Redox-driven quinone

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-... 

Most photosynthetic eubacteria appear to contain cyclic electron transfer pathways driven by the RCs. Electrons from the secondary acceptor of the RC are transferrred first to a quinone pool and then to the secondary donor (Cyt c) via a Cyt bic complex which stores some of the electron redox energy as potential energy in the form of a transmembrane proton gradient. Evidence for cyclic electron flow in the gram-positive line has not yet been found, but it would be surprising not to find it. 

In correlation with the nature of compounds that can induce ARE-driven transcription, many of the proteins whose expression is mediated by the ARE have an endogenous role in regulating cellular redox status and protecting the cell from oxidative damage. Enzymes such as GST, NQOl, and HO-1 function to detoxify harmful by-products of oxidative stress, including lipid and DNA base hydroperoxides (29,30), quinones (31), and heme-containing molecules (32). The induction of enzymes involved in GSH biosynthesis leads to an increase in cellular GSH levels that provides a buffer against oxidative insult (2). 

Transfer of calcium cations (Ca2 + ) across membranes and against a thermodynamic gradient is important to biological processes, such as muscle contraction, release of neurotransmitters or biological signal transduction and immune response. The active transport can be artificially driven (switched) by photoinduced electron transfer processes (Section 6.4.4) between a photoactivatable molecule and a hydroquinone Ca2 + chelator (405) (Scheme 6.194).1210 In this example, oxidation of hydroquinone generates a quinone to release Ca2+ to the aqueous phase inside the bilayer of a liposome, followed by reduction of the quinone back to hydroquinone to complete the redox loop, which results in cyclic transport of Ca2 +. The electron donor/acceptor moiety is a carotenoid porphyrin naphthoquinone molecular triad (see Special Topic 6.26). 

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