Whole-cell biotransformation systems for reduction of prochiral carbonyl compounds to chiral alcohol in Escherichia coli

Lactobacillus brevis alcohol dehydrogenase (Lb-ADH) catalyzes reduction of prochiral carbonyl compounds to chiral alcohol and meanwhile consumes its cofactor NADH into NAD+, while the cofactor regeneration can be catalyzed by Candida boidinii formate dehydrogenase (Cb-FDH). This work presents three different Escherichia coli whole-cell biocatalyst systems expressing recombinant ADH/FDH, FDH-LIN1-ADH and FDH-LIN2-ADH, respectively, all of which display very high efficacies of prochiral carbonyl conversion with respect to conversion rates and enantiomeric excess values. ADH/FDH represents co-expression of Lb-ADH and Cb-FDH under different promoters in a single vector. Fusion of Lb-ADH and Cb-FDH by a linker peptide LIN1 (GGGGS)2 or LIN2 (EAAAK)2 generates the two bifunctional enzymes FDH-LIN1-ADH and FDH-LIN2-ADH, which enable efficient asymmetric reduction of prochiral ketones in whole-cell biotransformation.

Bifunctional fusion proteins in which functional-coupled enzymes are connected through a linker have been constructed as an alternative to using two separated enzymes 10 . An adjacent active centre could reduce transfer distance of substrates and could decrease degradation of intermediate products 11,12 . A fusion protein of Mycobacterium vaccae FDH and Synechococcus KR [3-ketoacyl-(acyl-carrier-protein) reductase] displays an approximately two fold higher initial (1 h post-reaction) reaction rate in production of chiral alcohols than cells expressing the enzymes separately 13 .
In the present study, we constructed recombinant E. coli Rosetta (DE3) strains expressing bifunctional Lb-ADH-linker-Cb-FDH fusion enzymes to achieve high efficient whole-cell catalysis of prochiral carbonyl compounds into chiral alcohol ( Figure 1).
Enzymatic activity in whole-cell extracts. The whole-cell extracts of induced E. coli Rosetta (DE3) cells expressing the above recombinant enzyme proteins were subjected for determination of ADH or FDH enzymatic activity (Table 1). Co-expressed ADH/FDH displayed an ADH activity of 103.90 6 1.84 U/g CDW and a FDH activity of 32.47 6 1.84 U/g CDW , while the enzymatic activities of the ADH or FDH moiety in the above four difunctional fusion proteins were ranged from 70.9 to 77.2% or from 30.8 to 384.6%, respectively ( Table 1). Arrangement of ADH at the N-or C-terminus of the fusion proteins had little effect on its enzymatic activity; by contrast, the FDH moiety at N-terminus was much more active that at C-terminus.
Among these four bifunctional fusion enzymes, FDH-LIN2-ADH displayed the most active enzymatic activity and therefore was selected for analysis in detail using ADH/FDH as control. The residual ADH or FDH activity in whole-cell extract of Rosetta (DE3)-pETDuet-adh/fdh or Rosetta (DE3)-pETDuet-fdh-lin2-adh after incubation at 30 to 70uC for 10 min were determined; the ADH or FDH enzymatic stability for both ADH/FDH and FDH-LIN2-ADH decreased sharply with increasing of temperatures, and notably FDH-LIN2-ADH displayed slightly higher stability than ADH/ FDH at higher temperatures (40 to 70uC) ( Figure 2). In addition, 0.2 M phosphate buffer at pH 7.0 was shown to be the optimized buffer condition for enzymatic reaction for both ADH/FDH and FDH-LIN2-ADH ( Figure 3).

Discussion
The Cb-FDH monomer consists of 15 a-helices and 13 b-strands that are arranged into two domains, termed NAD 1 binding domain and catalytic domain, respectively 7 . Each domain contains three layers of polypeptide chain. Most of the 364 amino acid residues of Cb-FDH monomer are well-defined in electron density, but an exception is at the C-terminus, where 11 residues (354 to 364) are disordered 7 . The deletion of this 11-residue region in Cb-FDH leads to a .70% loss of its enzymatic activity (data not shown), highlighting the necessity of disordered C-terminus for Cb-FDH enzymatic activity. The catalytic activity of Lb-ADH depends strongly on the binding of Mg 21 . The magnesium ion is structurally coupled to the putative C-terminal hinge of the substrate-binding loop forming the substrate binding region 5 .
When two oligomeric enzymes are fused together with an appreciate linker peptide, a multimeric protein network of the two parent   14,15 . The E. coli Rosetta (DE3) strain compensates for a number of rare codon genes and thus is frequently used for high-level expression of eukaryotic or prokaryotic proteins. This work presents three different recombinant E. coli Rosetta (DE3) whole-cell biocatalyst systems expressing ADH/FDH, FDH-LIN1-ADH and FDH-LIN2-ADH, respectively, all of which display very high efficacies of prochiral carbonyl conversion. ADH/FDH represents co-expression of Lb-ADH and Cb-FDH under different promoters in a single vector. Fusion of Lb-ADH and Cb-FDH with LIN1 or LIN2 generates bifunctional fusion enzymes FDH-LIN1-ADH and FDH-LIN2-ADH. FDH-LIN1-ADH and FDH-LIN2-ADH consists of two oxidoreductases, of which one catalyzes reduction of prochiral ketones and the other one mediates regeneration of cofactors. For both FDH-LIN1-ADH and FDH-LIN2-ADH, the spatial separation of Lb-ADH and Cb-FDH subunits by an appropriate linker peptide enable two individual moieties fold independently and correctly and thereby ensure activity of both native enzymes. By contrast, there are probably hindered folding of FDH and thus reduced FDH activity for the two additional fusion proteins ADH-LIN1-FDH and ADH-LIN2-FDH constructed in this study.
The substrate conversion rates are very similar among ADH/FDH, FDH-LIN1-ADH and FDH-LIN2-ADH at 6 h post-reaction during whole cell biotransformation. Nevertheless, FDH-LIN1-ADH and FDH-LIN2-ADH exhibit higher e.e values than ADH/FDH when 4-Cl-AP is used as substrate, indicating that these two fusion proteins have elevated enantioselectivity compared with ADH/FDH. FDH-LIN1-ADH and FDH-LIN2-ADH exhibit very similar performances at 1 h and 6 h post-reaction during whole cell biotransformation, although FDH-LIN1-ADH has an FDH activity about 65% that of FDH-LIN2-ADH; this might be due to very similar balanced activities of FDH and ADH [the ideal ratio of activity of the two enzymes is 15n (n $ 1)] between FDH-LIN1-ADH and FDH-LIN2-ADH.

