Enantioselective, continuous (R)- and (S)-2-butanol synthesis: achieving high space-time yields with recombinant E. coli cells in a micro-aqueous, solvent-free reaction system.

The stereoselective production of (R)- or (S)-2-butanol is highly challenging. A potent synthesis strategy is the biocatalytic asymmetric reduction of 2-butanone applying alcohol dehydrogenases. However, due to a time-dependent racemisation process, high stereoselectivity is only obtained at incomplete conversion after short reaction times. Here, we present a solution to this problem: by using a continuous process, high biocatalytic selectivity can be achieved while racemisation is suppressed successfully. Furthermore, high conversion was achieved by applying recombinant, lyophilised E. coli cells hosting Lactobacillus brevis alcohol dehydrogenase in a micro-aqueous solvent-free continuous reaction system. The optimisation of residence time (τ) and 2-butanone concentration boosted both conversion (>99%) and enantiomeric excess (ee) of (R)-2-butanol (>96%). When a residence time of only τ=3.1 min was applied, productivity was extraordinary with a space-time yield of 2278±29g/(L×d), thus exceeding the highest values reported to date by a factor of more than eight. The use of E. coli cells overexpressing an ADH of complementary stereoselectivity yielded a synthesis strategy for (S)-2-butanol with an excellent ee (>98%). Although conversion was only moderate (up to 46%), excellent space-time yields of up to 461g/(L×d) were achieved. The investigated concept represents a synthesis strategy that can also be applied to other biocatalytic processes where racemisation poses a challenge.

Design of an activity and stability improved carbonyl reductase from Candida parapsilosis.

The carbonyl reductase from Candida parapsilosis (CPCR2) is an industrially attractive biocatalyst for producing chiral alcohols from ketones. The homodimeric enzyme has a broad substrate spectrum and an excellent stereoselectivity, but is rapidly inactivated at aqueous-organic interfaces. The latter limits CPCR2's application in biphasic reaction media. Reengineering the protein surface of CPCR2 yielded a variant CPCR2-(A275N, L276Q) with 1.5-fold increased activity, 1.5-fold higher interfacial stability (cyclohexane/buffer system), and increased thermal resistance (ΔT50=+2.7 °C). Site-directed and site-saturation mutagenesis studies discovered that position 275 mainly influences stability and position 276 governs activity. After single site-saturation of position 275, amino acid exchanges to asparagine and threonine were discovered to be stabilizing. Interestingly, both positions are located at the dimer interface and close to the active site and computational analysis identified an inter-subunit hydrogen bond formation at position 275 to be responsible for stabilization. Finally, the variant CPCR2-(A275S, L276Q) was found by simultaneous site-saturation of positions 275 and 276. CPCR2-(A275S, L276Q) has compared to wtCPCR2 a 1.4-fold increased activity, a 1.5-fold higher interfacial stability, and improved thermal resistance (ΔT50=+5.2 °C).

Reengineered carbonyl reductase for reducing methyl-substituted cyclohexanones.

The carbonyl reductase from Candida parapsilosis (CPCR2) is a versatile biocatalyst for the production of optically pure alcohols from ketones. Prochiral ketones like 2-methyl cyclohexanone are, however, only poorly accepted, despite CPCR2's large substrate spectrum. The substrate spectrum of CPCR2 was investigated by selecting five amino positions (55, 92, 118, 119 and 262) and exploring them by single site-saturation mutagenesis. Screening of CPCR2 libraries with poor (14 compounds) and well-accepted (2 compounds) substrates showed that only position 55 and position 119 showed an influence on activity. Saturation of positions 92, 118 and 262 yielded only wild-type sequences for the two well-accepted substrates and no variant converted one of the 14 other compounds. Only the variant (L119M) showed a significantly improved activity (7-fold on 2-methyl cyclohexanone; vmax = 33.6 U/mg, Km = 9.7 mmol/l). The L119M substitution exhibited also significantly increased activity toward reduction of 3-methyl (>2-fold), 4-methyl (>5-fold) and non-substituted cyclohexanone (>4-fold). After docking 2-methyl cyclohexanone into the substrate-binding pocket of a CPCR2 homology model, we hypothesized that the flexible side chain of M119 provides more space for 2-methyl cyclohexanone than branched L119. This report represents the first study on CPCR2 engineering and provides first insights how to redesign CPCR2 toward a broadened substrate spectrum.

Asymmetric reduction of ketones with recombinant E. coli whole cells in neat substrates.

The asymmetric reduction of ketones is performed by using lyophilized whole cells in neat substrates with defined water activity (a(w)). Ketones and alcohols prone to be unstable in aqueous media can now be converted via biocatalysis.