Subunit B of the Class III Extradiol ring-cleavage dioxygenase, 2,3-dihydroxyphenylpropionate 1,2-dioxygenase (MhpB), which catalyzes the oxidization and subsequent ring-opening of 2,3-dihydroxyphenylpropionate.2,3-dihydroxyphenylpropionate 1,2-dioxygenase (MhpB) catalyzes the oxidization and subsequent ring-opening of 2,3-dihydroxyphenylpropionate, yielding the product 2-hydroxy-6-oxo-nona-2,4-diene 1,9-dicarboxylate. It is an essential enzyme in the beta-phenylpropionic degradation pathway, in which beta-phenylpropionic is first hydrolyzed to produce 2,3-dihydroxyphenylpropionate. The enzyme is a member of the class III extradiol dioxygenase family, a group of enzymes which use a non-heme Fe(II) to cleave aromatic rings between a hydroxylated carbon and an adjacent non-hydroxylated carbon. LigAB-like class III enzymes are usually composed of two subunits, designated A and B, which form a tetramer composed of two copies of each subunit. This model represents the catalytic subunit, B. MhpB is likely to be a tetramer.