Oxidase synthetic applications

Ketoreductases catalyze the reversible reduction of ketones and oxidation of alcohols using cofactor NADH/NADPH as the reductant or NAD + /NADP+ as oxidant. Alcohol oxidases catalyze the oxidation of alcohols with dioxygen as the oxidant. Both categories of enzymes belong to the oxidoreductase family. In this chapter, the recent advances in the synthetic application of these two categories of enzymes are described. 

Compared with ketoreductases, the synthetic application of alcohol oxidases has been less explored. However, selective oxidation of primary alcohols to aldehydes is superior to the chemical methods in terms of conversion yields, selectivity, and environmental friendliness of reaction conditions. In addition, coupling of alcohol oxidase with other enzymes provides a tremendous opportunity to develop multi-enzyme processes for the production of complex molecules. Therefore, a growing impact of alcohol oxidases on synthetic organic chemistry is expected in the coming years. 

Galactose oxidase exhibits a surprisingly low specificity for the primary alcohol but is completely regioselective secondary alcohols are not substrates. This re-gioselectivity suggests potential synthetic applications (117) and has raised interest in the design of small molecule catalysts mimicking GO reactivity. 

A broad spectrum of chemical reactions can be catalyzed by enzymes Hydrolysis, esterification, isomerization, addition and elimination, alkylation and dealkylation, halogenation and dehalogenation, and oxidation and reduction. The last reactions are catalyzed by redox enzymes, which are classified as oxidoreductases and divided into four categories according to the oxidant they utilize and the reactions they catalyze 1) dehydrogenases (reductases), 2) oxidases, 3) oxygenases (mono- and dioxygenases), and 4) peroxidases. The latter enzymes have received extensive attention in the last years as bio catalysts for synthetic applications. Peroxidases catalyze the oxidation of aromatic compounds, oxidation of heteroatom compounds, epoxidation, and the enantio-selective reduction of racemic hydroperoxides. In this article, a short overview... 

Mazur AW (1991) Galactose Oxidase Selected Properties and Synthetic Applications. In Bednarski MD, Simon ES (eds) Enzymes in Carbohydrate Synthesis. American Chemical Society, Washington, vol. 466, p 99... 

The wide variety of known oxidases reflects the complexities of the different substrate classes such as carbohydrates, amino acids, lipids, amines, metabolites, alcohols, acids, and other chiral building blocks. An overview of synthetic applications of oxidases is given in Figure 20.3. 

Together with enantioselective hydrolysis/acylation reactions, enantioselective ketone reductions dominate biocatalytic reactions in the pharma industry [10], In addition, oxidases [11] have found synthetic applications, such as in enantioselective Baeyer-Villiger reactions [12] catalyzed by, for example, cyclohexanone monooxygenase (EC 1.14.13) or in the TEMPO-mediated oxidation of primary alcohols to aldehydes, catalyzed by laccases [13]. Hence, the class of oxidoreductases is receiving increased attention in the field of biocatalysis. Traditionally they have been perceived as difficult due to cofactor requirements etc, but recent examples with immobilization and cofactor regeneration seem to prove the opposite. 

Recently, laccases found some interest for synthetic application. Laccases are widely distributed in plants and fungi1131. The copper-containing enzymes are some of the few oxidases so far reported to reduce molecular oxygen to water (aside from cytochrome c oxidase and others). This ability was recently exploited in a novel regeneration concept for flavin-dependent enzymes (see Chapter 16.2)[14]. 

Among the enzymes catalyzing oxidations of carbon nitrogen bonds, the amino acid oxidases (AAO, E.C. 1.4.3.x) are the most interesting for synthetic applications. Compared to some specific amino acid oxidases such as aspartate oxidase or glutamate oxidase, the two d- and L-amino acid oxidases (E.C. 1.4.3.2 for l-AAO and E.C. 1.4.3.3 for d-AAO) are advantageous on account of their broad substrate... 

Bioelectrocatalysis involves the coupling of redox enzymes with electrochemical reactions [44]. Thus, oxidizing enzymes can be incorporated into redox systems applied in bioreactors, biosensors and biofuel cells. While biosensors and enzyme electrodes are not synthetic systems, they are, essentially, biocatalytic in nature (Scheme 3.5) and are therefore worthy of mention here. Oxidases are frequently used as the biological agent in biosensors, in combinations designed to detect specific target molecules. Enzyme electrodes are possibly one of the more common applications of oxidase biocatalysts. Enzymes such as glucose oxidase or cholesterol oxidase can be combined with a peroxidase such as horseradish peroxidase. 

The interest in catechol oxidase, as well as in other copper proteins with the type 3 active site, is to a large extent due to their ability to process dioxygen from air at ambient conditions. While hemocyanin is an oxygen carrier in the hemolymph of some arthropods and mollusks, catechol oxidase and tyrosinase utilize it to perform the selective oxidation of organic substrates, for example, phenols and catechols. Therefore, establishment of structure-activity relationships for these enzymes and a complete elucidation of the mechanisms of enzymatic conversions through the development of synthetic models are expected to contribute greatly to the design of oxidation catalysts for potential industrial applications. 

The coupling of these two enzymatic systems could find many more applications due to the avaUabihty of amino acid dehydrogenases of broader specificity [31]. A series of amino acid dehydrogenases with D-specificity for the preparation of D-amino acids has been applied to the reductive amination of a-keto acids [32]. However, the deracemization of rac-amino acids exploiting this type of enzyme requires an amino acid oxidase with L-specificily, which is a rare enzymatic activity. As an alternative the a-oxo acid, usually available through difficult synthetic procedures, can be used directly. 

Prom a practical point of view, the application of biomimetic models to processes of synthetic, or industrial interest depends strongly on the transformation of the current stoichiometric reactions into catalytic reactions (e.g., in the phenol oxygenation reactions), the stability of the catalysts, and the introduction of stereoselectivity characteristics in the reactions, by using structurally more sophisticated and possibly chiral ligands. In addition, the biomimetic chemistry of the more complex multinuclear Cu sites, such as the trinuclear Cu sites of blue oxidases, is still in its infancy, and important contributions are expected in the next few years from the study of new and suitable model compounds. 

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