Structure-guided protein engineering of ammonia lyase for efficient synthesis of sterically bulky unnatural amino acids.

Enzymatic asymmetric amination addition is seen as a promising approach for synthesizing amine derivatives, especially unnatural amino acids, which are valuable precursors to fine chemicals and drugs. Despite the broad substrate spectrum of methylaspartate lyase (MAL), some bulky substrates, such as caffeic acid, cannot be effectively accepted. Herein, we report a group of variants structurally derived from Escherichia coli O157:H7 MAL (EcMAL). A combined mutagenesis strategy was used to simultaneously redesign the key residues of the entrance tunnel and binding pocket to explore the possibility of accepting bulky substrates with potential application to chiral drug synthesis. Libraries of residues capable of lining the active center of EcMAL were then constructed and screened by an effective activity solid-phase color screening method using tyrosinase as a cascade catalyst system. Activity assays and molecular dynamics studies of the resultant variants showed that the substrate specificity of EcMAL was modified by adjusting the polarity of the binding pocket and the degree of flexibility of the entrance tunnel. Compared to M3, the optimal variant M8 was obtained with a 15-fold increase in catalytic activity. This structure-based protein engineering of EcMAL can be used to open new application directions or to develop practical multi-enzymatic processes for the production of various useful compounds.