Figure 1.

Nonoxidative Deamination of Phe.

(A) Reaction catalyzed by PAL from plants and fungi resulting in trans-cinnamate and ammonia. Besides, phenylalanine is processed by other enzymes to Tyr and to phenylpyruvate.

(B) Proposed autocatalytic formation of MIO from the tripeptide ²⁰²Ala-Ser-Gly by two water elimination steps. The sp³ conformation at 204-N is taken from the wild-type structure of HAL (Schwede et al., 1999). It is probably enforced by the surrounding polypeptide, increasing the electrophilicity of the 203-Cβ atom.