Changes in Cytochrome b559 During Photoinhibition

Cytochrome b559 (Cyt b559) is a well-known intrinsic component of the photosystem II (PSH) reaction center (RC) in all oxygen-evolving photosynthetic organisms. Although neither its structure nor its organization in PSII are known, it has been proposed that Cyt b559 is a heme-bridged protein heterodimer with two subunits, a and [3, of 9 and 4 kDa, respectively [1]. An important characteristic of this cytochrome is that it may exhibit different redox potential forms. Usually, Cyt b559 is present in two interconvertible redox forms: a pH-independent high-potential (HP) form (E ‘m = +380 mV) and a pH-dependent low-potential (LP) form (E ’ m = +140 mV, pH > 7.6) [2]. The HP form is labile and easily converted to the LP form by a variety of treatments which alter the structure of the membrane.