Concerning the enigmatic cytochrome b-559 of oxygenic photosynthesis

Although there is an extensive literature on the properties and possible electron transfer pathways of cytochrome b-559, which is a prominent subunit of the multi-subunit photosystem II complex which functions in oxygenic photosynthesis, there is presently no consensus on the function of b-559 in the photosynthetic electron transport chain. The inability in earlier times to define a redox-linked function of this cytochrome was, to a large extent, a consequence of an absence of biochemical and structure information to complement an extensive array of spectrophotometric studies of the cytochrome in situ. Based on the location of hetero-dimeric b-559 in the photosystem II reaction center complex, derived from crystal crystallographic structure analysis, and the absence of a necessary redox function for the cytochrome in PSII, it is proposed that the main function of cytochrome b-559 is linked to its role as a structure component in the PSII reaction center complex. This function resides in the association of b-559 through its heme histidine residues in the trans-membrane domains of the PsbE and PsbF subunits of the PSII reaction center. These subunits, along with PsbJ, are inferred, from the analysis of structure, to define the intra-membrane portal in the PSII reaction center for plastoquinol (PQH₂) export which, through the PSII complex, provides the redox link to the cytochrome b₆f complex in the electron transfer chain.