Next time you are cramming for finals and are up studying until midnight, take a glance at your Diet Coke can. Notice the ingredient panel contains a warning saying the drink contains phenylalanine. Phenylalanine is one of the ingredients in aspartame, which is an artificial sugar substitute. But that's is an essential amino acid that we already mentioned. What gives?
We didn't lie or lead you astray, pinky swear. It's just that some people have a genetic deficiency called phenylketonuria (PKU) where they lack phenylalanine hydroxylase (PAH), the enzyme that degrades the essential amino acid phenylalanine. PAH normally converts phenylalanine to tyrosine, another amino acid. Sithout PAH activity, phenylalanine builds up, creating an acid called phenylpyruvate that can cause mental retardation, seizures, and a musty odor in humans. If phenylketonurics (the name for those people with PKU) continue to eat the amino acid, they'll eventually develop blonde hair and pale skin since this acid will also cause a loss of pigmentation.
It's a pretty serious disease, so phenylketonurics have to avoid high-protein foods, like meats and beans. They've still got to get those amino acids, so they usually drink a specially designed liquid to supplement their diet. Something like this.
Phenylketonuria is a homozygous recessive disorder—that means both parents have to have the recessive gene for a child to inherit it. It's now required that all newborn babies receive a screening blood test for PKU before they leave the hospital so the disorder can be caught early.
If your mind is still thinking clearly after studying all night, you may ask, "Why can't they drink Diet Coke? There's no protein in soda." Aspartame—that's an artificial sugar substitute—is made up of phenylalanine. Aren't you glad you asked?