Towards an understanding of redox heterogeneity of the photosystem II cytochrome b559 in the native membrane

A complex redox titration pattern of cytochrome (Cyt) b559 in preparations of thylakoid membranes and photosystem (PS) II membrane fragments is commonly attributed to the presence of three conformational forms differing by a structure of the heme microenvironment. However, despite decades of research, structural determinants underlying differences between the redox forms of Cyt b559 have not been defined. In this work, we propose a different interpretation of redox heterogeneity in the native population of Cyt b559 assuming redox interaction between the Cyt b559 heme group and a nearby bound quinone (Q). The interacting quinone is supposed to be plastoquinone Q(C) present in the unusual singly protonated form (Q(C)H). The model successfully explains the unique redox properties of Cyt b559 and may provide a simple and effective mechanism of redox regulation of secondary electron transport in PS II. At the present time, the model of heme-quinone redox interaction can be considered as an alternative to the idea of conformational differences between the native redox forms of Cyt b559.