Preparation of optically active β-hydroxy-α-amino acid by immobilized Escherichia coli cells with serine hydroxymethyl transferase activity

Abstract

In this research, an improved method for preparation of optically pure β-hydroxy-α-amino acids, catalyzed by serine hydroxymethyl transferase with threonine aldolase activity, is reported. Using recombinant serine hydroxymethyl transferase (SHMT), an enzymatic resolution process was established. A series of new substrates, β-phenylserine, β-(nitrophenyl) serine and β-(methylsulfonylphenyl) serine were used in the resolution process catalyzed by immobilized Escherichia coli cells with SHMT activity. It was observed that the K (m) for L: -threonine was 28-fold higher than that for L: -allo-threonine, suggesting that this enzyme can be classified as a low-specificity L: -allo-threonine aldolase. The results also shows that SHMT activity with β-phenylserine as substrate was about 1.48-fold and 1.25-fold higher than that with β-(methylsulfonylphenyl) serine and β-(nitrophenyl) serine as substrate, respectively. Reaction conditions were optimized by using 200 mmol/l β-hydroxy-α-amino acid, and 0.1 g/ml of immobilized SHMT cells at pH 7.5 and 45°C. Under these conditions, the immobilized cells were continuously used 10 times, yielding an average conversion rate of 60.4%. Bead activity did not change significantly the first five times they were used, and the average conversion rate during the first five instances was 84.1%. The immobilized cells exhibited favourable operational stability.