Examples of reactions catalyzed by extradiol-cleaving catechol dioxygenases. (A) HPCD, homoprotocatechuate 2,3-dioxygenase [9]; (B) DHBD, 2,3-dihydroxybiphenyl 1,2-dioxygenase [10]; (C) 4,5-PCD, protocatechuate 4,5-dioxygenase [11]; (D) HAD, 3-hydroxyanthranilate-3,4-dioxygenase [2]; (E) HGD, homogentisate 1,2-dioxygenase[12]; (F) PcpA, 2,6-dichloro-p-hydroquinone 1,2-dioxygenase [4]. Four-letter codes adjacent to each reaction represent PDB files for respective enzymes.