Bioreduction of N-(3-oxobutyl)heterocycles with flexible ring by yeast whole-cell biocatalysts

This study explored the bioreduction of N-(3-oxobutyl)heterocycles with (partially) saturated heterocyclic moieties using whole-cell forms of wild-type yeast strains and commercially available baker's yeast (Saccharomyces cerevisiae). Eleven wild-type yeast strains and baker's yeast were screened for ketoreductase activity on a series of five flexible N-heterocycles with prochiral carbonyl group in the N-(3-oxobutyl) substituent. Among the yeast strains tested, Candida parapsilosis (WY12) proved to be the most efficient biocatalyst in the bioreductions, resulting in the corresponding enantiopure alcohols-being promising chiral fragments with high level of drug-likeness-with good to excellent conversions (83-99%) and high enantiomeric excess (ee > 99%). Other strains, such as Pichia carsonii (WY1) and Lodderomyces elongisporus (WY2), also showed promising ketoreductase activities with certain substrates. After screening as lyophilized whole cells, C. parapsilosis cells were immobilized in the form of calcium, zinc, nickel, and copper alginate beads. The whole-cell immobilization enabled recycling, with considerable residual activity of the biocatalyst over multiple cycles. Additionally, the study explored the scalability of these bioreductions, with immobilized C. parapsilosis delivering promising results. The use of immobilized cells simplified the work-up process and resulted in chiral alcohols with similar or even higher conversions to those observed in the screening reactions. Molecular docking of the five flexible N-heterocycles with prochiral carbonyl group into the active site of the experimental structure of the carbonyl reductase of C. parapsilosis rationalized their biocatalytic behavior and confirmed the assigned (S)-configuration of forming enantiopure alcohols. KEY POINTS: • Ketoreductase activity of eleven wild-type yeast strains and baker's yeast were examined. • Candida parapsilosis was subjected to whole-cell immobilization and recycling. • Enantiopure alcohols with flexible N-heterocyclic units were produced at preparative scale.

Microbial alcohol dehydrogenases: recent developments and applications in asymmetric synthesis

Alcohol dehydrogenases are a well-known group of enzymes in the class of oxidoreductases that use electron transfer cofactors such as NAD(P)+/NAD(P)H for oxidation or reduction reactions of alcohols or carbonyl compounds respectively. These enzymes are utilized mainly as purified enzymes and offer some advantages in terms of green chemistry. They are environmentally friendly and a sustainable alternative to traditional chemical synthesis of bulk and fine chemicals. Industry has implemented several whole-cell biocatalytic processes to synthesize pharmaceutically active ingredients by exploring the high selectivity of enzymes. Unlike the whole cell system where cofactor regeneration is well conserved within the cellular environment, purified enzymes require additional cofactors or a cofactor recycling system in the reaction, even though cleaner reactions can be carried out with fewer downstream work-up problems. The challenge of producing purified enzymes in large quantities has been solved in large part by the use of recombinant enzymes. Most importantly, recombinant enzymes find applications in many cascade biotransformations to produce several important chiral precursors. Inevitably, several dehydrogenases were engineered as mere recombinant enzymes could not meet the industrial requirements for substrate and stereoselectivity. In recent years, a significant number of engineered alcohol dehydrogenases have been employed in asymmetric synthesis in industry. In a parallel development, several enzymatic and non-enzymatic methods have been established for regenerating expensive cofactors (NAD+/NADP+) to make the overall enzymatic process more efficient and economically viable. In this review article, recent developments and applications of microbial alcohol dehydrogenases are summarized by emphasizing notable examples.

Direct observation of redox reactions in Candida parapsilosis ATCC 7330 by Confocal microscopic studies

Confocal microscopic studies with the resting cells of yeast, Candida parapsilosis ATCC 7330, a reportedly versatile biocatalyst for redox enzyme mediated preparation of optically pure secondary alcohols in high optical purities [enantiomeric excess (ee) up to >99%] and yields, revealed that the yeast cells had large vacuoles under the experimental conditions studied where the redox reaction takes place. A novel fluorescence method was developed using 1-(6-methoxynaphthalen-2-yl)ethanol to track the site of biotransformation within the cells. This alcohol, itself non-fluorescent, gets oxidized to produce a fluorescent ketone, 1-(6-methoxynaphthalen-2-yl)ethanone. Kinetic studies showed that the reaction occurs spontaneously and the products get released out of the cells in less time [5 mins]. The biotransformation was validated using HPLC.

Enantio- & chemo-selective preparation of enantiomerically enriched aliphatic nitro alcohols using Candida parapsilosis ATCC 7330

Enantiomerically pure β- and γ-nitro alcohols were prepared from their respective nitro ketones by asymmetric reduction using Candida parapsilosis ATCC 7330 under optimized reaction conditions (ee up to >99%; yields up to 76%).

Candida parapsilosis ATCC 7330 mediated oxidation of aromatic (activated) primary alcohols to aldehydes

A green, simple and high yielding [up to 86% yield] procedure is developed for the oxidation of aromatic (activated) primary alcohols to aldehydes using whole cells of Candida parapsilosis ATCC 7330.