Directed evolution of a designer enzyme featuring an unnatural catalytic amino acid

The impressive rate accelerations that enzymes display in nature often result from boosting the inherent catalytic activities of side chains by their precise positioning inside a protein binding pocket. Here we show that such fine‐tuning is also possible for catalytic unnatural amino acids. Specifically, we report the directed evolution of a recently described designer enzyme, which utilizes an aniline side chain to promote a model hydrazone formation reaction. Consecutive rounds of directed evolution identified a number of mutations in the promiscuous binding pocket, in which the unnatural amino acid is embedded in the starting catalyst. When combined, these mutations boost the turnover frequency (kcat) of the designer enzyme by almost 100‐fold. Crucially, these gains result from strengthening the catalytic contribution of the unnatural amino acid, as the engineered designer enzymes outperform variants, in which the aniline side chain is replaced with a catalytically inactive tyrosine residue, by >200‐fold.

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