Factors determining the special redox properties of photosynthetic cytochrome b559
2001
Roncel, Mercedes | Ortega, José M. | Losada, Manuel
Factors controlling the redox properties of the two conventional forms of cytochrome b559, i.e. the unstable highâpotential form and the stable lowâpotential form, have been further investigated using PSIIâenriched membranes from pea and spinach chloroplasts. The redox potential of the stable form of cytochrome b559 is pH independent both above pHâ7.5 (E′mâ≈â+110âmV) and below pHâ6.0 (E′mâ≈+203âmV), but it changes with a slope of 58âmV per pH unit between these two pH values. Thus, cytochrome b559 seems to have a single ionizing group influencing its redox potential, with a higher affinity for protons in the reduced form (pKredâ=â7.5) and a lower affinity in the oxidized form (pKoxâ=â6.0); consequently, one unprotonated lowâpotential form (LP) and one protonated intermediateâpotential form (IP). The redox potential of the highâpotential form (HP) is pHâindependent between pHâ5.0 and 8.0, but its relative content (compared to the total amount of protein) decreases progressively above pHâ7.0. This conversion to the stable LP form is interpreted as corresponding to the loss of a proton by one ionizing group, the protonation of which is essential for maintaining the unstable HP state. According to chemical modification experiments with diethylpyrocarbonate, one of the two histidine ligands of the heme seems to be the ionizing group responsible for the existence of both the protonated IP and HP forms. It is proposed that the difference between the IP and HP forms is due to the formation of an additional hydrogen bond between the protonated histidine and the protein in the HP state that stabilizes a special hydrophobic heme environment responsible for its high redox potential.
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