Methods
Bacterial strains and growth. For co-expression of L. brevis adh and C. boidinii fdh with a single expression vector, these two genes were cloned into the two different multiple cloning sites of pETDuet TM -1 (Novagen) to generate pETDuet-adh/fdh. These two genes were placed under two separate T7 lac promoters and expression of one gene would not affect that of the other 16 . All the primers designed in this study were listed in Table S1.
Each recombinant plasmid was transformed into E. coli Rosetta (DE3) for inducible over-expression of target recombinant proteins. Overnight cultures were diluted into fresh Luria-Bertani broth at an optical density at 600 nm (OD 600 ) of 0.1, and allowed for grow at 37uC with shacking at 200 rpm to an OD 600 of 0.8. After addition of 1 mM IPTG, the cell cultures were incubated at 20uC for 12 h with shacking at 200 rpm for cell harvest. 100 mg/m ampicillin and 20 mg/ml chloromycetin were added when necessary.

SDS-PAGE.
Harvested cells were resuspended in distilled water to an OD 600 of 3 and disrupted by sonication at 40 Hz and 4uC for 3 min, followed by centrifugation (10,000 3 g, 4uC and 30 min). The supernatants were mixed with 23 Laemmli buffer (300 mM Tris/HCl, pH6.8, 50% glycerol, 10% SDS, 5% 2-mercaptoethanol and 0.05% Bromophenol Blue), and equal volumes of 13 Laemmli buffer were used to resuspend the corresponding insoluble pellets. The resulting samples were heated to 100uC for 5 min and separated on the SDS-12% PAGE gels followed by Coomassie Blue staining.
Peptide mass fingerprinting (PMF). The presumed bands of target recombinant proteins on stained SDS-PAGE gels were carefully excised, destained and digested with trypsin. PMF assays were performed using an Ultraflex III TOF/TOF mass spectrometer (Bruker) at National Centre of Biomedical Analysis, Beijing, China.  Enzymatic activity assays. The enzymatic activity of ADH was determined photometrically at 340 nm in an 1 ml reaction mixture composed of 50 mM acetophenone (Sigma-Aldrich), 100 mM NADH (Roche), 1 mM MgCl 2 , 200 mM potassium phosphate buffer (pH 7.0) and 100 ml of crude whole-cell extract after an incubation at 30uC for 2.5 h 18 . The 1 ml reaction mixture for FDH activity assay contained 100 mM sodium formate, 500 mM NAD 1 (Roche), 200 mM potassium phosphate buffer (pH 7.0) and 100 ml of crude whole-cell extract 19 . For test of temperature stability, crude whole-cell extracts were incubated at 30 to 60uC for 10 min and then on ice for 5 min for further determination of residual enzymatic activity as above. For test of effect of phosphate or pH, the reaction mixtures with various concentrations of potassium phosphate (10 to 400 mM), or at different pH values (6 to 8), were subjected for enzymatic activity determination. One unit (U) of enzymatic activity was defined as the amount of enzyme catalyzing conversion of 1 mmol substrate/min/mg total protein at 30uC. The amounts of total protein in crude whole-cell extracts were determined with Thremo BCA Protein Assay Kit.
Whole-cell biotransformation. After induction with IPTG, bacteria cells were harvested by centrifugation at 5,000 3 g for 5 min and washed with 200 mM potassium phosphate buffer (pH 7.0). 0.08 g (w.t.) of cell pellets were resuspended in 500 ml of reaction solution (corresponding to 6 g/cdw) containing 200 mM sodium formate, 10 mM acetophenone or 2.5 mM 4-Cl-AP, 1 mM MgCl 2 , and 200 mM potassium phosphate buffer (pH 7.0). Biotransformation was conducted at 37uC for indicated times in a mute mixture to prevent cell sedimentation.
High-performance liquid chromatography (HPLC). 500 ml of biotransformation suspension were extracted with the same volume of n-hexane, and 10 ml of the resulting supernatant was analyzed using a HPLC chromatograph (Waters) connected to a CHIRALPAK OD-H column (Daicel Chemical Industries, Ltd., Tokyo, Japan). The mobile phase n-hexane and 2-propanol (90510, v/v) were used for quantitative analysis of the bioconversion products of acetophenone or 4-Cl-AP. The conversion rates were calculated with the formula '100% 3 volumes of product after conversion/volume of substrate before conversion'. The e.e values were calculated with the formula '100% 3 (R 2 S)/(R 1 S)'. R 5 R-phenylethanol or (R)-4-Chloro-1phenylethanol. S 5 S-phenylethanol or (S)-4-Chloro-1-phenylethanol.