An aminomutase, naturally catalyzing the interconversion of (S)-α-phenylalanine and (R)-β-phenylalanine, was converted into an ammonia lyase catalyzing the nonoxidative deamination of phenylalanine to cinnamic acid by a rational single-point mutation. It could be shown by crystal structures and kinetic data that the flexibility of the lid that covers the active site decides whether the enzyme acts as a lyase or a mutase. An Arg92Ser mutation destabilized the closed conformation of the lid structure and converted the mutase into a lyase that exhibited up to 44-fold increased reaction rates in the enantioselective deamination of (R)-β-phenylalanine. In addition, the amination rates of cinnamic acid yielding optically pure (S)-α- and (R)-β-phenylalanine were doubled. The applicability of the mutant enzyme for kinetic resolution and asymmetric amination could be shown by biocatalysis on a preparative scale.