Acceleration of an aldo-keto reductase by minimal loop engineering

Reductive enzymatic dynamic kinetic resolution affording 115 g/L (S)-2-phenylpropanol

Background

Published biocatalytic routes for accessing enantiopure 2-phenylpropanol using oxidoreductases afforded maximal product titers of only 80 mM. Enzyme deactivation was identified as the major limitation and was attributed to adduct formation of the aldehyde substrate with amino acid residues of the reductase.

Results

A single point mutant of Candida tenuis xylose reductase (CtXR D51A) with very high catalytic efficiency (43·10³ s⁻¹ M⁻¹) for (S)-2-phenylpropanal was found. The enzyme showed high enantioselectivity for the (S)-enantiomer but was deactivated by 0.5 mM substrate within 2 h. A whole-cell biocatalyst expressing the engineered reductase and a yeast formate dehydrogenase for NADH-recycling provided substantial stabilization of the reductase. The relatively slow in situ racemization of 2-phenylpropanal and the still limited biocatalyst stability required a subtle adjustment of the substrate-to-catalyst ratio. A value of 3.4 g_substrate/g_{cell-dry-weight} was selected as a suitable compromise between product ee and the conversion ratio. A catalyst loading of 40 g_{cell-dry-weight} was used to convert 1 M racemic 2-phenylpropanal into 843 mM (115 g/L) (S)-phenylpropanol with 93.1% ee.

Conclusion

The current industrial production of profenols mainly relies on hydrolases. The bioreduction route established here represents an alternative method for the production of profenols that is competitive with hydrolase-catalyzed kinetic resolutions.

Supplementary Information

The online version contains supplementary material available at 10.1186/s12896-021-00715-5.

Fluorescence-based screening for engineered aldo-keto reductase KmAKR with improved catalytic performance and extended substrate scope

BACKGROUND

Aldo-keto reductases-catalyzed transformations of ketones to chiral alcohols have become an established biocatalytic process step in the pharmaceutical industry. Previously, we have discovered an aldo-keto reductase (AKR) from Kluyveromyces marxianus that is active to the aliphatic tert-butyl 6-substituted (5R/S)-hydroxy-3-oxohexanoates, but it is inactive to aromatic ketones. In order to acquire an excellent KmAKRmutant for ensuring the simultaneous improvement of activity-thermostability toward tert-butyl 6-cyano-(5R)-hydroxy-3-oxohexanoate ((5R)-1) and broadening the universal application prospects toward more substrates covering both aliphatic and aromatic ketones, a fluorescence-based high-throughput (HT) screening technique was established.

MAIN METHODS AND MAJOR RESULTS

The directed evolution of KmAKR variant M5 (KmAKR-W297H/Y296W/K29H/Y28A/T63M) produced the "best" variant M5-Q213A/T23V. It exhibited enhanced activity-thermostability toward (5R)-1, improved activity toward all 18 test substrates and strict R-stereoselectivity toward 10 substrates in comparison to M5. The enhancement of enzymatic activity and the extension of substrate scope covering aromatic ketones are proposed to be largely attributed to pushing the binding pocket of M5-Q213A/T23V to the enzyme surface, decreasing the steric hindrance at the entrance and enhancing the flexibility of loops surrounding the active center. In addition, combined with 0.94 g dry cell weight (DCW) L^-1 glucose dehydrogenase from Exiguobacterium sibiricum (EsGDH) for NADPH regeneration, 2.81 g DCW L^-1 M5-Q213A/T23V completely converted (5R)-1 of up to 450 g L^-1 at 120 g g^-1 substrates/catalysts (S/C), yielding the corresponding optically pure tert-butyl 6-cyano-(3R,5R)-dihydroxyhexanoate ((3R,5R)-2, > 99.5% d.e._p ) with a space-time yield (STY) of 1.08 kg L^-1 day^-1 .

CONCLUSIONS

A fluorescence-based HT screening system was developed to tailor KmAKR's activity, thermostability and substrate scope. The "best" variant M5-Q213A/T23V holds great potential application for the synthesis of aliphatic/aromatic R-configuration alcohols.

Pushing the limits: Cyclodextrin-based intensification of bioreductions

The asymmetric reduction of ketones is a frequently used synthesis route towards chiral alcohols. Amongst available chemo- and biocatalysts the latter stand out in terms of product enantiopurity. Their application is, however, restricted by low reaction output, often rooted in limited enzyme stability under operational conditions. Here, addition of 2-hydroxypropyl-β-cyclodextrin to bioreductions of o-chloroacetophenone enabled product concentrations of up to 29 % w/v at full conversion and 99.97 % e.e. The catalyst was an E. coli strain co-expressing NADH-dependent Candida tenuis xylose reductase and a yeast formate dehydrogenase for coenzyme recycling. Analysis of the lyophilized biocatalyst showed that E. coli cells were leaky with catalytic activity found as free-floating enzymes and associated with the biomass. The biocatalyst was stabilized and activated in the reaction mixture by 2-hydroxypropyl-β-cyclodextrin. Substitution of the wild-type xylose reductase by a D51A mutant further improved bioreductions. In previous optimization strategies, hexane was added as second phase to protect the labile catalyst from adverse effects of hydrophobic substrate and product. The addition of 2-hydroxypropyl-β-cyclodextrin (11 % w/v) instead of hexane (20 % v/v) increased the yield on biocatalyst 6.3-fold. A literature survey suggests that bioreduction enhancement by addition of cyclodextrins is not restricted to specific enzyme classes, catalyst forms or substrates.

Crystal structure of yeast xylose reductase in complex with a novel NADP-DTT adduct provides insights into substrate recognition and catalysis

Aldose reductases (ARs) belonging to the aldo-keto reductase (AKR) superfamily catalyze the conversion of carbonyl substrates into their respective alcohols. Here we report the crystal structures of the yeast Debaryomyces nepalensis xylose reductase (DnXR, AKR2B10) in the apo form and as a ternary complex with a novel NADP-DTT adduct. Xylose reductase, a key enzyme in the conversion of xylose to xylitol, has several industrial applications. The enzyme displayed the highest catalytic efficiency for l-threose (138 ± 7 mm^-1 ·s^-1 ) followed by d-erythrose (30 ± 3 mm^-1 ·s^-1 ). The crystal structure of the complex reveals a covalent linkage between the C4N atom of the nicotinamide ring of the cosubstrate and the S1 sulfur atom of DTT and provides the first structural evidence for a protein mediated NADP-low-molecular-mass thiol adduct. We hypothesize that the formation of the adduct is facilitated by an in-crystallo Michael addition of the DTT thiolate to the specific conformation of bound NADPH in the active site of DnXR. The interactions between DTT, a four-carbon sugar alcohol analog, and the enzyme are representative of a near-cognate product ternary complex and provide significant insights into the structural basis of aldose binding and specificity and the catalytic mechanism of ARs. DATABASE: Structural data are available in the PDB under the accession numbers 5ZCI and 5ZCM.