From synthesis to process by overcoming enzyme limitations: Design of an asymmetric reductive amination route to L-β-methylphenylalanine at high substrate loading catalyzed by glutamate dehydrogenase

Enantiomerically pure non-canonical α-amino acids gain an increasing interest and are considered as valuable building blocks in drug discovery, e.g., when generating cyclic peptides. A compound class of specific interest are the diastereoisomers of L-β-methylphenylalanine. Addressing this demand, we explored the potential of amino acid dehydrogenases for their synthesis through reductive amination starting from a racemic α-keto acid. In spite of severe deactivation and substrate as well as product inhibition effects resulting in a low conversion below 10% under standard conditions, we could overcome such limitations through process engineering leading to 80% conversion when operating at a technically relevant substrate loading of 40 g/L (200 mM). In addition, we were able to demonstrate that this lab scale synthesis is scalable by a factor of 10, leading to a conversion of 63%, yield of 49% (340 mg), >99% ee, and 6% de.