Enzymatic production of L‐tryptophan from DL‐serine and indole by a coupled reaction of tryptophan synthase and amino acid racemase

Enzymatic production of L‐tryptophan from DL‐serine and indole by a coupled reaction of tryptophan synthase and amino acid racemase was studied. The tryptophan synthase (EC 4.2.1.20) of Escherichia coli catalyzed beta‐substitution reaction of L‐serine into L‐tryptophan and the amino acid racemase (EC 5.1.1.10) of Pseudomonas putida catalyzed the racemization of D‐serine simultaneously in one reactor. Under optimal conditions established for L‐tryptophan production, a large‐scale production of L‐tryptophan was carried out in a 200‐liter reactor using intact cells of E. coli and P. putida. After 24 h of incubation with intermittent indole feeding, 110 g liter‐1 of L‐tryptophan was formed in molar yields of 91 and 100% for added DL‐serine and indole, respectively. Continuous production of L‐tryptophan was also carried out using immobilized cells of E. coli and P. putida. The maximum concentration of L‐tryptophan formed was 5.2 g liter‐1 (99% molar yield for indole), and the concentration decreased to 4.2 g liter‐1 after continuous operation for 20 days.