Quinone binding

The electron on the bj heme facing the cytosolic side of the membrane is now passed to the bfj evcie on the matrix side of the membrane. This electron transfer occurs against a membrane potential of 0.15 V and is driven by the loss of redox potential as the electron moves from bj = — O.IOOV) to bn = +0.050V). The electron is then passed from bn to a molecule of UQ at a second quinone-binding site, Q , converting this UQ to UQ . The result-... 

A 6 positional state that is stabilized by the interaction of His 161 with a molecule of the inhibitor stigmatellin bound in the quinone binding pocket (41), which is supposed to mimic the hydrogen bonding pattern of the reaction intermediate, semiqui-none (43)... 

In the ci positional state, fast electron transfer from the Rieske protein to cytochrome Ci will he facilitated hy the close interaction and by the hydrogen bond between His 161 of the Rieske protein and a propionate group of heme Ci, but the Rieske cluster is far away from the quinone binding site. 

Hydroquinone will bind in a quinone binding site that is provided by cytochrome b. Before hydroquinone can be oxidized, it must first be deprotonated (step 2). 

Ghaim, J.B., Greiner, D.P., Meares, C.F., and Gennis, R.B. (1995) Proximity mapping the surface of a membrane protein using an artificial protease demonstration that the quinone-binding domain of subunit I is near the N-terminal region of subunit II of cytochrome bd. Biochemistry 34(36), 11311-11315. 

Vermass, W.F.J. and C.J. Arntzen (1984). Synthetic quinones influencing herbicide binding and photosystem II electron transport. The effects of triazine-resistance on quinone binding properties in thylakoid membranes. Biochim. Biophys. Acta., 725 483 -91. 

Recently, Schloss et al (33) showed that IM and TP were able to quantitatively displace a radiolabelled SU herbicide from ALS, indicating competitive binding. Curiously, the SU ligand was also displaced by the quinone, Qo. It was proposed that SU, TP, and IM bind to ALS in a vestigial quinone binding site associated with the evolution of ALS from pyruvate oxidase. This enzyme is an FAD-protein that catalyzes the oxidation of pyruvate to acetate. 

By exploiting the relative distance dependences of electron tunneling and diffusion, it seems possible that this restricted FeS diffusion may play a role in regulating the action of the n = 2 quinone binding... 

STRUCTURE AND FUNCTION OF QUINONE BINDING MEMBRANE PROTEINS... 

In both systems, membrane-bound ubiquinone plays crucial roles in the respiratory chain. Indeed, various quinones, including ubiquinone and menaquinone, are used to connect the redox reactions of various membrane proteins. In spite of the large amount of biochemical and biophysical data on quinone and quinone binding proteins, little structural... 

The reaction mechanism of cytochrome bc complex is known as the proton motive Qcycle originally proposed by Peter Mitchell (Mitchell, 1976). This mechanism is the basis of his chemiosmotic theory for which he was awarded the Nobel prize in 1978. Since then, the enzyme has been characterized extensively using various techniques. Redox centers have been characterized spectroscopically (for review, see Trumpower and Gennis, 1994), electron transfer pathways have been determined using kinetic experiments with specific inhibitors (De Vries 1986 Zhu et al., 1984), and the positions of quinone binding sites and redox centers have been determined using biochemical and mutational analysis (for review, see Esposti et al, 1993 Brasseur et al, 1996). As a result of these efforts, the latest modified Qcycle has been widely accepted by researchers in the field (for reviews, see Crofts et al, 1983 Trumpower, 1990 Berry et al, 2000). 

In the Qcycle mechanism, two different types of quinone binding sites play critical roles. We will analyze different types of the quinone-binding sites in Section III,C. 

Figure 5 is a view of the dimer from the mitochondrial intermembrane space. Transmembrane helices, hemes b i and b, inhibitors, and lipid molecules are shown. Site-specific inhibitors are superimposed in order to illustrate the positions of the quinone binding sites. The Qp site, characterized by myxothiazol, was located by heme b, toward the out side (the mitochondrial intermembrane space side) of the membrane, whereas the Qn site, characterized by antimycin A, was found by heme toward the in side (the matrix side) of the membrane. 

Fig. 7. (A) Quinone binding mode at the QN site. The residues involved in the binding are shown. (B) Possible quinone reduction...

Based on the positions of metal centers and quinone binding sites and the different ISP positions, the electron bifurcation can be well explained (Zhang et al., 1998 Iwata et al., 1998,1999 Crofts et al., 1999). 

The Q-cycle mechanism requires the presence of two separate quinone binding sites that are in contact with different sides of the membrane the hydroquinone oxidation (Q or Qp) site at the positive P side of the membrane and the quinone reduction (Q or Qn) site at the negative N side of the membrane. These sites have first been characterized by their different inhibitor binding properties [2] (see below) the existence of two distinct quinone binding sites was confirmed by the X-ray structure of the bc complex [3-6]. 

The X-ray structures of the bci complexes from bovine heart, chicken heart, and the yeast Saccharomyces cerevisiae have been determined at 3.0 - 2.3 A resolution [3-6] therefore, it is possible to explore the interaction between the complex and quinones and inhibitors at molecular level. First, we will give an overview of essential structural features of the bci complex. As the non-catalytic subunits are not involved in the coordination of the redox centers or the formation of quinone binding sites, only the structure of the three eatalytic subunits will be discussed in the following section (Figure 3) for a more detailed description see [19]. In order to avoid confusion, the residue numbering of the yeast bci complex will be used even if other organisms have been studied. 

The membrane spanning helices of cytochrome b are cotmected by four long (ab, cd, de, and ef) and three short (be, fg, gh) loops (loops are named according to the membrane spanning helices that they connect). The loops ab, cd, and ef at the outer side of the membrane contain peripheral helices which are oriented parallel to the membrane (aab, acdl, acd2, aef) the loops ab and de lie directly above or below the 4-a-helical bundle while the loops cd and ef form a lid on top of the Qo quinone binding site which is outside the 4-a-helical bundle (Figure 4). 

---