Isoleucine is an essential amino acid and is one of the three amino acids having branched hydrocarbon side chains. It is usually interchangeable with leucine and occasionally with valine in proteins. The side chains of these amino acids are not reactive and therefore not involved in any covalent chemistry in enzyme active centers; however, these residues are critically important for ligand binding to proteins and play central roles in protein stability.
Note also that the βetta carbon of isoleucine is optically active, just as the βetta carbon of threonine. These two amino acids, isoleucine and threonine, have in common the fact that they have two chiral centers.
Source: Department of Biochemistry and Molecular Biophysics University of Arizona