Class III extradiol dioxygenases with similarity to homoprotocatechuate 2,3-dioxygenase, which catalyzes the key ring cleavage step in the metabolism of homoprotocatechuate.
This subfamily of class III extradiol dioxygenases consists of two types of proteins with known enzymatic activities; 3,4-dihydroxyphenylacetate (homoprotocatechuate) 2,3-dioxygenase (HPCD) and 2-amino-5-chlorophenol 1,6-dioxygenase. HPCD catalyzes the key ring cleavage step in the metabolism of homoprotocatechuate (hpca), a central intermediate in the bacterial degradation of aromatic compounds. The enzyme incorporates both atoms of molecular oxygen into hpca, resulting in aromatic ring-opening to yield the product alpha-hydroxy-delta-carboxymethyl cis-muconic semialdehyde. 2-amino-5-chlorophenol 1,6-dioxygenase catalyzes the oxidization and subsequent ring-opening of 2-amino-5-chlorophenol, which is an intermediate during p-chloronitrobenzene degradation. The enzyme is probably a heterotetramer composed of two alpha and two beta subunits. Alpha and beta subunits share significant sequence similarity and both belong to this family. Like all Class III extradiol dioxygenases, these enzymes use a non-heme Fe(II) to cleave aromatic rings between a hydroxylated carbon and an adjacent non-hydroxylated carbon.