Bioelectrocatalysts

It seems worthwhile to explore these ideas further. It would need the investigation of the pathway and rds for the reduction of 02 on some artificial membranes that simulate suspected reaction centers in the mitochondrion. A pan-electrochemical explanation of degenerative diseases would be to see them in terms of a slowdown of the bioelectrocatalysts of the 02 reduction. To that extent that age and accumulated fragments reduce the velocity of the enzymatic reduction of 02, is it likely that an excess 02 remains left over to form peroxy radicals and damage cells. 

In c, d, and e we have the typical case of a bioelectrocatalyst where, through a mediator, there is electron transfer between the electrode and the enzyme active centre where the substrate is in its turn activated and reacts. In c the components are in solution in d and e the mediator or the enzyme are immobilized on the electrode surface, the electron transfer reaction occurring between mediator and electrode. In case/we have the ideal situation direct electron exchange between the electrode and active centre of the enzyme, the mediator being eliminated. It is, nevertheless, very difficult to reconcile the enzyme characteristics and the electrochemical process, and it continues to be important to find adequate mediators and enzyme immobilization procedures. 

As reconstitution proceeds, the surface coverage of the bioelectrocatalyst increased, and the resulting anodic current is enhanced. The anodic current reaches a saturation value after ca. 4 hours of reconstitution that corresponds to the time interval required to generate the optimal surface coverage of the protein at the stated conditions. The saturated surface-coverage of the enzyme was elucidated by electrochemical means and microgravimetric... 

The reconstitution method was suggested as a means to introduce the photoisomerizable unit into the vicinity of the biocatalytic redox-center, thereby generating a light-switchable bioelectrocatalyst that operates between fiilly switched ON and OFF states. Apo-glucose oxidase, apo-GOx, was reconstituted with the nitrospiropyran-FAD cofactor unit, (20), Fig. 3-30. 

Another important broadening of the reconstitution process is in the area of de novo synthesized proteins. The present study has demonstrated the feasibility of organizing bioelectronic systems based on reconstituted de novo proteins (Cf. Section 2.4). These examples spark the future possibilities in the field. By the reconstitution of new electroactive synthetic cofactors into pre-designed de novo proteins, new man-made bioelectrocatalysts may be envisaged. 

Fundamental research on the above problems will make it possible to work out concepts on the mechanism of bioelectrocatalysis and to elaborate the scientific basis for developing optimal bioelectrocatalysts. 

The results obtained on the activation of electroreduction of oxygen by adsorbed laccase, of hydrogen peroxide by adsorbed peroxidase, and of lactate by adsorbed cytochrome confirm the possibility of producing heterogeneous bioelectrocatalysts with activities significantly superior to those of known electrocatalysts. 

With respect to the above requirements for DET, laccase and BOx have been shown to be useful bioelectrocatalysts for O2 reduction. For both en mes, the substrates to be oxidized or reduced interact at different locations within the enzyme structure thus it is possible to orientate these enzymes without physically blocking access for the second substrate. In addition to the above-mentioned orientation of BOx by carb-ojq late groups at the surface of electrodes, the modification of electrodes with phenolic-type heterocycles (such as anthracene, anthraquinone and naphthoquinone derivatives) has been shown to significantly enhance the orientation of both enzymes to the electrode surface via their T1 Gu center, resulting in increased bioelectrocatalytic O2 reduction at the TNG. ° The phenolic modifications of the electrode constructs mimic the natural substrates of the enzymes, which results in docking of the enzymes to the electrode surface at their T1 Gu center in BFGs, this electrode then acts as the biocathode of the device, utilizing O2 as the oxidant and final electron acceptor. 

The utilization of enzyme cascades as bioelectrocatalysts to enhance the current density has been demonstrated in numerous studies. However, the performance of these systems is often limited by the mass... 

First we will discuss the similarities and differences between conventional chemical and microbial bioelectrocatalysts. Subsequently, the principal mechanisms of the extracellular electron transfer (EET) and the technology aiming at their exploitation will be introduced. Finally, the biological and electrochemical methods that allow the analysis of the microbial electron transfer and identification of the respective mediators, proteins, and microorganisms will be addressed. 

Microbial bioelectrocatalysts are living cells. As a consequence, they need a certain amount of energy for their maintenance and proliferation. Cells use energy gained to produce ATP and NAD(P)H, the two main energy carriers. In order to generate these, they need to build up internal potential gradients, which externally lead to a loss of useful potential. In the context of bioelectrocatalysis, this... 

Figure 8.1 Types of bioelectrocatalysts (a) enzymes, (b) organelles, and (c) microorganisms.

When comparing the microbial bioelectrocatalyst with a conventional, noble metal-based electrocatalyst for low-temperature fuel cells, it appears that the aforementioned energetic disadvantage could be compensated by a number of advantages (Table 8.1). 

Table 8.1 Operating conditions of a microbial bioelectrocatalyst and an electrocatalyst in a low-temperature fuel cell.

As already discussed, microorganisms extract a certain share of energy for their living from the maximum theoretically exploitable energetic difference. In case of anodic bioelectrocatalysis, the energy difference is situated between the microbial substrate, that is, the fuel, and the potential of the terminal electron transfer site. Furthermore, and like in conventional electrocatalytic systems [5], several energetic losses at the bioelectrocatalyst-electrode interface occur (Figure 8.2). 

The characterization of microbial bioelectrocatalysts and development of methods and techniques are therefore one of the most active and vital fields. One might distinguish two different types of methods (i) electrochemical methods (see Section 8.5.1) and (ii) biological methods (see Section 8.5.2). Whereas the former generally focus on the identification and characterization of the electron transfer, the latter are devoted to the identification, analysis, and spatial distribution of the bioelectrocatalytic cells and their environment. 

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