Indirect enzymatic reduction

Anaerobic bio-reduction of azo dye is a nonspecific and presumably extracellular process and comprises of three different mechanisms by researchers (Fig. 1), including the direct enzymatic reduction, indirect/mediated reduction, and chemical reduction. A direct enzymatic reaction or a mediated/indirect reaction is catalyzed by biologically regenerated enzyme cofactors or other electron carriers. Moreover, azo dye chemical reduction can result from purely chemical reactions with biogenic bulk reductants like sulfide. These azo dye reduction mechanisms have been shown to be greatly accelerated by the addition of many redox-mediating compounds, such as anthraquinone-sulfonate (AQS) and anthraquinone-disulfonate (AQDS) [13-15],... 

As discussed earlier, Azo biological decolorization are mainly reduced in a direct reduction or mediated/indirect reduction with nonspecial azo reductase or reduced enzyme cofactors (Figs. 1 and 3). According to the direct enzymatic reduction mechanism, nonspecial azo reductase can catalyze the transfer of reducing equivalents originating from the oxidation of original electron donor in the azo dyes. In... 

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. 

For enzymatic reductions with NAD(P)H-dependent enzymes, the electrochemical regeneration of NAD(P)H always has to be performed by indirect electrochemical methods. Direct electrochemical reduction, which requires high overpotentials, in all cases leads to varying amounts of enzymatically inactive NAD-dimers generated due to the one-electron transfer reaction. One rather complex attempt to circumvent this problem is the combination of the NAD+ reduction by electrogenerated and regenerated potassium amalgam with the electrochemical reoxidation of the enzymatically inactive species, mainly NAD dimers, back to NAD+ [51]. If one-electron... 

Nickel removed by coprecipitation can be remobilized by microbial action under anaerobic conditions (Francis and Dodge 1990). Remobilization results Ifom enzymatic reductive dissolution of iron with subsequent release of coprecipitated metals. A lowering of pH as a result of enzymatic reactions may indirectly enhance the dissolution of nickel. Experiments using mixed precipitates with goethite... 

Electro-generated and regenerated bis(bipyridine)rhodium(I) complexes were able to catalyze the selective non-enzymatically coupled electrochemical generation of NADH from NAD . The direct cathodic reduction even at very negative working potentials leads to the formation of large amounts of enzymatically inactive NAD dimers, while the indirect electrochemical reduction via the rhodium complex acting as... 

A second method for separating enzymatic and nonenzymatic Fe(III) reduction by H2S is to block S04 reduction with molybdate (Mo04 ). The technique has been used effectively to demonstrate the importance of enzymatic reduction in marine and freshwater sediments (Section 8.08.6.4.4). As with all inhibitor techniques, there is the possibility that molybdate additions directly or indirectly affect processes other than S04 reduction. For example, it could overestimate biotic Fe(III) reduction if the enzymatic process was stimulated by a cessation of competition with H2S for Fe(III) substrates, or underestimate if S04 reduction was not completely blocked. Despite these potential limitations, the molybdate method produces patterns that are consistent with other types of geochemical data, and it is therefore widely used. 

An indirect amperometric immimosensor for the detection of HCG was described by Chetcuti et al. [97]. The assay consisted of an anti-HCG monoclonal antibody immobilized on a GCE and the use of a sandwich assay with HRP conjugated to a second anti-HCG monoclonal antibody. Electrochemical detection of the enzymatic reduction of benzoquinone to... 

Figure 3. Principle of an indirect electrochemical cofactor or enzyme regeneration system for an enzymatic reduction process.

The reduced glutathione could indirectly provide the reducing power for many enzymatic reductions, but similar processes for the direct electrochemical reduction of NAD are now being developed. If they are successful some powerful catalysts for chemical reduction in aqueous media will become available. 

Redox mediators, such as flavins or quinones, are usually involved in the azo bond reduction. Therefore, the azo bond cleavage is a chemical, unspecific reaction that can occur inside or outside the cell, relying on the redox potential of the redox mediators and of the azo compounds. Also the reduction of the redox mediators can be both a chemical and an enzymatic process. As a consequence, it is an evidence that environmental conditions can affect the azo dyes degradation process extent both directly, depending on the reductive or oxidative status of the environment, and indirectly, influencing the microbial metabolism. 

Since HA is unstable in vivo , and is known to rapidly associate with the heme part of heme proteins , and possibly also with a variety of biological oxidants, such as the superoxide anion that is produced by many mammalian cells, it is difficult to demonstrate its accumulation in vivo. Already in 1932 Lindsey and Rhines discussed some analytical difficulties in the detection of HA, since when added externally, it disappeared rapidly from bacterial cultures this led to the conclusion that even if it is produced as an intermediate, its consumption is too fast to allow the accumulation of sufficient quantities for analytical demonstration. Compelling indirect evidence for the presence of HA as an intermediate in the enzymatically catalyzed reduction of nitrite (N02 ) to NH3 was provided by Einsle and colleagues , who characterized the crystal structure of the complex obtained by soaking cytochrome c-nitrite reductase with NH20H. ... 

In addition, technetium may be fixed by bacteri-aUy mediated reduction and precipitation. Several types of Fe(III)- and sulfate-reducing bacteria have been shown to reduce technetium, either directly (enzymatically) or indirectly through reaction with microbially produced Fe(II), native sulfur, or sulfide (Lyalikova and Khizhnyak, 1996 Lloyd and Macaskie, 1996 Lloyd et al, 2002). 

Whereas enzymatic one-electron oxidation or reduction to produce free radicals is common, one should be aware that some enzyme reactions can proceed by a mixture of one- and two-electron mechanisms, and also that free radicals can be formed indirectly subsequent to a two-electron enzymatic step. 

When considering the actual processes by which electrons are transferred in these reduction reactions, it should not be assumed that all reductions are caused directly by microorganisms. For example, bacteria may exude organic chemicals such as polyphenols, which in turn chemically reduce Mn oxides and other easily reducible compounds. Therefore, Mn oxide may in this sense be an indirect electron acceptor, whereas NO3" accepts electrons by a direct enzymatically catalyzed reaction within the cell. 

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