And that's exactly why these two amino acids, phenylalanine and tyrosine, are known as glucogenic and ketonic amino acids, because we can use them to ultimately form both glucose and ketone bodies. So let's begin by examining step one. And actually what step one shows us is we can transform phenylalanine directly into tyrosine. 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.