly unlikely that they participate directly in the primary processes of photochemical energy conversion.