In enzymology, a protocatechuate 4,5-dioxygenase () is an enzyme that catalyzes the chemical reaction protocatechuate + O2 4-carboxy-2-hydroxymuconate semialdehyde Thus, the two substrates of this enzyme are protocatechuate and O2, whereas its product is 4-carboxy-2-hydroxymuconate semialdehyde. This enzyme belongs to the family of oxidoreductases, specifically those acting on single donors with O2 as oxidant and incorporation of two atoms of oxygen into the substrate (oxygenases). The oxygen incorporated need not be derived from O2. The systematic name of this enzyme class is protocatechuate:oxygen 4,5-oxidoreductase (decyclizing). Other names in common use include protocatechuate 4,5-oxygenase, protocatechuic 4,5-dioxygenase, and protocatechuic 4,5-oxygenase. This enzyme participates in benzoate degradation via hydroxylation and 2,4-dichlorobenzoate degradation. It employs one cofactor, iron. Structural studies As of late 2007, two structures have been solved for this class of enzymes, with PDB accession codes and . References EC 1.13.11 Iron enzymes Enzymes of known structure