Chemical modification of some amino acid residues. Arginine can be methylated; the hydroxyl (–OH) group of serine is a frequent site of phosphorylation, as are those of tyrosine and threonine (not shown); lysine can be acetylated; asparagine can be glycosylated; and proline is sometimes hydroxylated. In each case, the modification alters the behavior of the protein containing the changed amino acid residue. Modification sites are indicated in red.