The Q<sub>o</sub>‐site inhibitor DBMIB favours the proximal position of the chloroplast Rieske protein and induces a p<i>K</i>‐shift of the redox‐linked proton

Most energy-transducing membranes contain a bc-type complex such as the cytochrome bc 1 complex or the cytochrome b 6 f complex. Members of this family couple electron transfer and proton translocation across the membrane. They also share some structural homologies, having three redox-active subunits in common: the Rieske iron-sulfur protein, a b-type cytochrome (cytochrome b or b 6 ), and a c-type cytochrome (cytochrome c 1 or f). Moreover, there is sequence homology between cytochrome b 6 , subunit IV, and the Rieske protein of the b 6 f complex, and the N-and C-terminal parts of cytochrome b, and the Rieske protein of the bc 1 complex, respectively. Cytochrome c 1 and cytochrome f, on the other hand, do not share any sequence homology. Spectroscopic and EPR properties of the hemes and of the iron-sulfur cluster are similar, although not identical, and some inhibitors affect both complexes equally (reviewed in Refs. 4 -7).The b 6 f complex is found in the thylakoid membrane of higher plants and algae and in the membrane of some cyanobacteria (for reviews, see . Besides the four high molecular weight subunits mentioned above, the b 6 f complex comprises four smaller subunits, PetG, PetL, PetM, and , that have no direct counterpart in the bc 1 complex. The cytochrome b 6 f is the middle component of the photosynthetic chain, coupling the electron transfer from photosystem II (via plastoquinol) to photosystem I (via plastocyanin or a c-type cytochrome).The most widely accepted mechanism for the bc-type complexes is the so-called "Q-cycle" mechanism proposed by Mitchell (18) and modified by Crofts et al. (19). According to this model, two electron paths are distinguished within the complex as follows: a high potential chain, composed of the iron-sulfur cluster of the Rieske protein and the heme of the c-type heme, and a low potential chain, composed of the two hemes of the b-type cytochrome, the low potential b L and high potential b H hemes. Two quinol/quinone-binding sites, located on opposite sides of the membrane, are also predicted: the Q o site, on the intermembrane space/lumenal side of the membrane, and the Q i site, on the matrix/stromal side of the membrane. A turnover of the Q o site comprises the oxidation of a two-electron carrier quinol, one electron reducing the Rieske protein, which in turn reduces the c-type cytochrome and which ultimately reduces plastocyanin or cytochrome c, and the other electron reducing b L , which in turn is oxidized by b H . These events are associated with the release of two protons in the intermembrane space/ lumen. A second turnover reduces another plastocyanine molecule and places both hemes of the b-type cytochrome in a reduced state. The reduction of a quinone at the Q i site results in the oxidation of the two b hemes and an uptake of two protons from the matrix/stroma. Thus, these complexes contribute to generating the proton gradient that drives ATP synthesis in respiration and photosynthesis.Recently, the structure of several bc 1 complexes from mitochondria have ...

mentioning

confidence: 78%

Conformational Changes in the Cytochromeb 6 f Complex Induced by Inhibitor Binding

Breyton

2000

“…X-ray data indicate that the Rieske protein of the bc 1 complex undergoes large scale displacements upon binding of inhibitors to the Q o site (2). Recent EPR measurements (23) and cryoelectromicroscopy data 2 show that similar events take place in the b 6 f complex. In bc 1 crystals, the movement of the Rieske protein is not accompanied by a significant displacement of Cyt c 1 .…”

Section: Arrangement Of the Two Hemes In Cyt Bmentioning

confidence: 88%

On the Spatial Organization of Hemes and Chlorophyll in Cytochrome b 6 f

Schoepp¹,

Chabaud²,

Breyton³

et al. 2000

Self Cite

The organization of chromophores in the cytochrome b 6 f from Chlamydomonas reinhardtii has been studied spectroscopically. Linear dichroism (LD) measurements, performed on the complex co-reconstituted into vesicles with photosynthetic reaction centers as an internal standard, allow the determination of the orientations of the chromophore with respect to the membrane plane. The orientations of the b H -and b L -hemes are comparable to those determined crystallographically on the cytochrome bc 1 . The excitonic CD signal, resulting from the interaction between b-hemes, is similar to that reported for the cytochrome bc 1 . LD and CD data are consistent with the differences between the b 6 f and bc 1 leaving the orientation of the b-hemes unaffected.

“…This inference is supported by orientation changes of the principal g values of the [2Fe-2S] system in the isolated b 6 f complex induced by the p-side quinone analogue inhibitor, 2,5-dibromo-3-methyl-6-isopropylbenzoquinone (23) or metal ions (24), and the systematic effect of increased ambient viscosity on the rate of reduction of cytochromes f and b 6 in thylakoid membranes (25).…”

mentioning

confidence: 89%

Electron Transfer from the Rieske Iron-Sulfur Protein (ISP) to Cytochrome f in Vitro

Soriano

Guo

Vitry

et al. 2002

(ISP-f) was independent of pH and ionic strength, implying no significant role of electrostatic interactions. Effective pK values of 6.2 and 8.3, respectively, of oxidized and reduced ISP were derived from the pH dependence of the amplitude of cytochrome f reduction. The firstorder rate constant, k 1 (ISP-f) , predicted from k 2 (ISP-f) is ϳ10 and ϳ150 times smaller than the millisecond and microsecond phases of cytochrome f reduction observed in vivo. It is proposed that in the absence of electrostatic guidance, a productive docking geometry for fast electron transfer is imposed by the guided trajectory of the ISP extrinsic domain. The requirement of a specific electrically neutral docking configuration for ISP electron transfer is consistent with structure data for the related cytochrome bc 1 complex.