Metabolic Engineering of Microbial Cell Factories for L-Tryptophan and Its Derivatives

L-Tryptophan (L-Trp) is used in nutrition, animal feed, pharmaceuticals, and biomaterial-related applications, and many of its derivatives are also commercially valuable. Microbial fermentation offers a practical route to L-Trp production, but wild-type strains do not naturally direct enough carbon to this product because precursor supply, feedback control, transcriptional regulation, and competing branches all restrict pathway output. This review examines how these limitations have been addressed in Escherichia coli. The strategies discussed include mutagenesis with high-throughput screening, targeted modification of known metabolic nodes, and modular approaches for balancing pathway flux. We also summarize work in which L-Trp-producing strains were used as starting points for the biosynthesis of 5-hydroxytryptophan, serotonin, melatonin, and violacein. Remaining issues relevant to larger-scale production include host tolerance, cofactor and transport limitations, and the narrow choice of production hosts. Opportunities in computer-assisted pathway design and alternative chassis, particularly Saccharomyces cerevisiae, are considered in this context.