The major structural difference between human monoamine oxidases A (MAO A) and B (MAO B) is that MAO A has a monopartite substrate cavity of 550 A ˚ 3 volume and MAO B contains a dipartite cavity struc-ture with volumes of 290 A ˚ 3 (entrance cavity) and 400 A ˚ 3 (substrate cavity).