Cytochrome b559 and cyclic electron transfer within photosystem II

Cytochrome b559 (Cyt b559), β-carotene (Car), and chlorophyll (Chl) cofactors participate in the secondary electron-transfer pathways in photosystem II (PSII), which are believed to protect PSII from photodamage under conditions in which the primary electron-donation pathway leading to water oxidation is inhibited. Among these cofactors, Cyt b559 is preferentially photooxidized under conditions in which the primary electron-donation pathway is blocked. When Cyt b559 is preoxidized, the photooxidation of several of the 11 Car and 35 Chl molecules present per PSII is observed. In this review, the discovery of the secondary electron donors, their structures and electron-transfer properties, and progress in the characterization of the secondary electron-transfer pathways are discussed. This article is part of a Special Issue entitled: Photosystem II.

► Cytochrome b559, b-carotenes, and chlorophylls are secondary donors in photosystem II. ► Cytochrome b559 is also photoreduced in photosystem II, forming a cyclic electron-transfer pathway. ► Cyclic electron transfer is believed to protect photosystem II from photodamage. ► The discovery and progress in characterization of the secondary donors in photosystem II are discussed.