Influence of deep eutectic solvents on redox biocatalysis involving alcohol dehydrogenases

Abstract

Redox biocatalysis plays an increasingly important role in modern organic synthesis. The recent integration of novel media such as deep eutectic solvents (DESs) has significantly impacted this field of chemical biology. Alcohol dehydrogenases (ADHs) are important biocatalysts where their unique specificity is used for enantioselective synthesis. This review explores aspects of redox biocatalysis in the presence of DES both with whole cells and with isolated ADHs. In both cases, the presence of DES has a significant influence on the outcome of reactions albeit via different mechanisms. For whole cells, DES was shown to be a useful tool to direct product formation or configuration - a process of solvent engineering. Whole cells can tolerate DES as media components for the solubilization of hydrophobic substrates. In some cases, DES in the growth medium altered the enantioselectivity of whole cell transformations by solvent control. For isolated enzymes, on the other hand, the presence of DES promotes substrate solubility as well as enhancing enzyme stability and activity. DES can be employed as a smart solvent or smart cosubstrate particularly for cofactor regeneration purposes. From the literatures examined, it is suggested that DES based on choline chloride (ChCl) such as ChCl:Glycerol (Gly), ChCl:Glucose (Glu), and ChCl:1,4-butanediol (1,4-BD) are useful starting points for ADH-based redox biocatalysis. However, each specific reaction will require optimisation due to the influence of several factors on biocatalysis in DES. These include solvent composition, enzyme source, temperature, pH and ionic strength as well as the substrates and products under investigation.

Keywords: Alcohol dehydrogenases; Biocatalysis; Deep eutectic solvents; Enantioselectivity; Solvent engineering.

Graphical abstract

Fig. 1

Nicotinamide adenine dinucleotide non-phosphorylated (NADH/NAD⁺) and its phosphorylated ribose (NADPH/NADP⁺) form. The adenine dinucleotide portion of the molecule is indicated as R. The site of phosphorylation, on the ribose moiety, is highlighted in red. The reversible transfer of a hydride ion is indicated. Note that the nicotinamide portion of the NAD(P) is the recipient or donor of a hydride ion during redox catalysis.

Fig. 2

Mechanism of ADH catalysed reduction and oxidation involving an active site metal (zinc). The nicotinamide cofactor is shown coordinated to the active site zinc. The binding of the alcohol compound leads to hydride transfer to the nicotinamide moiety of NAD⁺ and the binding of the carbonyl compound (reverse reaction) leads to the abstraction of hydride ion from the nicotinamide moiety of NADH. Note: The R group attached to the nicotinamide represents the remainder of the cofactor (see Fig. 1). R₁ and R₂ represent alkyl or aryl substituents. The hydride ion transferred during the redox reaction is highlighted in red. Adapted from Ref. [22].

Fig. 3

a) Classification of ADH based on the metal ion in the active site [39,40,53] from different species. b) 3D structure of horse liver ADH containing zinc ion, inset shows the zinc ion complexed to NADH and cyclohexyl formamide (PDB code-1LDY) [54]. Created with BioRender.com.

Fig. 4

Stereochemical recognition in asymmetric reduction reactions catalysed by ADHs. a) Prelog model, which was based on the addition/transfer of hydride ion (H⁻) to/from the cofactor to the substrate giving corresponding (R) alcohol (anti-Prelog ADH)/(S) alcohol (Prelog ADH) or the carbonyl compound [55]. b) Keinan model to explain enantiomeric outcomes in ADH-catalysed reactions which was based on the bulkiness of groups in the space surrounding the carbonyl moiety: above a certain bulk, some alkyl groups cannot bind to a smaller site (occupied by R₁ above) and will enter the larger site leading to the observed transition from R to S configuration [56]. Created with BioRender.com.

Scheme 1

Asymmetric reduction of 1-(2-(trifluoromethyl)phenyl)ethan-1-one with G. silvicola ZJPH1811. ee is enantiomeric excess.

Scheme 2

Ester formation in carboxylic acid-based DES during preparation and long-term storage.

Scheme 3

Synthesis of FDM from 5-hydroxymethylfurfural by P. putida S12 cells in betaine based DES.

Scheme 4

Synthesis of furfuryl alcohol from furfural with recombinant E. coli CF containing a reductase and cofactor recycling by formate dehydrogenase (FDH).

Scheme 5

Asymmetric reduction of 3-chloropropiophenone with immobilised Acetobacter sp. CCTCC M209061 cells.

Scheme 6

Enantioselective reduction of different ketones with Baker's yeast. The enantioselectivity varies with the reaction conditions [59].

Scheme 7

Asymmetric reduction of 3-acetyldihydrofuran-2(3H)-one with Y.lipolytica AM7 and C.viswanathi AM120. Kpi is potassium phosphate.

Scheme 8

Asymmetric reduction of cinnamaldehyde with HLADH in ChCl:Gly.

Fig. 5

Schematic illustraiton of thermomorphic multiphase system (TMS). T is the system temperature, T_LCST is the lower critical solution temperature. Created with BioRender.com.

Fig. 6

Overview of cofactor regeneration strategies over time with the introduction of DES in ADH catalysed reactions. a) Coupled enzyme approach in which the oxidation of phosphite by PTDH facilitates cofactor regeneration [110]. b) Smart cosubstrate, in which the 1,4-BD is getting oxidised to thermodynamically irreversible and kinetically inert coproduct facilitating the cofactor regeneration [108]. c) Extended coupled enzyme, in which PTDH, GDH, FDH are used for cofactor regeneration by oxidising phosphite, glucose, formate respectively [111]. d) smart solvent, in this case, glucose acts as HBD in DES and as a cosubstrate in the enzyme reaction for cofactor regeneration [112]. e) smart solvent and smart cosubstrate, 1,4-BD acts as HBD in the DES as well as being oxidised to a corresponding thermodynamically irreversible and kinetically inert coproduct [109]. The central yellow arrow denotes time. Abbreviations: BVMO is Baeyer-Villiger monooxygenases, HLADH is horse liver alcohol dehydrogenase, PTDH is phosphite dehydrogenase, GDH is glucose dehydrogenase and FDH is formate dehydrogenase. Created with BioRender.com.

Fig. 7

Advancement of the smart solvent concept from batch reaction to continuous flow reaction in ADH catalysed reactions. (a) Batch reaction in which glucose acts as both HBD of DES and as a cosubstrate for cofactor regeneration [112]. (b) Continuous flow system with the concept of smart solvent for the reduction of 3-oxo-3-(thiophen-2-yl)propanenitrile [113]. Imm. R. rubra is immobilised whole cells of Rhodotorula rubra MIM14. Created with BioRender.com.