Quantitation of 2,5-dibromo-3-methyl-6-isopropyl-p-benzoquinone binding sites in chloroplast membranes: Evidence for a functional dimer of the cytochrome b₆f complex

The binding of 2,5-dibromo-3-methyl-6-isopropyl-p-benzoquinone (DBMIB) to chloroplast thylakoid membranes was investigated by analyzing the inhibition of electron transfer by DBMIB according to a steady-state rate relationship for enzyme-catalyzed reactions in the presence of tightly binding reversible inhibitors. DBMIB interacts with the cytochrome b₆f complex in a manner best described by an apparent dissociation constant near 6 nm. The binding site titer is 1 mmol · mol chlorophyll^-1. This number of DBMIB binding sites approaches one-half the number of cytochrome b₆f complexes present in the membrane. These data suggest that the cytochrome b₆f complex may function in electron transfer as a dimer, plastoquinol oxidation being totally inhibited by the binding of a single DBMIB molecule to the dimer.