Electron shuttles

The second electron shuttle system, called the malate-aspartate shuttle, is shown in Figure 21.34. Oxaloacetate is reduced in the cytosol, acquiring the electrons of NADH (which is oxidized to NAD ). Malate is transported across the inner membrane, where it is reoxidized by malate dehydrogenase, converting NAD to NADH in the matrix. This mitochondrial NADH readily enters the electron transport chain. The oxaloacetate produced in this reaction cannot cross the inner membrane and must be transaminated to form aspartate, which can be transported across the membrane to the cytosolic side. Transamination in the cytosol recycles aspartate back to oxaloacetate. In contrast to the glycerol phosphate shuttle, the malate-aspartate cycle is reversible, and it operates as shown in Figure 21.34 only if the NADH/NAD ratio in the cytosol is higher than the ratio in the matrix. Because this shuttle produces NADH in the matrix, the full 2.5 ATPs per NADH are recovered. 

In mitochondria (Fig. lb), the electron acceptor protein is also a flavoprotein termed NADPH-adrenodoxin reductase (MW 50 kDa) because it was discovered in the adrenal cortex and because it donates its electrons not directly to the P450 but to the smaller redox protein adrenodoxin (MW 12.5 kDa). The two iron-sulphur clusters of this protein serve as electron shuttle between the flavoprotein and the mitochondrial P450. 

The prototype of this class of soluble ferredoxins was initially obtained from spinach chloroplasts and subsequently been shown to play a role in physiological electron shuttling between PSl and a number of redox proteins, most prominently ferredoxin-NADP-reduc-tase 13). Homologous proteins were purified from several cyanobac-... 

Platinum metal is often used as a passive electrode because platinum is one of the least reactive elements. Platinum has a large ionization energy, so it can act as an electron shuttle without participating in redox chemistry. 

The microbial degradation of contaminants under anaerobic conditions using humic acids as electron acceptors has been demonstrated. These included the oxidations (a) chloroethene and 1,2-dichloroethene to CO2 that was confirmed using C-labeled substrates (Bradley et al. 1998) and (b) toluene to CO2 with AQDS or humic acid as electron acceptors (Cervantes et al. 2001). The transformation of l,3,5-trinitro-l,3,5-triazine was accomplished using Geobacter metallireducens and humic material with AQDS as electron shuttle (Kwon and Finneran 2006). 

Kwon MJ, KT Finneran (2006) Microbially mediated biodegradation of hexahydro-l,3,5-trinitro-l,3,5-tri-azine by extracellular electron shuttling compounds. Appl Environ Microbiol 72 5933-5941. 

The high catalytic activity of enzymes has a number of sources. Every enzyme has a particular active site configured so as to secure intimate contact with the substrate molecule (a strictly defined mutual orientation in space, a coordination of the electronic states, etc.). This results in the formation of highly reactive substrate-enzyme complexes. The influence of tfie individual enzymes also rests on the fact that they act as electron shuttles between adjacent redox systems. In biological systems one often sees multienzyme systems for chains of consecutive steps. These systems are usually built into the membranes, which secures geometric proximity of any two neighboring active sites and transfer of the product of one step to the enzyme catalyzing the next step. 

Saito et al. (134) found that the cytosolic nitroreductase activity was due to DT-diaphorase, aldehyde oxidase, xanthine oxidase plus other unidentified nitroreductases. As anticipated, the microsomal reduction of 1-nitropyrene was inhibited by 0 and stimulated by FMN which was attributed to this cofactor acting as an electron shuttle between NADPH-cytochrome P-450 reductase and cytochrome P-450. Carbon monoxide and type II cytochrome P-450 inhibitors decreased the rate of nitroreduction which was consistent with the involvement of cytochrome P-450. Induction of cytochromes P-450 increased rates of 1-aminopyrene formation and nitroreduction was demonstrated in a reconstituted cytochrome P-450 system, with isozyme P-448-IId catalyzing the reduction most efficiently. 

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]. 

During the last two decades, more studies have been conducted to explore the catalytic effects of different redox mediators on the bio-transformation processes. Redox mediators, also referred to as electron shuttles, have been shown to play an important role not only as final electron acceptor for many recalcitrant organic compounds, but also facilitating electron transfer from an electron donor to an electron acceptor, for example, azo dyes [8, 11, 12], Redox mediators accelerate reactions by lowering the activation energy of the total reaction, and are organic molecules that can reversibly be oxidized and reduced, thereby conferring the capacity to serve as an electron carrier in multiple redox reactions. 

