Figure 1.

Metabolic pathway of phenylalanine (Phe). “The primary pathway (blue box) is the catalytic conversion of L-Phe to L-Tyr by phenylalanine hydroxylase (PAH). In phenylketonuria (PKU), the deficiency of PAH enzyme leads to the production of phenylketones by an alternative pathway (red box). A third pathway (green box) can be found in plants and yeast involving the enzyme (adopted from Ho G 2014) (2).