Stereoselective Reduction of Prochiral Cyclic 1,3-Diketones Using Different Biocatalysts

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.

Origin of the enantioselectivity of alcohol dehydrogenase

Alcohol dehydrogenases (ADH) are a family of enzymes that catalyse the interconversion between ketones/aldehydes and alcohols in the presence of NADPH cofactor. It is challenging to desymmetrise the substituted cyclopentane-1,3-dione by engineering an ADH, while the reaction mechanism of the metal independent ADH remains elusive. Here we measured the conversion of a model substrate 2-benzyl-2-methylcyclopentane-1,3-dione by LbADH and found it predominately gave the (2R,3R) product. Binding mode analysis of the substrate in LbADH from molecular dynamics simulations disclosed the origin of the enantioselectivity of the enzyme; the opening and closing of the loop 191-205 above the substrate are responsible for shaping the binding pocket to orientate the substrate, so as to give different stereoisomer products. Using QM/MM calculations, we elucidated the reaction mechanism of LbADH. Furthermore, we demonstrated the reaction profile corresponding to the production of different stereoisomers, which is in accordance with our experimental observations. This research here will shed a light on the rational engineering of ADH to achieve stereodivergent stereoisomer products.

Extended Scope and Understanding of Zinc-Dependent Alcohol Dehydrogenases for Reduction of Cyclic α-Diketones

Alcohol dehydrogenases (ADH) are important tools for generating chiral α-hydroxyketones. Previously, only the ADH of Thauera aromatica was known to convert cyclic α-diketones with appropriate preference. Here, we extend the spectrum of suitable enzymes by three alcohol dehydrogenases from Citrifermentans bemidjiense (CibADH), Deferrisoma camini (DecADH), and Thauera phenylacetica (ThpADH). Of these, DecADH is characterized by very high thermostability; CibADH and ThpADH convert α-halogenated cyclohexanones with increased activity. Otherwise, however, the substrate spectrum of all four ADHs is highly conserved. Structural considerations led to the conclusion that conversion of diketones requires not only the expansion of the active site into a large binding pocket, but also the circumferential modification of almost all amino acid residues that form the first shell of the binding pocket. The constellation appears to be overall highly specific for the relative positioning of the carbonyl functions and the size of the C-ring.

Directed evolution of an alcohol dehydrogenase for the desymmetric reduction of 2,2-disubstituted cyclopenta-1,3-diones by enzymatic hydrogen transfer

Directed evolution of carbonyl reductase TbADH created mutant Tb2 with balanced activity toward ethyl secodione and isopropanol, enabling the desymmetric reduction of ethyl secodione to give (13R,17S)-ethyl secol with 94% yield, >99% ee and >99% de.

Kinetics of enzyme-catalysed desymmetrisation of prochiral substrates: product enantiomeric excess is not always constant

The kinetics of enzymatic desymmetrisation were analysed for the most common kinetic mechanisms: ternary complex ordered (prochiral ketone reduction); ping-pong second (ketone amination, diol esterification, desymmetrisation in the second half reaction); ping-pong first (diol ester hydrolysis) and ping-pong both (prochiral diacids). For plausible values of enzyme kinetic parameters, the product enantiomeric excess (ee) can decline substantially as the reaction proceeds to high conversion. For example, an ee of 0.95 at the start of the reaction can decline to less than 0.5 at 95% of equilibrium conversion, but for different enzyme properties it will remain almost unchanged. For most mechanisms a single function of multiple enzyme rate constants (which can be termed ee decline parameter, eeDP) accounts for the major effect on the tendency for the ee to decline. For some mechanisms, the concentrations or ratios of the starting materials have an important influence on the fall in ee. For the application of enzymatic desymmetrisation it is important to study if and how the product ee declines at high conversion.

Structural insights into the desymmetrization of bulky 1,2-dicarbonyls through enzymatic monoreduction

Benzil reductases are dehydrogenases preferentially active on aromatic 1,2-diketones, but the reasons for this peculiar substrate recognition have not yet been clarified. The benzil reductase (KRED1-Pglu) from the non-conventional yeast Pichia glucozyma showed excellent activity and stereoselectivity in the monoreduction of space-demanding aromatic 1,2-dicarbonyls, making this enzyme attractive as biocatalyst in organic chemistry. Structural insights into the stereoselective monoreduction of 1,2-diketones catalyzed by KRED1-Pglu were investigated starting from its 1.77 Å resolution crystal structure, followed by QM and classical calculations; this study allowed for the identification and characterization of the KRED1-Pglu reactive site. Once identified the recognition elements involved in the stereoselective desymmetrization of bulky 1,2-dicarbonyls mediated by KRED1-Pglu, a mechanism was proposed together with an in silico prediction of substrates reactivity.

Biosynthesis of chiral cyclic and heterocyclic alcohols via CO/C–H/C–O asymmetric reactions

This review covers the recent progress in various biological approaches applied to the synthesis of enantiomerically pure cyclic and heterocyclic alcohols through CO/C–H/C–O asymmetric reactions.