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Details are presented of the structural analysis of the cytochromeb 6 f complex from the thermophilic cyanobacterium,Mastigocladus laminosus , in the presence of the electrochemically positive (p )-side quinone analogue inhibitor, 2,5-dibromo-3-methyl-6-isopropylbenzoquinone (DBMIB). One DBMIB binding site was found. This site is peripheral to the quinone binding space defined by the binding sites of otherp -side inhibitors previously resolved in cytochromebc 1 /b 6 f complexes. This high-affinity site resides in ap -side interfacial niche bounded by cytochromef , subunit IV, and cytochromeb 6 , is close (8 Å) to thep -side hemeb , but distant (19 Å) from the [2Fe-2S] cluster. No significant electron density associated with the DBMIB was found elsewhere in the structure. However, the site at which DBMIB can inhibit light-induced redox turnover is within a few Å of the [2Fe-2S] cluster, as shown by the absence of inhibition in mutants ofSynechococcus sp. PCC 7002 at iron sulfur protein-Leu-111 near the cluster. The ability of a minimum amount of initially oxidized DBMIB to inhibit turnover of WT complex after a second light flash implies that there is a light-activated movement of DBMIB from the distal peripheral site to an inhibitory site proximal to the [2Fe-2S] cluster. Together with the necessary passage of quinone/quinol through the small Qp portal in the complex, it is seen that transmembrane traffic of quinone-like molecules through the core of cytochromebc complexes can be labyrinthine.

proceedings of the national academy of sciences of the united states of america

Intraprotein transfer of the quinone analogue inhibitor 2,5-dibromo-3-methyl-6-isopropyl- p -benzoquinone in the cytochrome b 6 f complex