All amino acids have a carboxyl group and an amino group bonded to the same carbon atom (the α carbon). But, they differ from each other in their R group or the side chain group. The nature of the R-group differentiates each amino acid in terms of their acid/basic nature, solubility, stability, and ultimately the proteins.
Proteins in organisms may have some modified residues of the 20 amino acids. 4-hydroxyproline, a derivative of proline, 5-hydroxylysine, derived from lysine and α-carboxyglutamate from glutamic acid are some examples. Selenocysteine is a special case of having selenium rather than the sulfur of cysteine. Some 300 additional amino acids have been found in cells but they are not constituents of proteins. These amino acids are commonly known as uncommon amino acids.