Photosystem II

Abstract

Abstract Photosystem II (PSII) is a specialized protein complex that uses light energy to drive the transfer of electrons from water to plastoquinone, resulting in the production of oxygen and the release of reduced plastoquinone into the photosynthetic membrane. The key components of the PSII complex include a peripheral antenna system that employs chlorophyll and other pigment molecules to absorb light, a reaction centre at the core of the complex that is the site of the initial electron transfer reactions, an Mn 4 O x Ca cluster that catalyses water oxidation and a binding pocket for the reduction of plastoquinone. PSII is the sole source of oxygen production in all oxygenic photosynthetic organisms, which include plants, algae and cyanobacteria. In these organisms, PSII operates in series with other protein complexes, including the PSI reaction centre, to produce the reduced form of nicotenamide–adenine dinucleotide phosphate (NADPH) and adenosine triphosphate (ATP), which is used in the Calvin–Benson cycle to produce carbohydrates from carbon dioxide. Key concepts Photosystem II (PSII) is a membrane‐embedded protein–pigment complex, containing more than 20 subunits and approximately 100 cofactors. Antenna and reaction centre regions in PSII are in separate protein complexes. Light is absorbed by chlorophyll, carotenoid and phycobilin pigments in the antenna regions and the excitation energy is rapidly transferred to the reaction centre domain. PSII can switch among different modes to either utilize up to 90% of the incident light for charge separation (under low light conditions) or convert a large portion of the excess light into heat and light (fluorescence) (under high light conditions). The initial light‐induced charge separation results in the formation of a chlorophyll cation and a pheophytin anion which are approximately 10 Å apart; this charge separation is rapidly stabilized by transfer of the charges to other more distant cofactors. The oxidation of water occurs at an Mn 4 O x Ca cluster embedded in the protein environment of subunits D1 and CP43. To oxidize two molecules of water four oxidizing equivalents must be accumulated in the Mn 4 O x Ca cluster by four consecutive light‐induced charge separation(s). There are several conflicting proposals on the mechanism of water oxidation at the Mn 4 O x Ca cluster in PSII. The electrons and protons extracted from water by PSII are finally used to drive the reduction of NADP + and the production of ATP, respectively.