Crystallization Assisted Dynamic Kinetic Resolution for the Synthesis of (R)-β-Methylphenethylamine

This study explores a combination of the concept of enantioselective enzymatic synthesis of β-chiral amines through transamination with in situ product crystallization (ISPC) to overcome product inhibition. Using 2-phenylpropanal as a readily available and easily racemizing substrate of choice, (R)-β-methylphenethylamine ((R)-2-phenylpropan-1-amine) concentrations of up to 250 mM and enantiomeric excesses of up to 99 % are achieved when using a commercially available transaminase from Ruegeria pomeroyi in a fed-batch based dynamic kinetic resolution reaction on preparative scale. The source of substrate decomposition during the reaction is also investigated and the resulting unwanted byproduct formation is successfully reduced to insignificant levels.

Biocatalytic cascade to polysaccharide amination

BACKGROUND

Chitin, the main form of aminated polysaccharide in nature, is a biocompatible, polycationic, and antimicrobial biopolymer used extensively in industrial processes. Despite the abundance of chitin, applications thereof are hampered by difficulties in feedstock harvesting and limited structural versatility. To address these problems, we proposed a two-step cascade employing carbohydrate oxidoreductases and amine transaminases for plant polysaccharide aminations via one-pot reactions. Using a galactose oxidase from Fusarium graminearum for oxidation, this study compared the performance of CvATA (from Chromobacterium violaceum) and SpATA (from Silicibacter pomeroyi) on a range of oxidized carbohydrates with various structures and sizes. Using a rational enzyme engineering approach, four point mutations were introduced on the SpATA surface, and their effects on enzyme activity were evaluated.

RESULTS

Herein, a quantitative colorimetric assay was developed to enable simple and accurate time-course measurement of the yield of transamination reactions. With higher operational stability, SpATA produced higher product yields in 36 h reactions despite its lower initial activity. Successful amination of oxidized galactomannan by SpATA was confirmed using a deuterium labeling method; higher aminated carbohydrate yields achieved with SpATA compared to CvATA were verified using HPLC and XPS. By balancing the oxidase and transaminase loadings, improved operating conditions were identified where the side product formation was largely suppressed without negatively impacting the product yield. SpATA mutants with multiple alanine substitutions besides E407A showed improved product yield. The E407A mutation reduced SpATA activity substantially, supporting its predicted role in maintaining the dimeric enzyme structure.

CONCLUSIONS

Using oxidase-amine transaminase cascades, the study demonstrated a fully enzymatic route to polysaccharide amination. Although the activity of SpATA may be further improved via enzyme engineering, the low operational stability of characterized amine transaminases, as a result of low retention of PMP cofactors, was identified as a key factor limiting the yield of the designed cascade. To increase the process feasibility, future efforts to engineer improved SpATA variants should focus on improving the cofactor affinity, and thus the operational stability of the enzyme.

Stabilization of ω-transaminase from Pseudomonas fluorescens by immobilization techniques

Transaminases are a class of enzymes with promising applications for the preparation and resolution of a vast diversity of valued amines. Their poor operational stability has fueled many investigations on its stabilization due to their biotechnological relevance. In this work, we screened the stabilization of the tetrameric ω-transaminase from Pseudomonas fluorescens (PfωTA) through both carrier-bound and carrier-free immobilization techniques. The best heterogeneous biocatalyst was the PfωTA immobilized as cross-linked enzyme aggregates (PfωTA-CLEA) which resulted after studying different parameters as the precipitant, additives and glutaraldehyde concentrations. The best conditions for maximum recovered activity (29 %) and maximum thermostability at 60 ºC and 70 ºC (100 % and 71 % residual activity after 1 h, respectively) were achieved by enzyme precipitation with 90% acetone or ethanol, in presence of BSA (100 mg/mL) and employing glutaraldehyde (100 mM) as cross-linker. Studies on different conditions for PfωTA-CLEA preparation yielded a biocatalyst that exhibited 31 and 4.6 times enhanced thermal stability at 60 °C and 70 °C, respectively, compared to its soluble counterpart. The PfωTA-CLEA was successfully used in the bioamination of 4-hydroxybenzaldehyde to 4-hydroxybenzylamine. To the best of our knowledge, this is the first report describing a transaminase cross-linked enzyme aggregates as immobilization strategy to generate a biocatalyst with outstanding thermostability.