UniProt release 4.6

1 million cysteine residues in UniProtKB/Swiss-Prot

The total number of cysteine residues in Swiss-Prot has reached the 1 million mark. While there is nothing special about this number, we thought it was interesting in the context of the natural bias in the amino composition of proteins. There is more than a 8-fold difference between the frequency of the rarest amino acid (tryprophan at 1.15%) and that of the most frequent one (leucine at 9.64%). There are a number of reasons for this compositional bias, one of which is the degeneracy of the genetic code (which allows from 1 to 6 different triplets to code for a specific amino acid), and another one is the prevalence of hydrophobic aliphatic residues such as leucine or isoleucine in transmembrane domains and in signal sequences.