Efficient Synthesis of l-Asparagine by an Immobilized Three-Enzyme Cascade Reaction System

l-Asparagine (l-Asn) is an important amino acid with broad applications in food, medicine, fine chemicals, and environmental protection. However, its industrial production is limited by the high cost of raw materials and the low catalytic efficiency of enzymes. In this study, a three-enzyme cascade pathway (FDN) was designed to produce l-Asn, using fumaric acid as the raw material. Within this pathway, EcAsnA was identified as the rate-limiting enzyme and was subsequently engineered using a product rescue strategy to reduce product inhibition by opening the closed gate. The optimal mutant, L109 K/K58R, exhibited a 6.61-fold reduction in product inhibition and a 4.24-fold improvement in catalytic efficiency. This mutant was then integrated into Escherichia coli along with the other two enzymes to construct the optimal recombinant strain E. coli 17. Using diatomite-glutaraldehyde cross-linking immobilized E. coli 17 as a biocatalyst, 267.74 g of l-Asn (with 5.35 g·L^–1·h^–1 STY, > 99% ee) was produced across 50 batch feedings in 1 L reaction volume. The purity of l-Asn exceeded 99% after isolation and purification. This study demonstrates the effective integration of cascade reaction design, enzyme engineering, and cell immobilization, providing a promising approach for synthesizing high-value products from cost-effective substrates.