THE ENIGMATIC CYTOCHROME-B-559 OF OXYGENIC PHOTOSYNTHESIS

The ubiquitous and obligatory association of cytochrome b-559 with the photosystem II reaction center of oxygenic photosynthesis is a conund rum since it seems not to have a function in the primary electron tran sport pathway of oxygen evolution. A model for the cytochrome structur e that satisfies the cis-positive rule for membrane protein assembly c onsists of two short, non-identical hydrophobic membrane-spanning poly peptides (alpha and beta), each containing a single histidine residue, as ligands for the bridging heme prosthetic group that is on the side of the membrane opposite to the water splitting apparatus. The abilit y of the heterodimer, but not the single a-subunit, to satisfy the cis -positive rule implies that the cytochrome inserts into the membrane a s a heterodimer, with some evidence implicating it as the first membra ne-inserted unit of the assembling reaction center. The very positive redox potential of the cytochrome can be explained by a position for t he heme in a hydrophobic niche near the stromal aqueous interface wher e it is also influenced by the large positive dipole potential of the parallel a-helices of the cytochrome. The requirement for the cytochro me in oxygenic photosynthesis may be a consequence of the presence of the strongly oxidizing reaction center needed for H2O-splitting. This may lead to the need, under conditions of stress or plastid developmen t, for an alternate source of electrons when the H2O-splitting system is not operative as a source of reductant for the reaction center.