Enzymatic function of cytochrome b559 in photosystem II

Cytochrome b559 (cyt b559) is a heme-bridged protein heterodimer in photosystem II (PSII) of all oxygenic photosynthetic organisms. In spite of the fact that cyt b559 is strictly required for proper function of PSII, it is not involved in the linear electron transport chain from water to plastoquinone. Instead of that the participation of cyt b559 in the cyclic electron transport around PSII has been proposed mainly based on the ability of the heme iron to accept and donate an electron form the electron acceptor and to the electron donor side of PSII, respectively. In addition to the involvement of cyt b559 in the cyclic electron transport around PSII, several lines of evidence have been provided on the enzymatic function of cyt b559. The ability of oxygenic photosynthetic organisms to oxidize water and reduce plastoquinone is connected to the formation of reactive oxygen species (ROS) and thus required to develop an effective antioxidant defense system against ROS. The review attempts to summarize a recent progress on the role of cyt b559 as oxygen reductase, superoxide reductase, superoxide oxidase and plastoquinol oxidase. The focus is mainly given on the characterization of redox, redox potential and acid-base properties of the heme iron in the putative enzymatic cycles. The possible oxidase and reductase enzymatic activity of cyt b559 in protection from photoinhibition is discussed.