Asymmetric bioreduction of a ketosulfone to the corresponding trans-hydroxysulfone by the yeast Rhodotorula rubra MY 2169

Chemoenzymatic Oxosulfonylation-Bioreduction Sequence for the Stereoselective Synthesis of β-Hydroxy Sulfones

A series of optically active β-hydroxy sulfones has been obtained through an oxosulfonylation-stereoselective reduction sequence in aqueous medium. Firstly, β-keto sulfones were synthesized from arylacetylenes and sodium sulfinates to subsequently develop the carbonyl reduction in a highly selective fashion using alcohol dehydrogenases as biocatalysts. Optimization of the chemical oxosulfonylation reaction was investigated, finding inexpensive iron(III) chloride hexahydrate (FeCl3  ⋅ 6H2 O) as the catalyst of choice. The selection of isopropanol in the alcohol-water media resulted in high compatibility with the enzymatic process for enzyme cofactor recycling purposes, providing a straightforward access to both (R)- and (S)-β-hydroxy sulfones. The practical usefulness of this transformation was illustrated by describing the synthesis of a chiral intermediate of Apremilast. Interestingly, the development of a chemoenzymatic cascade approach avoided the isolation of β-keto sulfone intermediates, which allowed the preparation of chiral β-hydroxy sulfones in high conversion values (83-94 %) and excellent optical purities (94 to >99 % ee).

Highly enantioselective production of (R)-halohydrins with whole cells of Rhodotorula rubra KCh 82 culture

Biotransformation of ten α-haloacetophenones in the growing culture of the strain Rhodotorula rubra KCh 82 has been carried out. Nine of the substrates underwent an effective enantioselective reduction to the respective (R)-alcohols according to Prelog's rule, with the exception of 2-chloro-1,2-diphenylethan-1-one that was not transformed by this strain. The expected reduction proceeded without dehalogenation, leading to the respective (R)-halohydrins in high yields. The use of this biocatalyst yielded (R)-2-bromo-1-phenyl-ethan-1-ol (enantiomeric excess (ee) = 97%) and its derivatives: 4'-Bromo- (ee = 99%); 4'-Chloro- (ee > 99%); 4'-Methoxy- (ee = 96%); 3'-Methoxy- (ee = 93%); 2'-Methoxy- (ee = 98%). There were also obtained and characterized 2,4'-dichloro-, 2,2',4'-trichloro- and 2-chloro-4'-fluoro-phenyetan-1-ol with >99% of enantiomeric excesses.

Strain engineering for stereoselective bioreduction of dicarbonyl compounds by yeast reductases

Pure chiral molecules are needed in the pharmaceutical and chemical industry as intermediates for the production of drugs or fine chemicals. Microorganisms represent an attractive alternative to chemical synthesis since they have the potential to generate single stereoisomers in high enantiomeric excess (ee). The baker's yeast Saccharomyces cerevisiae can notably reduce dicarbonyl compounds (in particular alpha- and beta-diketones and keto esters) to chiral alcohols with high ee. However, products are formed at a low rate. Moreover, large amounts of co-substrate are required for the regeneration of NADPH that is the preferred co-factor in almost all the known dicarbonyl reductions. Traditionally, better ee, reduction rate and product titre have been achieved via process engineering. The advent of recombinant DNA technology provides an alternative strategy to improve productivity and yield by strain engineering. This review discusses two aspects of strain engineering: (i) the generation of strains with higher reductase activity towards dicarbonyl compounds and (ii) the optimisation of co-substrate utilisation for NADPH cofactor regeneration.

Efficient anaerobic whole cell stereoselective bioreduction with recombinant Saccharomyces cerevisiae

In this study we investigate the NADPH-dependent stereoselective reduction of the bicyclic diketone bicyclo[2.2.2]octane-2,6-dione (BCO2,6D) to the chiral ketoalcohol (1R,4S,6S)-6-hydroxybicyclo[2.2.2]octane-2-one (BCO2one6ol). Our aim was to develop a whole cell batch process for reduction of carbonyl substrates with (i) a high cosubstrate yield (formed product/consumed cosubstrate) and (ii) a high conversion rate under anaerobic conditions with Saccharomyces cerevisiae as biocatalyst and glucose as cosubstrate. Five open reading frames (ORFs), YMR226c, YDR368w, YOR120w, YGL157w, and YGL039w, encoding reductases involved in the conversion of BCO2,6D were identified using cell-free extract from strains belonging to the ExClone collection (yeast ORF expression clones; ResGen, Invitrogen Corp., UK). We report the one-step purification and characterization of three major BCO2,6D reductases, YMR226cp, YDR368wp (YPR1p), and YOR120wp (GCY1p). The reductases were overexpressed under a strong constitutive promoter and the impact on cosubstrate yield, conversion time, glucose consumption rate, and reduction rate was investigated when reductases were overexpressed either alone or in combination with low phosphoglucose isomerase activity (encoded by YBR196c). Combining overexpression of BCO2,6D reductase with reduced glycolytic rate (low phosphoglucose isomerase activity) offers a fast whole cell stereoselective bioreduction system useful for facilitated anaerobic batch conversions.

