And this oxygen is shown here. Now, the other oxygen atom goes to form water, and that's exactly why water is released here. So phenylalanine hydroxylase is a mixed function oxygenase. Now, in order for the phenolaline hydroxylase to be able to catalyze this step, it has to use the reducing power of an electron carrier molecule we call tetrahydrobiopterin. Now, tetrahydrobiopterin is actually not a vitamin because our cells can synthesize this molecule. And to synthesize this molecule, we basically begin with dihydrobiopterin.