Stigmatellin, binding

The third mechanism is based on the inference of the different binding modes for quinone species at Qosite from the different binding positions of the two classes of QoSite inhibitors as discussed in previous section. It was proposed by Crofts and Berry (1998) that after transfer of the first electron to Rieske protein at the stigmatellin binding site, the hydrogen bond breaks and the semiquinone flips into the position of the MOA inhibitors before passing the second electron to haem bL. This model predicts high occupancy of a semiquinone under conditions of oxidant-induced reduction, and the failure to observe the radieal by EPR speetroseopy under these conditions needs to be explained (Junemann et al., 1998). 

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)... 

Only a few of the many known cytochrome bci complex inhibitors will be mentioned in this chapter. Cytochrome bci inhibitors that bind at the Qo site (p side, near the intermembrane space) include myxothiazol (MYX) and stigmatellin (SMA) (see Figure 7.29). MYX and SMA inhibit electron transfer from ubiquinol (QH2) to cytochrome c (outside the bci complex) or to other domains of the bci complex. Antimycin A (see Figure 7.29), a fungicide, binds... 

The movement of the catalytic domain of the Rieske protein is related to the binding of Qo site inhibitors. Stigmatellin or 5-undecyl-6-hydroxy-4,7-dioxobenzothiazol (UHDBT) bind directly to tiie exposed HislSl of the Rieske protein [6] and fix the Rieske protein in the h position , while binding of MOA-stilbene or myxothiazol favors tiie ci position [23]. 

It was unexpected that the extrinsic domain of Rieske protein was found at different positions in different crystal forms. In bovine structure IQCR (see Table 2), the location of the Rieske extrinsic domain was identified to be close to cytochrome b (Xia et al., 1997). This domain was suggested to have high mobility based on further inhibitor binding experiments and anomalous scattering studies (Kim et al., 1998). In the native chicken bc structure (IBCC), the Rieske domain was found to be far away from haem Ll and close to cytochrome c. However, in the chicken bci complex co-crystallized with Qo site inhibitor stigmatellin (structure 2BCC and 3BCC) the Rieske domain was close to haem Ll and far away from cytochrome Cl, which is approximately the position found in the structure... 

Fig. 14. Rotation of the mobile [2Fe 2S] cluster in the Cyt be, complex upon binding of the inhibitor stigmatellin. Cytb[bH,orb(HP) and bi or b(LP)], Cyt c and ISP (the [2Fe 2S]) are as indicated. (A) Arrangement oftheCyt bo, complex subunits when stigmatellin is bound to the Qo site. (B) Arrangement of the Cyt be, complex subunits in the absence of the inhibitor. Figure source Izrailev, Crofts, Berry and Schulten (1999) Steered molecular dynamics simulation of the Rieske subunit motion in the cytochrome be, complex. Biophys J 77 1754.

Other recent reports further confirmed the different locations occupied by the extrinsic [2Fe 2S] domain in different crystals. An intermediate location was reported by Iwata et alf in a new crystal form which places this domain 27.5 A from Cyt ci and 13 A from the Qo site. Kim, (D) Xia, (C-A) Yu, (J-Z) Xia, Kachurin, Zhang, (L) Yu and Deisenhofer further examined crystal stmctures of bovine mitochondrial Cyt-bc complexes with inhibitors bound to the Qo and/or Qj (j.e., Q ) sites. Binding of the Qq-site inhibitor UHDBT (5-undecyl-6-hydroxy-4,7-dioxobenzothiazol) and stigmatellin was found to result in the [2Fe 2S] domain being at the Cyt b interface, whereas binding of myxothiazol or MOAS (methoxyacrylate-stilbene) led to the release of this domain. The native complex with a vacant Qo site was found to have a structure intermediate between these two extremes. 

Neither photosystem I nor II were implicated in desaturation, as light is not required. Supporting this concept are the observations that neither DCMU nor atrazine (which prevent reduction of PQ at the Qb site of PS II, fig. 4) inhibited desaturation (fig. 2). Because PQ is involved in "reverse" electron transport, the effects of inhibitors of its function were determined. DBMBIB, UHDBT, BPA, stigmatellin (STG), and DNP-INT interfere with oxidation of reduced PQ by blocking the Qz site (fig. 4), and in fact the first three were able to Inhibit desaturation (fig. 3 and table 5). Both DNP-INT and STG may bind at an overlapping but more... 

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