Subunit B of Class III Extradiol ring-cleavage dioxygenases
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. Two major groups of dioxygenases have been identified 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 further 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. This model represents the catalytic subunit B of extradiol dioxygenase class III enzymes. Enzymes belonging to this family include Protocatechuate 4,5-dioxygenase (LigAB), 2'-aminobiphenyl-2,3-diol 1,2-dioxygenase (CarB), 4,5-DOPA Dioxygenase, 2,3-dihydroxyphenylpropionate 1,2-dioxygenase, and 3,4-dihydroxyphenylacetate (homoprotocatechuate) 2,3-dioxygenase (HPCD). There are also some family members that do not show the typical dioxygenase activity.