Amine oxidases (AO) are enzymes that catalyze the oxidation of a wide range of biogenic amines including many neurotransmitters, histamine and xenobiotic amines. There are two classes of amine oxidases: flavin-containing and copper-containing .
Copper-containing AO act as a disulphide-linked homodimer. They catalyse the oxidation of primary amines to aldehydes, with the subsequent release of ammonia and hydrogen peroxide, which requires one copper ion per subunit and topaquinone as cofactor :
RCH2NH2 + H2O + O2 = RCHO + NH3 + H2O2
Copper-containing amine oxidases are found in bacteria, fungi, plants and animals. In prokaryotes, the enzyme enables various amine substrates to be used as sources of carbon and nitrogen [9048544, 9405045]. In eukaryotes they have a broader range of functions, including cell differentiation and growth, wound healing, detoxification and cell signalling .
The copper amine oxidases occur as mushroom-shaped homodimers of 70-95 kDa, each monomer containing a copper ion and a covalently bound redox cofactor, topaquinone (TPQ). TPQ is formed by post-translational modification of a conserved tyrosine residue. The copper ion is coordinated with three histidine residues and two water molecules in a distorted square pyramidal geometry, and has a dual function in catalysis and TPQ biogenesis. The catalytic domain is the largest of the 3-4 domains found in copper amine oxidases, and consists of a beta sandwich of 18 strands in two sheets. The active site is buried and requires a conformational change to allow the substrate access. The two N-terminal domains share a common structural fold, its core consisting of alpha-beta(4), where the helix is packed against the coiled anti-parallel beta-sheets.