And this is precisely how our cells can synthesize tyrosine by beginning with phenylalanine. Now, the enzyme that catalyzes step one is phenylalanine hydroxylase. And this enzyme is part of a category of enzymes we call mixed function oxygenases. So this is a mixed function oxygenase. And what that means is it uses a diatomic oxygen. It takes one of the oxygen atoms within this diatomic molecule, places it on this, reactant the phenylalanine, and this basically forms the tyrosine.