Function in biochemistry revolves around a reversible protein-ligand interaction. A molecule bound reversibly by a protein is called a ligand'. A ligand binds at a site on the protein called the `binding site'. which is complementary to the ligand in size, charge and hydrophobic or hydrophilic character. As long as the ligand-binding residues are preserved to allow interaction between the ligand and the binding sites the function will essentially be preserved.