Mild detergent treatment ofCandida tropicalis reveals a NADPH-dependent reductase in the crude membrane fraction, which enables the production of pure bicyclic exo-alcohol

This study demonstrated the occurrence of a NADPH-dependent exo-alcohol reductase in the crude membrane fraction of Candida tropicalis. Cytosolic endo-alcohol reductase activity could be separated from the membrane-bound exo-alcohol activity by means of detergent treatment, enabling the preparation of pure exo-alcohol via the enzymatic conversion of the bicyclic diketone, bicyclo[2.2.2]octane-2,6-dione. The exo-alcohol reductase is, to our knowledge, the first membrane-bound NADPH-dependent reductase accepting a xenobiotic carbonyl substrate that was not a steroid. When C. tropicalis was grown on D-sorbitol, a two-fold increase in the exo-reductase activity was observed as compared to when grown on glucose. An in silico comparison at the protein level between putative xenobiotic carbonyl reductases in Candida albicans, C. tropicalis and Saccharomyces cerevisiae was performed to explain why Candida species are often encountered when screening yeasts for novel stereoselective reduction properties. C. albicans contained more reductases with the potential to reduce xenobiotic carbonyl compounds than did S. cerevisiae. C. tropicalis had many membrane-bound reductases (predicted with the bioinformatics program, TMHMM), some of which had no counterpart in the two other organisms. The exo-reductase is suspected to be either a beta-hydroxysteroid dehydrogenase or a polyol dehydrogenase from either the short chain dehydrogenase family or the dihydroflavonol reductase family.

Asymmetric ketone reduction with immobilized yeast in hexane: biocatalyst deactivation and regeneration

There is a need to develop methods for producing enantiomerically pure pharmaceuticals because the racemic mixtures made today will probably not be allowed in the future. Synthetic chiral catalysts are being developed for this purpose, as well as new product separation techniques. Another possible option is to use biocatalysts, such as purified enzymes or whole microbial cells, since these can result in the production of mostly a single enantiomer. This study emphasizes the use of alginate-entrapped yeast cells to catalyze the reduction of ketones as a model system. The emphasis is on the factors that might limit the reactivity of such cells, such as equilibrium conditions, substrate or product inhibition, solvent toxicity, loss of cell viability, or the degradation of intracellular levels of enzymes or cofactors. It was found that there was a progressive loss of catalytic activity of the immobilized yeast cells, which appeared to be mainly associated with a loss of cell viability and a decline of intracellular NAD(H) levels during the reaction. The other factors investigated did not have a large effect. A regeneration scheme was developed in order to replenish the intracellular NAD(H) lost during the reaction, which involved removing the biocatalyst from the reaction and supplying the cells with a nutrient source. This resulted in an increase in the NAD(H) to initial levels and also resulted in a maintenance of the ketone reduction rate over time.

Implementation of a rapid microbial screening procedure for biotransformation activities

A rapid and efficient microbial screening procedure was developed utilizing a 24-well plate format in conjunction with an automated liquid handling system and an HPLC. For the evaluation of this miniaturized and automated screening system, we selected the bioreduction of 6-bromo-beta-tetralone to 6-bromo-beta-tetralol. This procedure employed both yeast and rhodococci libraries, representing a culture collection comprised of several hundred strains, from which to screen for desirable bioconversion activity. Most of these strains had demonstrated bioreducing activity during previous screens to insure a "hit rate" as high as possible. The cultivation of microbes in the plate format was facile, time saving, and efficient compared to the standard method of screening utilizing larger volumes, such as test tubes or shake flasks. This improved method of screening for bioconversion activity, employing pre-selected microbial libraries based on microtiter plates and a fully roboticized analytical system, proved to rapidly yield valuable leads which compared advantageously with a more classical approach. A total of 192 yeast strains and 48 rhodococci strains were screened using this procedure. Analytical data revealed that 78% of the strains tested bioconverted the tetralone to the desired alcohol.