Whole-cell catalysts oxidation reactions

The use of enzymes and whole cells as catalysts in organic chemistry is described. Emphasis is put on the chemical reactions and the importance of providing enantiopure synthons. In particular kinetics of resolution is in focus. Among the topics covered are enzyme classification, structure and mechanism of action of enzymes. Examples are given on the use of hydrolytic enzymes such as esterases, proteases, lipases, epoxide hydrolases, acylases and amidases both in aqueous and low-water media. Reductions and oxidations are treated both using whole cells and pure enzymes. Moreover, use of enzymes in sngar chemistiy and to prodnce amino acids and peptides are discnssed. 

Reduction with isolated enzymes avoids difficulties associated with diffusion limitations and also avoids the presence of many different enzymes, present in the whole cell, which can cause side reactions or reduced enantioselectivity. The main drawback, however, is the instability of the isolated enzyme and the requirement for added co-factor NAD(H) or NADP(H), which are the oxidized (or reduced) forms of nicotinamide adenine diphosphate or its 2 -phosphate derivative. These co-factors are expensive, but can be used as catalysts in the presence of a co-reductant such as formate ion HCOO or an alcohol (e.g. isopropanol or ethanol). The reduction of ketones occurs by transfer of hydride from the C-4 position of the dihydropyridine ring of NADH or NADPH (7.105). Only one of the two hydrogen atoms is transferred and this process occurs within the active site of the enzyme to promote asymmetric reduction. 

Divergent reaction conditions are well exemplified in the case of reduction and oxidation steps. Consequently it is easier to run such systems sequentially [53]. This leads to the possibility of a linear-cyclic-parallel system for oxidation and reduction, proposed by Oberleitner and coworkers [54]. Here oxidative and reductive catalysts were combined to balance redox, followed by a further oxidation reaction. For example, alcohol dehydrogenase and enoate reductase, followed by Baeyer-Villiger monooxygenase, have been demonstrated in such a system. The last oxidation needs to be run with a parallel system or alternatively in a whole cell for cofactor recycle. 

Since bacterial cells behave like bags of enzymes in catalyzing redox reactions of substrates, using artificial redox compounds as electron acceptors or donors, they may work as catalysts to produce catalytic currents for the electrocatalytic oxidation or reduction of substrates in the presence of appropriate electron-transfer mediators. In fact, whole bacterial cells both in a suspension and in an immobilized state produce similar bioelectro-catalytic currents to those obtained with enzymes. 

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