Computer-aided directed evolution ofl-threonine aldolase for asymmetric biocatalytic synthesis of a chloramphenicol intermediate.

l-Threonine aldolases (LTAs) employing pyridoxal phosphate (PLP) as cofactor can convert low-cost achiral substrates glycine and aldehyde directly into valuable ß-hydroxy-α-amino acids such as (2R,3S)-2-amino-3-hydroxy-3-(4-nitrophenyl) propanoic acid ((R,S)-AHNPA), which is utilized broadly as crucial chiral intermediates for bioactive compounds. However, LTAs' stereospecificity towards the ß carbon is rather moderate and their activity and stability at high substrate load is low, which limits their industrial application. Here, computer-aided directed evolution was applied to improve overall activity, selectivity and stability under desired process conditions of a l-threonine aldolase in the asymmetric synthesis of (R,S)-AHNPA. Selectivity and stability determining regions were computationally identified for structure-guided directed evolution of LTA-variants under efficient biocatalytic process conditions using 40% ethanol as cosolvent. We applied molecular modeling to rationalize selectivity improvement and design focused libraries targeting the substrate binding pocket, and we also used MD simulations in nonaqueous process environment as an effective and promising method to predict potential unstable loop regions near the tetramer interface which are hot-spots for cosolvent resistance. An excellent LTA variant EM-ALDO031 with 18 mutations was obtained, which showed âˆ¼ 30-fold stability improvement in 40% ethanol and diastereoselectivity (de) raised from 31.5% to 85% through a three-phase evolution campaign. Our fast and efficient data-driven methodology utilizing a combination of experimental and computational tools enabled us to evolve an aldolase variant to achieve the target of 90% conversion at up to 150 g/L substrate load in 40% ethanol, enabling the biocatalytic production of ß-hydroxy-α-amino acids from cheap achiral precursors at multi-ton scale.

Continuous asymmetric ketone reduction processes with recombinant Escherichia coli.

The reduction of methyl acetoacetate was carried out in continuously operated biotransformation processes catalyzed by recombinant Escherichia coli cells expressing an alcohol dehydrogenase from Lactobacillus brevis. Three different cell types were applied as biocatalysts in three different cofactor regeneration approaches. Both processes with enzyme-coupled cofactor regeneration catalyzed by formate dehydrogenase or glucose dehydrogenase are characterized by a rapid deactivation of the biocatalyst. By contrast the processes with substrate-coupled cofactor regeneration by alcohol dehydrogenase catalyzed oxidation of 2-propanol could be run over a period of 7 weeks with exceedingly high substrate and cosubstrate concentrations of up to 2.5 and 2.8 mol L(-1), respectively. Even under these extreme conditions, the applied biocatalyst showed a good stability with only marginal leakage of intracellular cofactors.

Study on mesophilic and thermophilic alcohol dehydrogenases in gas-phase reaction.

The initial reaction rate and the thermostability of the mesophilic alcohol dehydrogenase (ADH) from Lactobacillus brevis (LBADH), and the thermophilic ADH from Thermoanaerobacter sp. (ADH T) in gas-phase reaction were compared. The effects of water activity, cofactor-to-protein molar ratio, and reaction temperature on the reduction of acetophenone to 1-phenylethanol were studied. An optimal water activity of 0.55 in terms of productivity was found for both ADHs. The cofactor-to-protein molar ratio was chosen slightly higher than equimolar to increase both activity and thermostability. An excellent optimal productivity of 1,000 g x L(-1) x d(-1) for LBADH and 600 g x L(-1) x d(-1)for ADH T was found at 60 degrees C, while the highest total turnover numbers with respect to the enzyme were achieved at 30 degrees C and amounted to 4.2 million for LBADH and 1.7 million for ADH T, respectively. Interestingly, the ADH from the mesophilic L. brevisshowed the higher thermostability in the nonconventional medium gas phase.

Optimization of adsorptive immobilization of alcohol dehydrogenases.

In this work, a systematic examination of various parameters of adsorptive immobilization of alcohol dehydrogenases (ADHs) on solid support is performed and the impact of these parameters on immobilization efficiency is studied. Depending on the source of the enzymes, these parameters differently influence the immobilization efficiency, expressed in terms of residual activity and protein loading. Residual activity of 79% was achieved with ADH from bakers' yeast (YADH) after optimizing the immobilization parameters. A step-wise drying process has been found to be more effective than one-step drying. A hypothesis of deactivation through bubble nucleation during drying of the enzyme/glass bead suspension at low drying pressure (<45 kPa) is experimentally verified. In the case of ADH from Lactobacillus brevis (LBADH), >300% residual activity was found after drying. Hyperactivation of the enzyme is probably caused by structural changes in the enzyme molecule during the drying process. ADH from Thermoanaerobacter species (ADH T) is found to be stable under drying conditions (>15 kPa) in contrast to LBADH and YADH.

Optimization of enzymatic gas-phase reactions by increasing the long-term stability of the catalyst.

