7.2 Nonstandard Protein Residues

A few proteins incorporate selenocysteine, H₃N⁺-CH(CH₂Se)-CO₂^-, side chain pka = 5.43, as the 21st amino acid. Like cysteine, it occurs as the (R)-L- isomer. Selenocystine, a dimer of selenocysteines joined by a Se-Se bond, occurs by analogy with normal cystine.

Selenocystine (Sec).

Hydroxyproline is an important part of collagen. It is originally proline when the protein is first made, then an enzyme adds a hydroxy group on the gamma carbon with (R)- chirality.

Hydroxyproline (Hyp).

Selenomethionine, having the same structure as normal methionine but with selenium in place of sulfur, occurs in small amounts in plants and animals. It behaves mostly the same as its standard congener in the body, and wherever a protein would normally have normal methionine, occasionally a selenomethionine residue appears instead. Too much of this amino acid is toxic, but too little in the diet causes a deficiency.

Selenomethionine (SeMet).

Pyrrolysine is used by some bacteria and archaea as a proteinogenic amino acid. It has a complex molecular structure and performs catalytic functions inside enzymes.

Pyrrolysine (Pyl).

Selenocysteine has the letter code U when it occurs in a protein sequence. Hydroxyproline and pyrrolysine are both indicated by the letter O, so it is important to know from context which one is meant, or else clarify the meaning of O separately.

Next Lesson | Table of Contents | Main