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Non‐canonical amino acids (ncAAs) are useful synthons for the development of new medicines, materials, and probes for bioactivity. Recently, enzyme engineering has been leveraged to produce a suite of highly active enzymes for the synthesis of β‐substituted amino acids. However, there are few examples of biocatalyticN‐substitution reactions to make α,β‐diamino acids. In this study, we used directed evolution to engineer the β‐subunit of tryptophan synthase, TrpB, for improved activity with diverse amine nucleophiles. Mechanistic analysis shows that high yields are hindered by product re‐entry into the catalytic cycle and subsequent decomposition. Additional equivalents ofl‐serine can inhibit product reentry through kinetic competition, facilitating preparative scale synthesis. We show β‐substitution with a dozen aryl amine nucleophiles, including demonstration on a gram scale. These transformations yield an underexplored class of amino acids that can serve as unique building blocks for chemical biology and medicinal chemistry.

Non‐canonical amino acids (ncAAs) are useful synthons for the development of new medicines, materials, and probes for bioactivity. Recently, enzyme engineering has been leveraged to produce a suite of highly active enzymes for the synthesis of β‐substituted amino acids. However, there are few examples of biocatalyticN‐substitution reactions to make α,β‐diamino acids. In this study, we used directed evolution to engineer the β‐subunit of tryptophan synthase, TrpB, for improved activity with diverse amine nucleophiles. Mechanistic analysis shows that high yields are hindered by product re‐entry into the catalytic cycle and subsequent decomposition. Additional equivalents ofl‐serine can inhibit product reentry through kinetic competition, facilitating preparative scale synthesis. We show β‐substitution with a dozen aryl amine nucleophiles, including demonstration on a gram scale. These transformations yield an underexplored class of amino acids that can serve as unique building blocks for chemical biology and medicinal chemistry.