Now, to regenerate back the tetrahydrobiopterin so that it can be used again in this reaction, we use an enzyme called dihydropyridine reductase. And this enzyme takes the reducing power of NADPH, transfers it onto this molecule to form back the tetrahydrobiopter. And so that again, we can use the reducing power of this molecule to undergo this first step. So again, in the first step, we utilize a phenylalamine, a diatomic water molecule. We use NADPH to basically give us this. And by using the reducing power of this molecule, we transform the phenylalanine into tyrosine.