Carbonyl reductases systems

The enantioselective reduction of alkyl 3-oxobutanoates by carbonyl reductase (SI) from C. magnoliae was also performed in organic-aqueous two-phase reaction system (Figure 8.15) [llc,d]. 

Kragl and Wandrey made a comparison for the asymmetric reduction of acetophenone between oxazaborolidine and alcohol dehydrogenase.[59] The oxazaborolidine catalyst was bound to a soluble polystyrene [58] and used borane as the hydrogen donor. The carbonyl reductase was combined with formate dehydrogenase to recycle the cofactor NADH which acts as the hydrogen donor. Both systems were run for a number of residence times in a continuously operated membrane reactor and were directly comparable. With the chemical system, a space-time yield of 1400 g L"1 d"1 and an ee of 94% were reached whereas for the enzymatic system the space-time yield was 88 g L 1 d"1 with an ee of >99%. The catalyst half-life times were... 

There are a number of findings suggesting that agents that facilitate elimination of protein carbonyls (by either proteolytic elimination or by enzymically mediated chemical reduction) may suppress neurodegenerative conditions in model systems (Botella et al., 2004). Consequently, as carnosine may also react with protein carbonyls, it is theoretically possible that it could suppress formation and /the reactivity of protein carbonyls in the brain. Whether carnosine participates in carbonyl reductase activity has not been investigated but it is also a reasonable speculation. 

A more efficient and simple bioreduction system, which avoids the problems with the system involving naturally-occurring microorganisms or cell-free systems described above, has been required. A novel bioreduction system, in which Esherichia coli transformant cells coexpressing the genes of an NAD(P)H-dependent carbonyl reductase and GDH, as a cofactor regenerator, were used as the catalyst, has been constructed (Kataoka et al., 2003). The production of chiral CHBEs is a typical successful example with this bioreduction system. 

This bioreduction system is applicable to the production of many other useful chiral alcohols by replacing the carbonyl reductase gene with that of another appropriate enzyme for carbonyl reduction (Fig. 19.6). A good library of microbial carbonyl reductases with different substrate and stereospecifici-... 

Figure 19.6. Multi-purpose bioreduction system, involving recombinant microorganisms coexpressing carbonyl reductase and cofactor regenerator genes.

Kansal, H. and Banerjee, U.C. (2009) Enhancing the biocatalytic potential of carbonyl reductase of Candida visivanathii using aqueous-organic solvent system. Bioresour. Technol., 100,1041-1047. 

Fig. 11. Priniciple of stereospecific reduction of carbonyl compounds coupled with cofacter regeneration (a) and outline of the stereospecific reduction of ethyl 4-chloroacetoacetate (CAAE) by Sporobolomyces aldehyde reductase (AR) with glucose dehydrogenase (GDH) as a cofactor regenerator in a water-organic solvent two-phasic system (b). CHBE, ethyl 4-chloro-3-hydroxybutanoate...

A number of functional groups, such as nitro, diazo, carbonyls, disulfides, sulfoxides, and alkenes, are susceptible to reduction. In many cases it is difficult to determine whether these reactions proceed nonenzymatically by the action of biological reducing agents such as NADPFI, NADH, and FAD or through the mediation of functional enzyme systems. As noted above, the molybdenum hydroxylases can carry out, in vitro, a number of reduction reactions, including nitro, azo, A-oxidc, and sulfoxide reduction. Although the in vivo consequences of this are not yet clear, much of the distribution of reductases described below may be, in whole or in part, the distribution of molybdenum hydroxylases. 

Intriguingly, the blue copper sites, especiaUy those with a carbonyl oxygen at the axial coordination position, display high affinity for Zn + ions. Mutants in which the Met is replaced by Gin or Glu preferentiaUy bind Zn + when expressed in heterologous systems, e.g., Escherichia coli. Examples include azurin, amicyanin, nitrite reductase, and possibly also plastocyanin (Diederix et al., 2000 Hibino et al., 1995 Murphy et al., 1995 Nar et al., 1992a Romero et al., 1993). In the case of azurin it has been shown that both wild-type and the Met—Gin mutant have the same affinity for both Zn +and Cu + (Romero ci a/., 1993). In addition, EXAFS studies showed that some preparations of blue copper proteins purihed from their natural sources also contain small fractions of Zn derivatives (DeBeer George, personal communication). 

This enzyme consists of three different proteins. A, B, and C, in which a polypeptide with MW 12,000 (also known as selenoprotein A) contains selenium as selenocysteine in a unique site and the protein B with an MW of 200,000 possesses an essential carbonyl group, probably pyruvate . The system has several characteristics similar to proline reductase and may function by a related mechanism. It remains unclear, however, why selenium would be required for such a mechanism, nor is it obvious how the ATP is produced. 

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