Q pool

Yang Q, Poole SI, Borkovich KA, A G-protein beta subunit required for sexual and vegetative development and maintenance of normal G alpha protein levels in Neurospora crassa, Eukaryot Cell 1 378-390, 2002. 

Q-pool outside. The minimum distance between myxothiazol and an-timycin A molecules is only 18 A this indicates that the two sites can exchange a quinol/quinone molecule very quickly by simply flipping the head group. 

The oxidation of plastoquinol results in the release of two protons into the thylakoid lumen. In the second half of the Q cycle (Section 18.3.4). cytochrome hf reduces a second molecule of plastoquinone from the Q pool to plastoquinol, taking up two protons from one side of the membrane, and then reoxidizes plastoquinol to release these protons on the other side. The enzyme is oriented so that protons are released into the thylakoid lumen and taken up from the stroma, contributing further to the proton gradient across the thylakoid membrane (Figure 19.18). 

More recently a modified Q cycle scheme has been proposed by Crofts and colleagues [38], in which the actual existence of Q was invalidated. The apparent displacement of the midpoint potential of Q, as compared to the Q pool was interpreted as due to the large size of the pool, so that a sufficient supply of electron to the oxidoreductase could be provided by a small reduced fraction of the pool. In this scheme the RCs are proposed to function in pairs as compared to the Z>/c, complexes, so that per flash a doubly reduced quinol is available to the oxidoreductase the quinol is then oxidized at the site (now visualized as a site in rapid exchange equilibrium with the pool) in a concerted reaction on the FeS center, reducing the b cytochrome sequence and cytochrome c,. After the oxidation of two quinols two electrons are delivered to the secondary electron donors of two RCs. The other two electrons are utilized to reduce a molecule of oxidized Q in a second reducing site of the oxidoreductase, either following two turnovers of cyt. b reduction, or via the concerted action of a dimer of the b/c complex (evidence for a dimeric structure of the complex is available for N. crassa mitochondria and for... 

Figure 18.12 Q cycle. In the first half of the cycle, two electrons of a bound QH are transferred, one to cytochrome c and the other to a bound Q in a second binding site to form the semiquinone radical anion Q. The newly formed Q dissociates and enters the Q pool. In the second half of the cycle, a second QH also gives up its electrons, one to a second molecule of cytochrome c and the other TO reduce Q to QH. This second electron transfer results in the uptake of two protons from the matrix. The path of electron transfer is shown in red. 

Genova, M.L., Bianchi, C. and Lenaz, G., Supercomplex organization of the mitochondrial respiratory chain and the role of the Coenzyme Q pool pathophysiological implications. Biofactors 25 (2005) 5-20. 

Cunha F Q, Poole S, Lorenzetti B B, et al. (1992). The pivotal role of tumour necrosis factor alpha in the development of inflammatory hyperalgesia. British J. Pharmacol. 107 660-664. 

Sachs D, Cunha F Q, Poole S, et al. (2002). Tumour necrosis factor-alpha, interleukin-Ibeta and interleukin-8 induce persistent mechanical nociceptor hypersensitivity. Pain. 96 89-97. 

G. Lenaz, A Critical Appraisal of the Mitochondrial Coenzyme Q Pool. FEBS Lett., 509, 151-155,2001. 

The duration of dark adaption before measurement has great influence on the beginnig of the induction kinetics. A short dark adaption leads to disappearence of the first oxygen outburst. (Fig.6b) We tried to simulate a short dark adaption by a higher reduced primary acceptor Q of PS II. (Fig.7) We assumed that during a short dark period Q and PQ will not get oxidized very much. With a higher reduced Q-pool the initial oxygen outburst is reduced in the experiment as well as in computer simulation. 

There exist several hypotheses as to how the ubiquinone functions in the membrane. The most common one is that coenzyme Q floats freely in the membrane or forms a mobile Q pool or Q cycle Ubiquinones differ in the length of the side isoprenoid chain, consisting usually of 6-10 carbon atoms. 

---