Biocatalysis redox enzymes

Combinatorial biocatalysis, where redox enzymes are used in mulit-component systems for new molecule discovery. 

The use of organic solvents as reaction media for biocatalytic reactions can not only overcome the substrate solubility issue, but also facilitate the recovery of products and biocatalysts as well. This technique has been widely employed in the case of lipases, but scarcely applied for biocatalytic reduction processes, due to the rapid inactivation and poor stability of redox enzymes in organic solvents. Furthermore, all the advantages for nonaqueous biocatalysis can take effect only if the problem of cofactor dependence is also solved. Thus, bioreductions in micro- or nonaqueous organic media are generally restricted to those with substrate-coupled cofactor regeneration. 

It also has to be stressed that many useful redox enzymes for asymmetric oxidations are not commercially available as isolated enzymes—alcohol dehydrogenases are the main exception here— but have to be isolated from the wild-type organism or produced in recombinant form. For that reason, applied biocatalysis is a multidisciplinary field where the expertise of biologists, chemists, and engineers is required. 

During the last few decades, biocatalysis has become an important tool in performing organic synthesis. This chapter has shown how redox enzymes comprise a broad number of different biocatalysts that can perform an ample number of synthetic transformations under environmentally friendly and rather diverse reaction conditions. Likewise, many of these enzymes can accept other substrates different from their natural ones. All these mentioned properties are crucial for the application of enzymes at a practical level. As also reported in this chapter, some of these redox enzymes are currently being used on a commercial scale for the production of important chiral building blocks. 

Modified electrodes for biocatalysis use either electron mediators or promoters immobilized on the electrode surface -. In both cases, redox enzyme molecules are in solution and in contact with the common electron mediators for redox enzymes such as cytochrome c and ferredoxin. An electron promoter is not a mediator since it does not take part in electron transfer in the potential region of interest. An electrode modified with promoter molecules has enables some redox enzymes to directly transfer electrons. It has been shown that 4,4 -bipyridyl, bis(4-pyridyl)sulfide, and bis 4-pyridyl)disulfide are excellent promoters of electron transfer of cytochrome c. Cytochrome c gives a reversible cyclic voltammogram at gold electrodes modified with these promoters. 

At present, studies to extend this strategy to other enzymes are being conducted in our groups and we hope that this concept may become an appealing solution to the problem of co-factor regeneration in redox biocatalysis. 

In MET, the thermodynamic redox potentials of the enzyme and the mediator should be accurately matched. The tuning of these potentials is of critical importance to EFC design as this will have a major bearing on cell voltage and catalytic current. When compared to the redox potential of the enzyme, the mediator should have a redox potential that is more positive for oxidative biocatalysis (at anode) and more negative for reductive biocatalysis (at cathode). For efficient electron transfer. 

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