When ferredoxin is the reductant of cytochrome b-563 in the chloroplast cytochrome b-563/f complex, the reaction rate varies with pH in a manner which permits identification of a plastoquinone/plastosemiquinone couple as an intermediate in the reaction. The requirement for the inhibitor, DNP-INT, in this reaction, suggests that there are two sites in the complex where plastosemiquinone can react with cytochrome(s) b-563. The ineffectiveness of DBMIB in promotion of ferredoxin-mediated cytochrome b-563 reduction has led to a model where DBMIB and DNP-INT inhibit different electron transfer steps at the same, or overlapping, sites in the complex.
