Domain movement of iron sulfur protein in cytochrome bc1 complex is facilitated by the electron transfer from cytochrome bL to bH
FEBS Letters 582 (2008) 523-526
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Domain movement of iron sulfur protein in cytochrome bc1 complex is facilitated by the electron transfer from cytochrome bL to bH
3 comments:
Let's see if I understand the logic, how the results reported here lead to the conlusion presented in the title:
The ISP extrinsic domain movement is believed to be controlled by conformational changes in the surface of cyt b ("conformational switch" hypothesis, Esser et al. PNAS).
P-site inhibitors are classed in two groups: Pm inhibitors favor the conformation which promotes mobility while Pf inhibitors favor the conformation that results in fixation.
Here it is shown that a Pm inhibitor binds faster and tighter if cyt b(H) is reduced
A Pf inhibitor binds slightly less tightly when bH is reduced (provided the ISP is reduced in both cases). Therefore reduction of cyt bH must favor the conformation that binds Pm inhibitors, i.e. the conformation that promotes mobility.
Reduction of cyt b would thus be the trigger to release the ISP and perhaps not allow it to rebind until cyt b is oxidized again, preventing it from taking the second electron and short-circuiting the Q-cycle.
Takingthe results at face value, can this account for the strict bifurcation of electron flow at Qo? For example, how do we explain the inhibition by antimycin, and oxidant- induced reduction?
Under conditions of oxidant-induced reduction in the presence of antimycin, cyt bH is fully reduced, so the ISP would be in its mobile state. This seems all wrong because a mobile ISP could move back and forth between the Qo site and cyt c1, taking electrons alternately from QH2 and b(L) (or rather from the semiquinone after it takes an electron back from bL). However if we interpret mobile state as not simply free to move but also unable to dock at Qo, it might work. The complex is hung up with cyt b reduced and ISP + cyt c1 oxidized, ISP in a distal position so the Qo site cannot oxidize another quinol. Sounds reasonable, but:
*How did b(L) get reduced? Both b's go rapidly reduced under these conditins. If the Qo site cannot turnover when b(H) is reduced, whence came the second electron to reduce b(L)?
*The observed effects are much to small: factors of 2-5 in rates and binding constants. Such a weak preference for one conformation could not result in the 100-fold or greater inhibition of electron transfer by antimycin.
*Superoxide is produced at Qo under the oxidant-induced reductant conditions. If the ISP cannot dock when cyt b is reduced, then what oxidized quinol to produce SQ to make superoxide (OK, some assumptions taken there). Or if the semiquinone was left behind from the first turnover, how did the ISP dissociate before the signal from the electron arriving on b(H?) released it?
*Many groups have looked at the binding of these inhibitors, and at the effect of the inhibitors on Em of cyt b(H), which gives a thermodynamic measure of preferential binding to one redox state. The authors acknowledge this but instead of discussing the previous work, they refer to the difficulty of the measurements and dependence of the results on method used and the preparation of bc1. If you actually look up the references, previous work indicates binding constants nearly 3 orders of magnitude tighter than seen here. This is outside the bounds of expected variability and raises serious doubts as to just what the ITC results are measuring.
I have a hard time believing that the Daldal or Dutton laboratories work did not contribute to any of the conclusion put forth here. Yet, referenced they are not!
Well, Neil Howell and Dan Robertson (Then with Daldal and Dutton) looked at the effects of inhibitor binding on the midpoint potential
of the b hemes. For heme bH, stigmatellin had no effect (consistent with no significant effect of redox state of bH on stigmatellin binding observed here). Myxothiazol (a Pm inhibitor like MOAS) increased the Em by a barely significant 15-25 mv implying tighter biding to the reduced form, as reported here.
They could have cited:
Howell N, Robertson DE. Biochemistry. (1993) 32:11162-72
Electrochemical and spectral analysis of the long-range interactions between the Qo and Qi sites and the heme prosthetic groups in ubiquinol-cytochrome c oxidoreductase.
10 years earlier, Kunz and Konstantinov also reported a slight increase in midpoint potential of bH in the presence of myxothiazol: FEBS Lett 155, p237
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