Sites of cytochromeb‐563 reduction, and the mode of action of DNP‐INT and DBMIB in the chloroplast cytochromeb‐563/fcomplex

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Abstract
When ferredoxin is the reductant of cytochromeb‐563 in the chloroplast cytochromeb‐563/fcomplex, 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 cytochromeb‐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.