The acceleration mechanism of redox mediators are presumed by van der Zee [15]. Redox mediators as reductase or coenzymes catalyze reactions by lowering the activation energy of the total reaction. Redox mediators, for example, artificial redox mediators such as AQDS, can accelerate both direct enzymatic reduction and mediated/indirect biological azo dye reduction (Fig. 3). In the case of direct enzymatic azo dye reduction, the accelerating effect of redox mediator will be due to redox mediator enzymatic reduction in addition to enzymatic reduction of the azo dye. Possibly, both reactions will be catalyzed by the same nonspecific periplasmic enzymes. In the case of azo dye reduction by reduced enzyme cofactors, the accelerating effect of redox mediator will either be due to an electron shuttle between the reduced enzyme cofactor and redox mediator or be due to redox mediator enzymatic reduction in addition to enzymatic reduction of the coenzymes. In the latter case, the addition of redox mediator simply increases the pool of electron carriers. 

Additionally, it was found that the double reductive alkylation of the 2,5-diester 66 could be achieved under Birch conditions (Li/NH3) to produce the 3-pyrroline 67. On the basis of a mechanistic postulate that such reductions do not involve transfer of a proton from ammonia, the authors discovered that the same reduction could be performed in THF (no ammonia) with lithium metal and catalytic amounts of naphthalene as an electron shuttle, thereby making this reaction more practicable on a large scale <00TL1327>. 

Among the main goals of electrochemical research are the design, characterization and understanding of electrocatalytic systems, (1-2) both in solution and on electrode surfaces. (3.) Of particular importance are the nature and structure of reactive intermediates involved in the electrocatalytic reactions.(A) The nature of an electrocatalytic system can be quite varied and can include activation of the electrode surface by specific pretreatments (5-9) to generate active sites, deposition or adsorption of metallic adlayers (10-111 or transition metal complexes. (12-161 In addition the electrode can act as a simple electron shuttle to an active species in solution such as a metallo-porphyrin or phthalocyanine. 

Electron transport in electrode coatings containing redox centers is a necessary ingredient of their functioning as a catalytic device. They indeed serve as an electron shuttle between the electrode and the catalyst present inside the film. As discussed in the next section, the same molecule may play the role of catalyst and of electron carrier, since as shown earlier, redox catalysis is possible in these multilayered coatings. They may also be different, as exemplified is Section 4.3.6. 

Combining an Electron-Shuttling Mediator with a Chemical Catalyst in a Multilayer Electrode Coating... 

Immobilizing the catalyst on the electrode surface is useful for both synthetic and sensors applications. Monomolecular coatings do not allow redox catalysis, but multilayered coatings do. The catalytic responses are then functions of three main factors in addition to transport of the reactant from the bulk of the solution to the film surface transport of electrons through the film, transport of the reactant in the reverse direction, and catalytic reaction. The interplay of these factors is described with the help of characteristic currents and kinetic zone diagrams. In several systems the mediator plays the role of an electron shuttle and of a catalyst. More interesting are the systems in which the two roles are assigned to two different molecules chosen to fulfill these two different functions, as illustrated by a typical experimental example. 

Other examples have shown that pristine fullerenes serve as electron shuttles in liquid-based electrolytes [100,101], In this context, clusters of pristine fullerenes were deposited on top of Ti02 electrodes that were modified with ruthenium (II) complex, porphyrins, and fluorescein. All of these examples featured overall enhancements of... 

Figure 5. Possible pathways by which Fe isotopes may be fractionated during dissimilatory Fe(III) reduction (DIR). Dissolution, if it occurs congruently, is unlikely to produce isotopic fractionation (Afi. If Fe(II) is well complexed in solution and conditions are anaerobic, precipitation of new ferric oxides (A3) is unlikely to occur. Significant isotopic fractionation is expected during the reduction step (A2), possibly reflecting isotopic fractionation between soluble pools of Fe(III) and Fe(II). The soluble Fe(III) component is expected to interact with the cell through an electron shuttle compound and/or an outer membrane protein, and is not part of the ambient pool of aqueous Fe. Sorption of aqueous or soluble Fe(II) to the ferric oxide/hydroxide substrate (A4) is another step in which isotopic fractionation may occur. Modified from Beard et al. (2003a).

Nevin KP, Lovley DR (2000) Lack of production of electron-shuttling compounds or solubilization of Fe(III) during reduction of insoluble Fe(III) oxide by Geobacter metallireducens. App Environ Microbio 66 2248-2251... 

Shyu JBH, Lies DP, Newman DK (2002) Protective role of tolC in efflux of the electron shuttle anthraquinone-2,6-disulfonate. JBacteriol 184 1806-1810... 

Figure 1-12-5. Oxidation of Cytopiasmic NADH in the ETC invoivesTwo Electron Shuttles...

A proposed electron shuttle In which the oxidative power of O2 can be conveyed below the oxic zone In marine sediments. 

Chen, R Pignatello, JJ. Role of quinone intermediates as electron shuttles in Fenton and photoassisted Fenton oxidations of aromatic compormds. Environmental Science Technology, 1997 31, 2399-2406. 

Pseudomonas putida were coexpressed in an E. coli strain. CamA is an NAD-dependent flavin-containing protein which requires CamB (an iron sulfur protein) as an electron shuttle [65]. 

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