Cytochrome b(559) and cyclic electron transfer within photosystem II

Cytochrome b(559) (Cyt b(559)), beta-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 b(559) is preferentially photooxidized under conditions in which the primary electron-donation pathway is blocked. When Cyt b(559) 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 (C) 2011 Elsevier B.V. All rights reserved.