Here's how it works:

* Phenylalanine is an essential amino acid found in proteins.

* PAH converts phenylalanine into tyrosine, another amino acid. This conversion requires a co-factor called tetrahydrobiopterin (BH4).

* Tyrosine is then used in various metabolic pathways, including the production of neurotransmitters like dopamine and norepinephrine.

Deficiency of PAH leads to phenylketonuria (PKU), a genetic disorder where phenylalanine builds up in the body, causing serious health problems.