domains,<unk> e tetramerization domain (pink) and<unk> e catalytic domain (blue).<unk> e regulatory domain (not shown) would sit somewhere on<unk> e right hand side of<unk> e image where also<unk> e enzyme's substrate would enter from.<unk>  is a tetramer of four identical subunits (homotetramer). Each subunit consists of<unk> ree domains. At<unk> e carboxyl terminal of<unk> e peptide chain<unk> ere's a short alpha helix domain<unk> at allows tetramerization.<unk> e central ~300 amino acids make up a catalytic core, in which all<unk> e residues necessary for catalysis are located, along with a non-covalently bound iron atom.<unk> e iron is held in place by two histidine residues and one glutamate residue, making it a non-heme, non-iron-sulfur iron-containing enzyme.<unk> e amino terminal ~150 amino acids make up a regulatory domain,<unk> ught to control access of substrates to<unk> e active site. In humans<unk> ere are<unk> ught to be four different versions of<unk> is regulatory domain, and<unk> us four versions of<unk> e enzyme, depending on alternative splicing,<unk> ugh none of<unk> eir structures have yet been properly determined. It has been suggested<unk> at<unk> is domain might be an intrinsically unstructured protein, which has no clearly defined tertiary structure, but so far no evidence has been presented supporting<unk> is claim. It has however been shown<unk> at<unk> e domain has a low occurrence of secondary structures, which doesn't weaken suspicions of it having a disordered overall structure. As for<unk> e tetramerization and catalytic domains<unk> eir structure was found with rat<unk>  using X-ray crystallography.<unk> is has shown how its structure is very similar to<unk> at of phenylalanine hydroxylase and tryptophan hydroxylase; together<unk> e<unk> ree make up a family of homologous aromatic amino acid hydroxylases.
Tyrosine hydroxylase
Dopamine beta-hydroxylase
Phenylalanine hydroxylase
Catechol-O-methyltransferase
Aromatic L-amino acid decarboxylase
Monoamine oxidase
Glutamate dehydrogenase
Monoamine oxidase B