A New Value for the Redox Potential of Cytochrome c550 in Photosystem II from Thermosynechococcus Elongatus

Abstract

Cytochrome c550 (cyt c550), which is one of the extrinsic proteins of photosystem II (PSII), is only present in cyanobacteria and red algae. Although this cytochrome has been reported to stabilize the binding of Ca²⁺ and Cl⁻ ions, which are essential for activity of PSII, the specific function of heme is not yet clear. The reported negative values of the midpoint redox potential (E _m) of cyt c550 (−300 mV in the soluble state and −80 mV when associated with PSII) appear to be incompatible with a redox function in PSII. It has been reported that the E _m of Q_A in PSII-enriched membranes was affected by the presence of redox mediators at low ambient potentials. We have carried out new measurements of E _m of cyt c550 associated to PSII changing the type and number of redox mediators used. We have determined that the E _m of cyt c550 is about +200 mV in the absence of mediators or in the presence of a very limited number of mediators. Our results suggest that the highly reducing conditions reached in the presence of mediators, favor the reduction of a PSII component, most likely the Mn cluster, thereby inducing alterations in protein, the heme environment and consequently the E _m of the heme. The new value of E _m of cyt c550 opens the possibility of a redox function for this protein.