A ketoreductase

ADH alcohol dehydrogenase (alternative name for a ketoreductases or KREDs)... 

There are basically two approaches to the synthesis of enantiomerically pure alcohols (i) kinetic resolution of the racemic alcohol using a hydrolase (lipase, esterase or protease) or (ii) reduction mediated by a ketoreductase (KRED). Both of these processes can be performed as a cascade process. The first approach can be performed as a dynamic kinetic resolution (DKR) by conducting an enzymatic transesterification in the presence of a redox metal [e.g. a Ru(ll) complex] to catalyze in situ racemization of the unreacted alcohol isomer [11] (Scheme 6.1). We shall not discuss this type of process in any detail here since it forms the subject of Chapter 1. 

MSA does not contain any chiral carbon centers. Before the aromatization of the six-membered ring occurs, two prochiral carbons (C-2 and C-4 in the six-carbon intermediate) evolve, each of which loses a hydrogen in the process of the dehydratization/aromatization steps. In addition, C-3 of the six-carbon intermediate forms a chiral center when the ketone is reduced to a hydroxyl by a ketoreductase activity (Fig. 5). The chirality of this hydroxyl carbon is unclear since the intermediate has not been isolated. It is also unknown if this carbon retains its chirality in an eight-carbon intermediate or whether the hydroxyl is eliminated by dehydration prior to the third condensation reaction. The stereospecificity at the prochiral C-2 and C-4 carbons in the reaction intermediates was addressed using chemically synthesized (] )- and (S)-[1- C, 2- H]malonate precursors which were enzymatically converted into CoA derivatives via succinyl CoA transferase [127,128]. Thus, the prochiral methylene in malonyl CoA was replaced by chiral, double-labeled (S)- or (J )-[1- C, 2- H]malonyl CoA substrates in the reaction mixture with 6-MSAS. The condensation is expected to occur with inversion of configuration and the intact methylene... 

Figure 3 (a) Traditional view of reductive processing at the f-ketone position in the growing polyketide chain. Presence of a ketoreductase domain leads... 

Keatinge-Clay AT, Stroud RM. The structure of a ketoreductase determines the organization of the beta-carbon processing enzymes of modular synthases. Structure 2006 14 737-748. 

E. coli fell into the PKS associated gene. These included an AGP, four /3-ketoacyl AGP synthases, an aminotransferase, an AGP reductase, and a ketoreductase (Figure 9). Two transcription factors (a LysR-type response regulator and a IclR-type response regulator) located adjacent to the predicted biosynthetic genes... 

Hanson RL, Goldberg S, Goswami A, et al. Purification and cloning of a ketoreductase used for the preparation of chiral alcohols. Advanced Synthesis and Catalysis 347(7-1-8), 1073, 2004. 

The majority of the anthracyclines belong to the class of reduced polyketides, meaning that after formation of the polyketide chain the keto group at C-9 (numbered from the enzyme-bound carboxy termini) is reduced after cyclization of the first ring between C-7 and C-12 by a ketoreductase (Scheme 2, step 2). The ketoreduction occurs in all anthracycline biosynthetic pathways apart from steffimycin, where the lack of this enzymatic step leads... 

Life uses enzymes to catalyse asymmetric reactions, so the question is—can chemists The answer is yes, and there are many enzymes that can be produced in quantities large enough to be used in the catalytic synthesis of enantiomerically pure molecules. This field—known as biocatalysis—melds ideas in chemistry and biology, and we do not have the space here to discuss it in detail. We leave you with just one example the reduction of a ketone to an alcohol with an enzyme known as a ketoreductase. 

Essential for the economics of the overall process is a simple way to make the amino-component available. A pivotal intermediate is therefore the R)-4-cyano-3-hydroxybutyrate ester, which is accessible by various routes. [397] In variant a) ethyl 4-chloroacetoacetate is reduced enantioselectively with a ketoreductase, and the chlorine is replaced enzymatically by cyanide (Codexis technology). [398]... 

The nascent polyketide chain must then be folded into the tetracyclic structure. Again, the exact details of chain folding have not been deduced for any polyketide structure, but a ketoreductase (for which Actlll is the paradigm) and aromatase/cyciase enzyme (or enzymes) are known to be important (102). For oxytetracycline (Figure 7), 0lcY2-l (ketoreductase) and otcDl (aromatase) are the likely candidates, although others may also be involved. 

For both FAS types, the catalytic domains include an AT, a ketosynthase (KS), a ketoreductase (KR), a dehydratase (DH), an enoylreductase (ER), an acyl carrier protein (ACP), and a thioesterase (TE). Both FAS I and FAS II utilize Ac-CoA and malonyl-CoA as substrates that are converted into enzyme-bound thioesters. 

Recently, Codexis devdoped a ketoreductase which catalyzed the reduction of 80 to (lS,2R)-79. Wild type recombinant ketoreductase gave only 5% conversion and 80% enantiopuiity after 24 h at substrate input of 20 g/L and enzyme input of 5 g/L. The enzyme was evolved based on ProSAR generated library and screening of entire library of mutants. They developed a highly stereoselective and diastereosdective reduction process with isopropanol in water as the solvent and NADP+ as the cofactor by a custom-evolved ketoreductase. The evolved enzyme operated at 200 g/L substrate, 1 g/L enzyme and 0.01 g/L NADP inputs with 99.9% conversion and >99% enantiomeric purity in 24 h reaction time [144]. 

An enzymatic alternative for reducing ketone 79 was created by Codexis. A ketoreductase (KRED) was developed by directed evolution using high-throughput screens that mimicked the actual process conditions. Beneficial mutations obtained during each round were recombined and new mutations were introduced, guided by ProSAR. The productivity of the final enzyme was improved 2000-fold and stability was also substantially increased [109,110]. 

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