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. So in the presence of NADPH and an H plus ion, the enzyme Dihydrophobate reductase basically takes the dihydrobiopterine and transfers the reducing power from NADPH onto this molecule to give us tetrahydrobiopterin. And then this mixed function oxygenates, this enzyme, phenylaline hydroxylase, uses the reducing power of tetrahydrobiopterin to basically form tyrosine. And of course, we also use up the reducing power of this molecule to form Quinnode dihydrobiopterin.