Identification of tryptophanase from Escherichia coli for the synthesis of S-allyl-L-cysteine and related S-substituted cysteine derivatives

A wide variety of S-substituted cysteine derivatives occur in plant metabolites. For example, S-allyl-L-cysteine (SAC), mainly contained in garlic, gathers huge interest because of its favorable bioactivities for human health. However, conventional methods for preparing SAC suffer from several drawbacks with regard to efficiency and toxicity, which highlights the need for improved processes for SAC synthesis. This study aims to develop a novel bioprocess to produce SAC by microbial enzymes from easily available substrates. We found that Escherichia coli had the ability to synthesize SAC from allyl mercaptan, pyruvic acid, and ammonium sulfate. An enzyme purification through 3-step column chro-matography, followed by determination of the N-terminal amino acid sequence revealed that tryptophanase (TnaA) was the enzyme responsible for SAC formation. Although the enzyme catalyzed the reversible reaction for synthesizing and degrading SAC, the degradation proceeded significantly faster than the synthesis. Interestingly, TnaA catalyzed the synthesis of a wide range of S-substituted cysteines with alkyl chains or aromatic rings, some of which are present in Allium and Petiveria plants. Our results showed a novel substrate specificity of TnaA toward various S-substituted cysteine. TnaA is a promising biocatalyst for developing a new process to supply various valuable S-substituted cysteine derivatives for medicinal and health-promoting applications. (C) 2022, The Society for Biotechnology, Japan. All rights reserved.

Automated summary

This study found that E. coli has the ability to synthesize S-allyl-L-cysteine (SAC) through the enzyme tryptophanase (TnaA). TnaA also showed the ability to synthesize a wide range of S-substituted cysteines with various chains or rings. These findings have important implications for the development of new processes for synthesizing valuable S-substituted cysteine derivatives.