Biocatalyzed reduction

Besides the broad applications of electrically contacted enzyme electrodes as amperometric biosensors, substantial recent research efforts are directed to the integration of these functional electrodes as biofuel cell devices. The biofuel cell consists of an electrically contacted enzyme electrode acting as anode, where the oxidation of the fuel occurs, and an electrically wired cathode, where the biocatalyzed reduction of the oxidizer proceeds (Fig. 12.4a). The biocatalytic transformations occurring at the anode and the cathode lead to the oxidation of the fuel substrate and the reduction of the oxidizer, with the concomitant generation of a current through the external circuit. Such biofuel cells can, in principle, transform chemical energy stored in biomass into electrical energy. Also, the use... 

Figure 12.3 Coupling of photoinduced electrical interaction of cytochrome c (cyt c) with the photoisomerizable monolayer modified Au electrode to the biocatalyzed reduction of 02 by cytochrome oxidase (COX) [33]...

The hydrogen-based MBfR was developed by Lee and Rittmann (2000). They focused on denitrification of drinking water using autohydrog-enotrophic bacteria. Hydrogen is an ideal electron donor for biocatalyzed reduction of oxidized contanfinants in water since it results in low biomass... 

Scheme 4.4 Biocatalyzed reduction of interesting diketones through ADH-mediated processes.

Since most of the biocatalyzed reduction reactions involve cofactor-dependent redox enzymes, coupling of the synthetic reaction with an enzymatic cofactor regeneration reaction is very common. However, as cofactor regeneration in biocatalyzed reductions will be described elsewhere in this book, this topic will not be discussed herein to avoid undesired overlapping. 

Therefore, this chapter will focus on multienzymatic processes where at least one of the coupled reactions is a biocatalyzed reduction, apart from the necessary cofactor regeneration reactions. 

Cascade Processes Including Biocatalyzed Reductive Amination Steps... 

Various examples were described, including whole cell-biocatalyzed reductions [4], especially employing baker s yeast [la,5]. However, there was always the problem that several active enzymes with different selectivities were present, although in particular cases excellent conversions and stereoselectivities were achieved. Later on, by directed evolution techniques [6], improvement of stereoselectivities and substrate specificities of enzymes was achieved, as well as several enzymatic properties (e.g., thermal stability) were modified. Moreover, large amounts of biocatalysts have been produced by fermentation of recombinant bacteria that express the gene, when their corresponding DNA has been doned. The drawback of the expensive cofactor recyding has also been overcome by the devdopment of effident techniques [7]. 

This book is intended to describe the state of the art of these efforts for the advantage of both the academic and the industrial audience. The book will focus on the current available toolbox of biocatalyzed reductions of C=0, C=C, and formal C=N double bonds, in order to show (i) which are the reliable biocatalyzed transformations to be used by organic chemists involved in the development of manufacturing processes, and (ii) which are the biotransformations still requiring improvements and investigations. Bioreductions have been chosen as the main topic of the book, because of their widespread applications in organic synthesis and their versatility in the creation of stereogenic centers in chiral molecules. 

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