Candida parapsilosis: A versatile biocatalyst for organic oxidation-reduction reactions

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.

Hydroxynitrile lyase engineering for promiscuous asymmetric Henry reaction with enhanced conversion, enantioselectivity and catalytic efficiency

Arabidopsis thaliana hydroxynitrile lyase (AtHNL) engineering has uncovered variants that showed up to 12-fold improved catalytic efficiency than the wild-type towards asymmetric Henry reaction. The AtHNL variants have displayed excellent enantioselectivity, up to >99%, and higher conversion in the synthesis of 13 different (R)-β-nitroalcohols from their corresponding aldehydes. Using cell lysates of Y14M/F179W, we demonstrated a preparative scale synthesis of (R)-1-(4-methoxyphenyl)-2-nitroethanol, a tembamide chiral intermediate, in >99% ee and 52% yield.

Batch and Flow Nitroaldol Synthesis Catalysed by Granulicella tundricola Hydroxynitrile Lyase Immobilised on Celite R-633

Granulicella tundricola hydroxynitrile lyase (GtHNL) catalyses the synthesis of chiral (R)-cyanohydrins and (R)-β-nitro alcohols. The triple variant GtHNL-A40H/V42T/Q110H (GtHNL-3V) was immobilised on Celite R-633 and used in monophasic MTBE saturated with 100 mM KPi buffer pH 7 for the synthesis of (R)-2-nitro-1-phenylethanol (NPE) in batch and continuous flow systems. Nitromethane was used as a nucleophile. A total of 82% of (R)-NPE and excellent enantioselectivity (>99%) were achieved in the batch system after 24 hours of reaction time. GtHNL-3V on Celite R-633 was successfully recycled five times. During more recycling steps a significant decrease in yield was observed while the enantioselectivity remained excellent over eight cycles. The use of a flow system enabled the continuous synthesis of (R)-NPE. A total of 15% formation of (R)-NPE was reached using a flow rate of 0.1 mL min−1; unfortunately, the enzyme was not stable, and the yield decreased to 4% after 4 hours on stream. A similar yield was observed during 15 hours at a rate of 0.01 mL min−1. Surprisingly the use of a continuous flow system did not facilitate the process intensification. In fact, the batch system displayed a space-time-yield (STY/mgenzyme) of 0.10 g L−1 h−1 mgenzyme−1 whereas the flow system displayed 0.02 and 0.003 g L−1 h−1 mgenzyme−1 at 0.1 and 0.01 mL min−1, respectively. In general, the addition of 1 M nitromethane potentially changed the polarity of the reaction mixture affecting the stability of Celite-GtHNL-3V. The nature of the batch system maintained the reaction conditions better than the flow system. The higher yield and productivity observed for the batch system show that it is a superior system for the synthesis of (R)-NPE compared with the flow approach.

Imine reduction by an Ornithine cyclodeaminase/μ-crystallin homolog purified from Candida parapsilosis ATCC 7330

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Novel imine reductase from yeast Candida parapsilosis purified and characterized.

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CpIM1 belongs to unexplored Ornithine cyclodeaminase/Mu crystallin protein family

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CpIM1 catalyzed stereospecific alkylamination of α-ketoacids/ketoesters.

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CpIM1 also reduced cyclic and aryl imines.

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First report on enzymatic alkylamination of α-ketoesters and reduction of arylimines.

We report a stereospecific imine reductase from Candida parapsilosis ATCC 7330 (CpIM1), a versatile biocatalyst and a rich source of highly stereospecific oxidoreductases. The recombinant gene was overexpressed in Escherichia coli and the protein CpIM1 was purified to homogeneity. This protein belongs to the Ornithine cyclodeaminase/ μ-crystallin (OCD-Mu) family of proteins which has only a few characterized members. CpIM1 catalyzed the alkylamination of α-keto acids/esters producing exclusively (S)-N-alkyl amino acids/esters e.g. N-methyl-l-alanine with > 90% conversion and > 99% enantiomeric excess (ee). The enzyme showed the highest activity for the alkylamination of pyruvate and methylamine leading to N-methyl-l-alanine with an apparent K_M of 15.04 ± 2.8 mM and V_max of 13.75 ± 1.07 μmol/min/mg. CpIM1 also catalyzed (i) the reduction of imines e.g. 2-methyl-1-pyrroline to (S)-2-methylpyrrolidine with ∼30% conversion and 75% ee and (ii) the dehydrogenation of cyclic amino acids e.g. l-Proline (as monitered by reduction of cofactor NADP⁺ spectrophotometrically).

