Phenylalanine Hydroxylase (PAH)

Phenylalanine hydroxylase is the rate-limiting enzyme of the metabolic pathway which degrades excess phenylalanine. The other substrates in the reaction are molecular oxygen and tetrahydrobiopterin. Tetrahydrobiopterin is a member of the group of redox biochemicals known as pteridines.

PAH is the gene that encodes for phenylalanine hydroxylase. It was the research on phenylalanine hydroxylase by Seymour Kaufman that led to the discovery of tetrahydrobiopterin as a biological cofactor. Phenylalanine hydroxylase is a tetramer composed of four monomers, that is, composed of 4 identical subunits. Each subunit is in turn composed of three domains, a regulatory domain, a catalytic domain, and a tetramerization domain.