Biocatalysis oxidoreductases

In order to broaden the field of biocatalysis in ionic liquids, other enzyme classes have also been screened. Of special interest are oxidoreductases for the enan-tioselective reduction of prochiral ketones [40]. Formate dehydrogenase from Candida boidinii was found to be stable and active in mixtures of [MMIM][MeS04] with buffer (Entry 12) [41]. So far, however, we have not been able to find an alcohol dehydrogenase that is active in the presence of ionic liquids in order to make use of another advantage of ionic liquids that they increase the solubility of hydrophobic compounds in aqueous systems. On addition of 40 % v/v of [MMIM][MeS04] to water, for example, the solubility of acetophenone is increased from 20 mmol to 200 mmol L ... 

Oxidoreductases, which catalyze oxidation-reduction reactions and are acting, for example, on aldehyde or keto groups. An important application is the synthesis of chiral molecules, especially chiral PFCs (22 out of 38 chiral products produced on large industrial scale are already made using biocatalysis). 

HNLs comprise a heterogenous enzyme family, since hydroxynitrile lyase activity has evolved in different structural frames by convergent evolution [17, 18]. Thus, (S) -specific HNLs based on an a/P-hydrolase fold framework from Manihot esculmta (cassava) [19-21], Hevea hrasilensis (rubber tree) [22-26], and Sorghum hicolor (millet) [27-33] have been described. (R)-specific HNLs based on the structural framework of oxidoreductases were isolated from Linum usitatissimum (flax) [30, 34-37] and Rosaceae (e.g., bitter almonds) [31, 38]. Despite their potential in biocatalysis only few HNLs (from cassava and rubber tree) are available by recombinant gene expression, which is a prerequisite for their technical application [20, 24]. Thus, cloning, recombinant expression, and... 

The goal of this review is to highlight a progress in the transition-metal chemistry of some enzymes that catalyze oxidative and reductive reactions. These enzymes are referred to as oxidoreductases (1,2) and transition metals are usually found in their active sites. However, the discussion will not be devoted to these metals, which are absolutely essential for biocatalysis. Such information is brilliantly summarized in several recent fundamental reviews and monographs (3-9). 

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. 

An important technical issue is the large-scale applicability of co-factor-dependent enzymatic systems. It is generally accepted that, e.g., NADH-requiring oxidoreductases can easily be used in whole-cell biocatalysis such as baker s yeast-mediated reductions, where the cofactor recycling step is simultaneously performed within the intact cell, driven by the reduction equivalents introduced via the external carbon and energy source (glucose). 

Enoate reductases (EC 1.3.1.31) are flavin-dependent and iron-sulfur-contain-ing proteins found among others in Clostridium species. Members of this NADHrflavin oxidoreductase/NADH oxidase family are distinguished from other ERs due to their high stereospecificity and strict regioselectivity for the reduction of double bonds of monoacids and monoesters, as well as reducing classical substrates such as a,p-unsaturated aldehydes, cyclic ketones, and methyl ketones. However, these enzymes are extremely oxygen sensitive, so they have not been employed in biocatalysis so far [2,3]. 

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