Oxidoreductases carbonyl reductases

For reduction of acetylenic ketones, two oxidoreductases were used [25]. Lactobacillus brevis alcohol dehydrogenase (LBADH) gave the (R)-alcohols and Candida parapsilosis carbonyl reductase (CPCR) afforded the (S)-isomer, both in good yield and excellent enantioselectivity. By changing the steric demand of the substituents, the enantiomeric excess values can be adjusted and even the configurations of the products can be altered (Figure 8.34). 

Studies on metabolic stability using hepatocyte suspensions are not feasible for automation/HTS, but these studies do provide rather complete profiles of hepatic biotransformation without the supplements of cofactors and cosubstrates. The use of S9 in metabolic stability studies can be evaluated in a manner similar to that used for the microsomal assays, but with the possible addition of a broader panel of cofactors or cosubstrates. These include NADPH for CYP/FMO-mediated reactions, NADH for xanthine oxidoreductase and quinone oxidoreductase 2, NADPH-dependent reductions by carbonyl reductases, and NADPH/NADH-dependent reductions catalyzed by aldo-keto reductases, uridine 5 -diphosphate... 

The reductive biotransformation of drugs has been one of the least studied reactions, and many of the enzymes that are involved have not been well characterized. Some of the enzymes that catalyze reductive reactions of drugs are the cytochrome P450s, molybdenum reductases, alcohol dehydrogenases, carbonyl reductases, NADPH cytochrome P450 reductase, NAD(P)H— quinone oxidoreductases, and enzymes of the intestinal microflora (Matsunaga et al., 2006 Rosemond and Walsh, 2004). 

The biocatalytic counterpart for this transformation is done by the alcohol dehydrogenases [ADHs, EC 1.1.1.x., also called ketoreductases (KREDs) or carbonyl reductases (CRs)], which are able to perform stereoselective carbonyl reductions or enantioselective alcohol oxidations [5-8]. These enzymes are probably the most employed oxidoreductases and make use of a nicotinamide cofactor such as NADH or NADPH to transfer electrons into and from the target substrate. Depending on their substrate scope, ADHs can be divided into primary alcohol dehydrogenases, preferentially reducing aldehydes, and secondary alcohol dehydrogenases that have... 

Until now, several oxidoreductases or microorganisms have been used in the preparation of chiral alcohols including NAD -dependent alcohol dehydrogenases (ADHs) from yeast and horse liver [11] (EC 1.1.1.1), Candida parapsilosis [12] and Pseudomonas sp. [13], and NADP -dependent ADHs from yeast [14], Thermoanaer-obium brockii [15] and Lactobacillus kefir [16], aldehyde reductases from Sporobolo-myces salmonicolor (EC 1.1.1.2) [17] and Penicillium citrinum (EC 1.1.1.21) [18], and carbonyl reductase (EC 1.1.1.184) from Candida magnoliae [19]. Our research group has reported an efficient method for producing both enantiomers of chiral alcohols... 

Aldose reductase (ALR2 EC 1.1.1.21) is an 36 kDa enzyme that catalyzes the reduction of a wide range of carbonyl-containing compounds to their corresponding alcohols. It is a member of an extensive aldo-keto oxidoreductase enzyme family, a collection of structurally similar proteins expressed in both animals and plants. Most members of the enzyme family possess similarities in molecular mass, pH optimum, coenzyme dependence, and demonstrate overlapping specificity for many substrates and inhibitors. 

Oxidoreductases these enzymes catalyze redox reactions. Examples are oxidases that catalyze oxidation of a substrate by reducing molecular oxygen (02), and peroxidases that reduce H202. Laccases (EC 1.10.3.2) are oxidases that catalyze the oxidation of (poly)phenolic substrates. Reductases and dehydrogenases (EC 1.1.1) catalyze the reduction of carbonyls, using NADH/NADPH cofactors. Catalases (EC 1.11.1.6) catalyze the decomposition of H202 to 02 and H20. 

The remainder of the biosynthesis takes place in the peroxisomes. The substrate is imported by the cassette-transporter comatose (CTS). The hydrogenation of the endocyclic double bond is carried out by 12-oxophytodienic acid reductase this enzyme belongs to a small group of flavin-dependent oxidoreductases. In accord with the reaction mechanism, which was suggested for the related Old Yellow Enzyme of yeast, two hydrogen bridges from histidine (His-186 and His-189 in the 12-oxophytodienic acid reductase from mouse-ear cress Arabidopsis thaliana)) towards the carbonyl group polarise the double bond, so that a hydride of the reduced flavin cofactor can be transferred to C-10. The carbanion in the 11 -position is then protonated by Tyr-191. 

Forrest et al. (183) isolated and sequenced a gene from liver encoding an NADPH-dependent carbonyl (i.e., n-oxo-anthracycline) reductase. The enzyme belonged to a class of secondary alcohol oxidoreductases and was related to the Streptomyces sp. strain C5 ketoreductase encoded by arfl (280 renamed in this chapter to doHlJ). Interestingly, Forrest et al. (183) indicated that the liver enzyme also catalyzed reduction of the anthra eye line quinone as well. 

Initiating another branch of the flavonoid pathway, C4 of dihydroflavonol 16 can be reduced from a carbonyl group to a hydroxyl group by the oxidoreductase enzyme dihydroflavonol reductase (DFR), producing leucoanthocyanidins, or the colorless precursors to anthocyanins 17. Leucoanthocyanidins are unstable and are quickly converted to anthocyanidins by anthocyanidin... 

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