* Phenylalanine is the simplest of the aromatic amino acids: its side chain is a benzene ring attached at the beta carbon. To remember its one letter code, remember that phenylalanine begins with an F-sound even though it starts with a P.
Tyrosine is phenylalanine with an alcohol group attached at position 4 on the benzene ring. Tyrosine and phenylalanine are closely related. In fact, tyrosine is synthesized in the body from phenylalanine.
Clinical correlation: phenylalanine hydroxylase converts phenylalanine to tyrosine. Its deletion or inactivation results in
phenylketonuria, a congenital metabolic disorder that can lead to toxic build-up of phenylalanine in the blood and brain.
- Histidine is an aromatic amino acid with a five-membered ring.
- Histidine’s side chain can carry a charge because its ring structure makes the second nitrogen also able to accept a proton. To remember this, we add a hydrogen atom and a positive charge to the double-bonded nitrogen atom in histidine.
- Tryptophan has two rings.
- You can remember that the one-letter code of tryptophan is W by noticing that the side of the bulky indole ring has a bit of a W shape.
