Structural Biochemistry/Other Non-Common Amino Acids
There are a vast majority of non-common amino acids. There are typically presented as twenty to twenty two common amino acids. The most common amino acids are Glycine, Alanine, Valine, Leucine, Isoleucine, Methionine, Proline, Phenylalanine, Tyrosine, Tryptophan, Serine, Threonine, Asparagine, Glutamine, Cysteine, Lysine, Arginine, Histidine, Asparatate, and Glutamate. There are also two amino acids that aren't typically included in the standard and they are selenocysteine and pyrrolysine.
Selenocysteine - Sel/U
This amino acid is considered one of the primary amino acids. There is typically considered twenty-two primary amino acids. This one is similar to Cysteine although it does not contain sulfur. Instead the R group on this amino acid is -CH2SeH. The sulfur that is typically present in the amino acid cysteine is now replaced by a selenium atom.
Pyrrolysine - Pyl/O
This amino acid is also considered one of the primary amino acids but is relatively not included. It is obviously has similarities to lysine in the fact that it's R group is lysine but it has an added extra bit to the end of the lysine. The added end ring is considered a pyrroline ring. This is where it gets it's name pyrrolysine from by combining the two names together.
Other Amino Acids
There are a number of other amino acids but are not considered primary amino acids. These amino acids are usually the result of changes are the amino acid is joined into a protein. These are called post translational modifications and when this happens it can change the structure of the amino acid.