title: Intraprotein transfer of the quinone analogue inhibitor 2,5-dibromo-3-methyl-6-isopropyl-p-benzoquinone in the cytochrome b6f complex.
abstract: Details are presented of the structural analysis of the cytochrome b(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 other p-side inhibitors previously resolved in cytochrome bc(1)/b(6)f complexes. This high-affinity site resides in a p-side interfacial niche bounded by cytochrome f, subunit IV, and cytochrome b(6), is close (8 A) to the p-side heme b, but distant (19 A) 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 A of the [2Fe-2S] cluster, as shown by the absence of inhibition in mutants of Synechococcus 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 Q(p) portal in the complex, it is seen that transmembrane traffic of quinone-like molecules through the core of cytochrome bc complexes can be labyrinthine.
authors.affiliation: Department of Biological Sciences, Purdue University, West Lafayette, IN 47907, USA.
authors.forename: Jiusheng
authors.initials: J
authors.lastname: Yan
authors.forename: Genji
authors.initials: G
authors.lastname: Kurisu
authors.forename: William A
authors.initials: WA
authors.lastname: Cramer
mesh_terms: D001665:Binding Sites / Q000235:genetics; D003461:Crystallography; D000458:Cyanobacteria / Q000737:chemistry*; D045346:Cytochrome b6f Complex / Q000737:chemistry* / Q000378:metabolism; D003991:Dibromothymoquinone / Q000737:chemistry* / Q000378:metabolism; D004563:Electrochemistry; D008027:Light; D008956:Models, Chemical; D008958:Models, Molecular; D021381:Protein Transport / Q000502:physiology; D013057:Spectrum Analysis
chemical_list: D003991:Dibromothymoquinone; D045346:Cytochrome b6f Complex