Redox loop

The pH is typically controlled by acid/alkali feeds. Dissolved oxygen and redox loops are controlled as a cascade loop utilising air flow, agitation, pressure, auxiliary feed or a combination of these controllers. 

To explain how H+ transfer occurred across the membrane Mitchell suggested the protons were translocated by redox loops with different reducing equivalents in their two arms. The first loop would be associated with flavoprotein/non-heme iron interaction and the second, more controversially, with CoQ. Redox loops required an ordered arrangement of the components of the electron transport system across the inner mitochondrial membrane, which was substantiated from immunochemical studies with submitochondrial particles. Cytochrome c, for example, was located at the intermembranal face of the inner membrane and cytochrome oxidase was transmembranal. The alternative to redox loops, proton pumping, is now known to be a property of cytochrome oxidase. 

That redox loop, potential large, runs exergonically With membrane-separated charge, and thence to ATP. 

These reactions are the terminal electron-transfer reactions during anaerobic respiration, the enzymes being part of a redox loop generating a proton-motive force capable of driving ATP synthesis. Periplasmic nitrate reductase (Nap) participates in cellular redox processes, aerobic denitrification, and nitrate scavenging. ... 

The mechanism of proton translocation is not understood. Since the stoicheiome-try is almost certainly higher that one H /e— (Table 3.1 Section 2.2), the prototype of a Mitchellian redox loop (see Refs. 39. 41) may be rather safely excluded. 

Schematic representation of a transmembrane redox loop, in which 2H" are ejected into the intermembrane space as the substrate is oxidized on the matrix side.

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). 

Q-cyde a cycle devised by P. Mitchell [FEBS Lett. 56 (1975) 1-6 S9 (1975) 137-139] to overcome the requirement of the redox loop mechanism (see Che-miosmotic hypothesis) for a H electron carrier in the cytochrome hq-containing Complex III of the mitochondrial electron transport chain, the Q.c. proposed that ubiquinone (coenzyme Q), the only mobile, hydrophobic redox component of the chain, participates in electron transfer from cytochrome b to cytochrome c, within Complex III by one-electron steps involving the fully reduced quinol-form (QH2), a stabilized free-radical semiquinone-form (QH ) and the fully oxidized quinone-form (Q). It also made use of the observation that cytochrome b appears to be a dimer composed of b- (b and b (b, which is buried deeply in the membrane with probably on the cytosolic side and by. on the matrix side. In the Hg., outlining the proposed mechanism, it can be seen that two protons are pumped across the membrane (steps 1 9 for uptake from the matrix and steps 3... 

Although the redox loop mechanism is no longer believed to explain how electron flow through an electron transport chain causes the generation of a trans-membrane H gradient, there is considerable evidence that the Q.c. operates in mitochondria, bacteria and possibly chloroplasts under low light conditions. 

---