[hseguma]

Hi, I have a question related to the amino acid tryptophan.

I know that the common 20 amino acids are coded by several kinds of codon except for methionine (which is coded by AUG) and tryptophan (which is coded by UGG).

I understand that because methionine is the one to send ribosome a signal to start a translation process, it is coded by only AUG. If more than one amino acid can decide a start point, the coding sequence can be interrupted from unwanted locations.

However, tryptophan is neither a start nor stop amino acid, and is the one building block of polypeptide. Why does tryptophan have to be coded by only one codon? If there is any one who understand the reason for this, I would greatly appreciate an explanation for it.

All the best,

Hiro 

[majorjp]

There are several theories, some of which conclude that the number of codons for an amino acid is relative to that amino acid's usage. A more scientific approach is based on the evolution of primative amino acids. Tryptophan has an aromatic R-group and is considered to be one of the most complex amino acids. It is theorize that due to that fact that the creation of tryptophan requires more energy and molecules that other amino acids that it is a derivative amino acid of the primative amino acids. This could explain the fact that only the UGG codon is for tryptophan.

But all of this is theoretically and hypothetically, it has yet to have been confirmed as far as I'm aware of