An overview of biological production of L-theanine.

L-Theanine (γ-glutamylethylamide) is a unique non-protein amino acid that is naturally found in tea plants. It contributes to the umami taste and unique flavor to green tea infusion, and thus its content in tea leaves highly impacts the tea quality and price. In addition to the graceful taste, it has been proved to have many beneficial physiological effects, especially promoting relaxation and improving concentration and learning ability. Based on these promising advantages, L-theanine has been commercially developed as a valuable ingredient for use in food and beverages to improve and/or maintain human health. L-Theanine can be obtained by chemical synthesis or isolation from tea, while chemical synthesis of L-theanine is hard to be accepted by consumers and is not allowed to use in food industry, and isolation of L-theanine in high purity generally involves time-consuming, cost-ineffective, and complicated operational processes. Accordingly, the biological production of L-theanine has recently attracted much attention. Four kinds of bacterial enzymes, including L-glutamine synthetase, γ-glutamylmethylamide synthetase, γ-glutamyltranspeptidase, and L-glutaminase, have been characterized to have L-theanine-producing ability. Herein, an overview of recent studies on the biological production of L-theanine was presented.