title: 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.
abstract: 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 X 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.
mesh_terms: D002736:Chloroplasts / Q000187:drug effects / Q000378:metabolism*; D003573:Cytochrome b Group / Q000037:antagonists & inhibitors / Q000378:metabolism*; D045346:Cytochrome b6f Complex; D003991:Dibromothymoquinone / Q000378:metabolism* / Q000494:pharmacology; D004579:Electron Transport / Q000187:drug effects; D007425:Intracellular Membranes / Q000378:metabolism; D010084:Oxidation-Reduction / Q000187:drug effects; D010944:Plants; D011809:Quinones / Q000378:metabolism*
chemical_list: D003573:Cytochrome b Group; D011809:Quinones; D003991:Dibromothymoquinone; D045346:Cytochrome b6f Complex