Side-Path Electron Donors: Cytochrome b559, Chlorophyll Z and β-Carotene

Abstract

β-Carotene (Car), cytochrome (Cyt) b559 and a monomeric chlorophyll (Chl) designated as chlorophyll Z, all undergo oxidation in Photosystem (PS) II under some illumination conditions. These components are not part of the direct electron transfer that leads to water oxidation and plastoquinone reduction and are thus designated ‘side-path electron donors.’ Under the usual conditions of PS II function, the quantum yield for the oxidation of these components is low; however, under certain experimental conditions, particularly low temperatures, the dominant reactions can be those involving the side-path donors. Car is a branch point in the side-path electron donation, being oxidized by P+ (the kinetically competent Chl cation radical), and reduced by Cyt b559, which is itself reduced by electrons from the pool of plastoquinol, possibly through the QB site. This all occurs on the D2-side of the reaction center. When the Cyt b559 is pre-oxidized, Car+ is reduced by Chl Z. There are two candidates for Chl Z, the more obvious candidate on the D2 side and the less straightforward candidate on D1 side of the reaction center. The side-pathway is usually rationalized as a photoprotective cycle aimed at removing long-lived P+ and thus limiting oxidative damage. Based on the low quantum yields, we consider this unlikely. Instead we suggest that the side-path constitutes a photoprotective cycle in which the aim is to reduce the Car cation, rather than P+, returning the carotene cation to its unoxidized state, preventing adventitious reactions and allowing it to play its a role as a singlet O2 quencher in the heart of PS II.