Enzymatic gas-phase reactions are usually performed in continuous reactors, and thus very stable and active catalysts are required to perform such transformations on cost-effective levels. The present work is concerned with the reduction of gaseous acetophenone to enantiomerically pure (R)-1-phenylethanol catalyzed by solid alcohol dehydrogenase from Lactobacillus brevis (LBADH), immobilized onto glass beads. Initially, the catalyst preparation displayed a half-life of 1 day under reaction conditions at 40 degrees C and at a water activity of 0.5. It was shown that the observed decrease in activity is due to a degradation of the enzyme itself (LBADH) and not of the co-immobilized cofactor NADP. By the addition of sucrose to the cell extract before immobilization of the enzyme, the half-life of the catalyst preparation (at 40 degrees C) was increased 40 times. The stabilized catalyst preparation was employed in a continuous gas-phase reactor at different temperatures (25-60 degrees C). At 50 degrees C, a space-time yield of 107 g/L/d was achieved within the first 80 h of continuous reaction.

Laboratory evolution of Pyrococcus furiosus alcohol dehydrogenase to improve the production of (2S,5S)-hexanediol at moderate temperatures.

There is considerable interest in the use of enantioselective alcohol dehydrogenases for the production of enantio- and diastereomerically pure diols, which are important building blocks for pharmaceuticals, agrochemicals and fine chemicals. Due to the need for a stable alcohol dehydrogenase with activity at low-temperature process conditions (30 degrees C) for the production of (2S,5S)-hexanediol, we have improved an alcohol dehydrogenase from the hyperthermophilic archaeon Pyrococcus furiosus (AdhA). A stable S-selective alcohol dehydrogenase with increased activity at 30 degrees C on the substrate 2,5-hexanedione was generated by laboratory evolution on the thermostable alcohol dehydrogenase AdhA. One round of error-prone PCR and screening of approximately 1,500 mutants was performed. The maximum specific activity of the best performing mutant with 2,5-hexanedione at 30 degrees C was tenfold higher compared to the activity of the wild-type enzyme. A 3D-model of AdhA revealed that this mutant has one mutation in the well-conserved NADP(H)-binding site (R11L), and a second mutation (A180V) near the catalytic and highly conserved threonine at position 183.

Modelling the reaction course of N-acetylneuraminic acid synthesis from N-acetyl-D-glucosamine--new strategies for the optimisation of neuraminic acid synthesis.

In this work, a model describing the complete enzyme catalysed synthesis of N-acetylneuraminic acid (Neu5Ac) from N-acetyl-D-glucosamine (GlcNAc) is presented. It includes the combined reaction steps of epimerisation from GlcNAc to N-acetyl-D-mannosamine (ManNAc) and the aldol condensation of ManNAc with sodium pyruvate yielding Neu5Ac. The model is expedient to predict the reaction course for various initial and feed concentrations and therefore to calculate reaction times and yields. The equilibrium constants calculated from the kinetic constants via the Haldane relationship correspond with experimental values very well (0.26 calculated and 0.24 experimental value for the epimerisation, 27.4 l mol(-1) calculated and 28.7 l mol(-1) experimental for the aldol condensation). The actual relevance of the model is shown by a scale-up. Using the model, an optimisation of reaction conditions in consideration of different targets is possible. Exemplarily, it is presented how the optimal ratio of the two enzymes in the reaction can be determined and how the composition of the reaction solution in a fed-batch reactor can be designed to meet downstream processing needs.

Kinetic modeling of acetophenone reduction catalyzed by alcohol dehydrogenase from Thermoanaerobacter sp.

NADPH-dependent alcohol dehydrogenase (ADH) from Thermoanaerobacter sp. was kinetically characterized using reduction of acetophenone as a model. To achieve 98% conversion of acetophenone, cofactor regeneration by oxidation of 2-propanol with the same enzyme was used. The enzyme was stable in the batch reactor. It was enantioselective towards (S)-1-phenylethanol (ee>99.5%). Due to its high deactivation in continuously operated stirred tank reactor (kd=0.0141 min-1) there was no way to keep high conversion of acetophenone at 98%. The deactivation occurred in the repetitive batch as well. A mathematical model for the acetophenone reduction with cofactor regeneration describing the behaviour in a batch, repetitive-batch and continuously stirred tank reactor was developed.

A novel convenient procedure for extractive work-up of whole-cell biotransformations using de-emulsifying hydrolases.

Extractive work-up of whole-cell biotransformations generally suffers from the formation of stable gels and slimes upon addition of the organic solvent to the cell suspension and the cell-free solution, respectively. This problem has been overcome by enzymatic lysis of emulsifying agents present in the medium through addition of hydrolases. Of these agents, proteases have exhibited the most powerful de-emulsifying activity. Enzyme treatment of cell-free culture media of Saccharomyces cerevisiae with pronase E drastically reduced phase separation time (t(p)) from 1 week to 30 min without significantly affecting product integrity. Yeast cell suspensions were de-emulsified best with protease N-01, where phase separation was complete after 1 h. As was exemplified with cell-free culture media of Lactobacillus kefir, wherein addition of pronase E or protease N-01 reduced t(p) from 1 week to 2 h each, this practical, ready-to-use method is appropriate for both fungal and bacterial biocatalysts.