The stereochemistry of a phenylalanine aminomutase (PAM) on the andrimid biosynthetic pathway in Pantoea agglomerans (Pa) is reported. PaPAM is a member of the 4-methylidene-1H-imidazol-5(4H)-one (MIO)-dependent family of catalysts and isomerizes (2S)-alpha-phenylalanine to (3S)-beta-phenylalanine, which is the enantiomer of the product made by the mechanistically similar aminomutase TcPAM from Taxus plants. The NH(2) and pro-(3S) hydrogen groups at C(alpha) and C(beta), respectively, of the substrate are removed and interchanged completely intramolecularly with inversion of configuration at the migration centers to form P-phenylalanine. This is a contrast to the retention of configuration mechanism followed by TcPAM.