On turkey and tryptophan

Since Thursday was Thanksgiving in the US — a day when one eats lots of Turkey — I can’t resist talking about turkey and tryptophan. Every one knows what a turkey is, and actually many people have heard that tryptophan is the compound in turkey that makes one sleepy, but what is tryptophan?

Tryptophan is an amino acid — one of the 20 amino acids commonly found in proteins in fact.  The chemical structure of tryptophan (or Trp as it is abbreviated — spoken as "Trip") is shown in the right.  It turns out that Trp is one of the most bulky amino acids, with its ring structure.  Being a bulky ring, Trp is one of the most hydrophobic amino acids, and is thus present in protein cores. 

Moreover, Trp has a very important properties in the study of protein folding.  Due to its ring structure, Trp fluoresces (i.e. gives off light at a particular frequency, when hit with light of another frequency), and so Trp is a very important probe of protein folding.  When a protein is unfolded, usually its Trp residues are exposed to solvent, but when folded, Trp is buried.  This difference leads to differences in how it fluoresces, which can be measured very precisely.

Finally, it’s interesting to ask where does Trp come from?  Most animals can’t make Trp on their own and instead get it from plants, but where do plants get Trp?  Many plants can make it themselves, using an enzyme called Trp Synthase (shown on the right).  As with all enzymes, Trp Synthase is itself a protein, so we have proteins synthesizing amino acids, which are used to build proteins!