Q-cycle mechanism

Such a process is supposed to occur within the limits of Q-cycle mechanism (Figure 23.2). In accord with this scheme ubihydroquinone reduced dioxygen in Complex III, while superoxide producers in Complex I could be FMN or the FeS center [12]. Zhang et al. [24] also suggested that the Q-cycle mechanism is responsible for the superoxide production by the succinate-cytochrome c reductase in bovine heart mitochondria and that FAD of succinate dehydrogenase is another producer of superoxide. Young et al. [25] concluded that, in addition to Complex III, flavin-containing enzymes and FeS centers are also the sites of superoxide production in liver mitochondria. 

FIGURE 23.2 Redox cycling of ubiquinone in mitochondria. (Q-cycling mechanism). (Adapted from Y Li, H Zhu, MA Trush. Biochim Biophys Acta 1428 1-12, 1999.)... 

The coupling of electron transfer and proton translocation is described by the proton-motive Q-cycle mechanism first proposed by Peter Mitchell [1], In the bc complex, hydroquinone is oxidized at a reaction site which is at the positive side of the membrane,... 

Figure 1. The Q-cycle mechanism of the cytochrome bc complex (cf. section 1.1). The points of inhibition of three classes of inhibitors are indicated.

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]. 

Although the Q-cycle mechanism successfully explained most of the experimental data, still our understanding of the mechanism of bcj complexis function was hindered by lack of the detailed structural infor-... 

Crofts, A. R., Meinhardt, S. W., Jones, K. R., and Snozzi, M., 1983, The role of the quinone pool in the cyclic electron transfer chain of Rhodopseudomonas sphaeroides. A modified Q-cycle mechanism. Biochim. Biophys. Acta, 723 2029218. 

Evidence for the existence of a Q cycle mechanism also in the intersystem chain of higher plant chloroplasts has been obtained this evidence rests, however, on more indirect experimental approaches, such as a slow phase of the electrochromic shift of carotenoids and an H /e stoicheiometry of proton translocation higher than one, as would be postulated by a linear loop including plastoquinone as a transmembrane proton translocator. 

The increase in amplitude of cytochrome bs photoreduction and inhibition of the slow electrochromic phase by the compound NQNO was an important observation with regard to the understanding of function of the cytochrome 5-/complex (1). The inferred analogy to the action of antimycin A provided support for a Q cycle mechanism. The basis of this analogy and the evidence supporting inteiheme transfer associated with the Q cycle is examined in this study. 

In the absence of FCCP, temporary decrease of pH was observed besides the proton uptake. It was inhibited by antimycin or myxothiazol. The result support the idea that cytochrome b- complex is involved in the electron transfer of the TMAO respiration. Protons translocated by the Q-cycle mechanism probably contributed to the observed pH increase. 

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