Electron transport substrate shuttles

Ferric iron can act as an electron acceptor under the anaerobic conditions the azo dye is in. Like sulfate, it was found that addition of ferric iron to the reactor stimulates the azo dye reduction. Indeed, the reactions are dealing with the redox couple Fe (III)/Fe (II), which can act as an electron shuttle for transferring electrons from electron donor to the electron accepting azo dye. Meanwhile, reactions of both reduction of Fe (III) to Fe (II) and oxidation of Fe (II) to Fe (III) facilitate the electron transport from the substrate to azo dye, thus acting as an extracellular redox mediator [31]. 

Under aerobic conditions, the hydrogen atoms of NtUDH are oxidised within the mitochondrion pyruvate is also oxidised in the mitochondrion (Figure 9.15). However, NADH cannot be transported across the inner mitochondrial membrane, and neither can the hydrogen atoms themselves. This problem is overcome by means of a substrate shuttle. In principle, this involves a reaction between NADH and an oxidised substrate to produce a reduced product in the cytosol, followed by transport of the reduced product into the mitochondrion, where it is oxidised to produce hydrogen atoms or electrons, for entry into the electron transfer chain. Finally, the oxidised compound is transported back into the cytosol. The principle of the shuttle is shown in Figure 9.16. 

The final reactions to be considered in the metabolism of ethanol in the liver are those involved in reoxidation of cytosolic NADH and in the reduction of NADP. The latter is achieved by the pentose phosphate pathway which has a high capacity in the liver (Chapter 6). The cytosolic NADH is reoxidised mainly by the mitochondrial electron transfer system, which means that substrate shuttles must be used to transport the hydrogen atoms into the mitochondria. The malate/aspartate is the main shuttle involved. Under some conditions, the rate of transfer of hydrogen atoms by the shuttle is less than the rate of NADH generation so that the redox state in the cytosolic compartment of the liver becomes highly reduced and the concentration of NAD severely decreased. This limits the rate of ethanol oxidation by alcohol dehydrogenase. 

The ultimate source of redox equivalents in microbial reduction reactions is usually a carbohydrate. Since the majority of it is metabolized by the cells and only a minor fraction (typically 0.5-2%) is used for the delivery of redox equivalents onto the substrate, the productivity of such processes is usually low and side-reactions are common. In order to avoid the undesired metabolism of the auxiliary substrate, nondegradable organic dye molecules such as viologens have been used as shuttles ( mediators ) for the electron-transport from the donor to the oxidized cofactor [1019]. Provided that the mediators are accepted by the ene-reductases and the recycling enzymes, the productivities were improved by one to three orders of magnitude. 

This transformation occurred because electrons were shown to be shuttled from one site to the next through multilayered polymeric films, and finally to species in solution, some distance from the surface of the electrode. This mechanism is similar to that for a redox enzyme linked to an electron transport chain. Substrate species can also enter the polymeric phase and exchange... 

In red blood cells, which lack mitochondria, reoxidation of NADH formed in glycolysis cannot be by way of the substrate shuttles discussed above (section 5.4.1.1) and the electron transport chain. 

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