Biocatalytic approaches for enantio and diastereoselective synthesis of chiral β-nitroalcohols

Chiral β-nitroalcohols find significant application in organic synthesis due to the versatile reactivity of hydroxyl and nitro functionalities attached to one or two vicinal asymmetric centers. They are key building blocks of several important pharmaceuticals, bioactive molecules, and fine chemicals. With the growing demand to develop clean and green methods for their synthesis, biocatalytic methods have gained tremendous importance among the existing asymmetric synthesis routes. Over the years, different biocatalytic strategies for the asymmetric synthesis of β-nitroalcohol stereoisomers have been developed. They can be majorly classified as (a) kinetic resolution, (b) dynamic kinetic resolution, (c) Henry reaction, (d) retro-Henry reaction, (e) asymmetric reduction, and (f) enantioselective epoxide ring-opening. This review aims to provide an overview of the above biocatalytic strategies, and their comparison along with future prospects. Essentially, it presents an enzyme-toolbox for the asymmetric synthesis of β-nitroalcohol enantiomers and diastereomers.

Understanding (R) Specific Carbonyl Reductase from Candida parapsilosis ATCC 7330 [CpCR]: Substrate Scope, Kinetic Studies and the Role of Zinc

CpCR, an (R) specific carbonyl reductase, so named because it gave (R)-alcohols on asymmetric reduction of ketones and ketoesters, is a recombinantly expressed enzyme from Candida parapsilosis ATCC 7330. It turns out to be a better aldehyde reductase and catalyses cofactor (NADPH) specific reduction of aliphatic and aromatic aldehydes. Kinetics studies against benzaldehyde and 2,4-dichlorobenzaldehyde show that the enzyme affinity and rate of reaction change significantly upon substitution on the benzene ring of benzaldehyde. CpCR, an MDR (medium chain reductase/dehydrogenase) containing both structural and catalytic Zn atoms, exists as a dimer, unlike the (S) specific reductase (SRED) from the same yeast which can exist in both dimeric and tetrameric forms. Divalent metal salts inhibit the enzyme even at nanomolar concentrations. EDTA chelation decreases CpCR activity. However, chelation done after the enzyme is pre-incubated with the NADPH retains most of the activity implying that Zn removal is largely prevented by the formation of the enzyme-cofactor complex.

[Yeasts from the CTG clade (Candida clade): Biology, impact in human health, and biotechnological applications]

Among the subdivision of Saccharomycotina (ascomycetes budding yeasts), the CTG clade (formerly the Candida clade) includes species that display a particular genetic code. In these yeasts, the CTG codon is predominantly translated as a serine instead of a leucine residue. It is now well-known that some CTG clade species have a major impact on human and its activities. Some of them are recognized as opportunistic agents of fungal infections termed candidiasis. In addition, another series of species belonging to the CTG clade draws the attention of some research groups because they exhibit a strong potential in various areas of biotechnology such as biological control, bioremediation, but also in the production of valuable biocompounds (biofuel, vitamins, sweeteners, industrial enzymes). Here we provide an overview of recent advances concerning the biology, clinical relevance, and currently tested biotechnological applications of species of the CTG clade. Future directions for scientific research on these particular yeasts are also discussed.

Genome hunting of carbonyl reductases from Candida glabrata for efficient preparation of chiral secondary alcohols

In this work, genome hunting strategy was adopted in screening for reductases from Candida glabrata. A total of 37 putative reductases were successfully expressed in E. coli BL21(DE3). A substrate library containing 32 substrates was established for characterization of each reductase by average specific activity and Shannon-Wiener index. Among them, Cg26 was identified with the highest efficiency and wider substrate spectrum in the reduction of prochiral ketones, with average activity and Shannon-Wiener index of 8.95U·mg^-1 and 2.82. Cg26 is a member of 'extended' short chain dehydrogenase/reductase superfamily. Ni²⁺ could improve its activity. As much as 150g·L^-1 ethyl 2-oxo-4-phenylbutyrate could be completely converted by 10g·L^-1 Cg26. This study provides evidence for this newly identified Cg26 in the preparation of chiral secondary alcohols.

A carbonyl reductase from Candida parapsilosis ATCC 7330: substrate selectivity and enantiospecificity

A purified carbonyl reductase (SRED) asymmetrically reduces ketones and α-ketoesters to (S)-alcohols with a potential application in the synthesis of industrially important chiral molecules.