Extradiol dioxygenases catalyze the incorporation of both atoms of molecular oxygen into substrates using a variety of reaction mechanisms, resulting in the cleavage of aromatic rings. There are two major groups of dioxygenases according to the cleavage site of the aromatic ring. Intradiol enzymes cleave the aromatic ring between two hydroxyl groups, whereas extradiol enzymes cleave the aromatic ring between a hydroxylated carbon and an adjacent non-hydroxylated carbon. Extradiol dioxygenases can be divided into three classes. Class I and II enzymes are evolutionary related and show sequence similarity, with the two domain class II enzymes evolving from the class I enzyme through gene duplication. Class III enzymes are different in sequence and structure and usually have two subunits, designated A and B, which form a tetramer composed of two copies of each subunit. This model represents subunit A of class III extradiol dioxygenase enzymes. The A subunit is the smaller, non-catalytic subunit. Enzymes that belong to this family include Protocatechuate 4,5-dioxygenase (LigAB) A subunit, 2'-aminobiphenyl-2,3-diol 1,2-dioxygenase (CarB) A subunit, Gallate Dioxygenase and proteins of unknown function.