| Description | Protocatechuate (3,4-dihydroxybenzene, PCA) is an aromatic compound which is a key intermediate in the degradation of the plant biopolymer lignin and other aromatic compounds. The key step of PCA degradation is the ring-cleavage performed by dioxygenases adding both atoms from molecular oxygen to specific carbon atoms within the ring. This step can be performed by two distinct mechanisms; intradiol cleavage and extradiol cleavage. In intradiol cleavage the oxygen atoms are added to the carbons carrying the hydroxyl groups, producing two carboxylate groups. In extradiol cleavage the oxygens are added to one carbon carrying a hydroxyl group and another carrying a hydrogen, resulting in the formation of a carboxylate group and an aldehydic group. For further information see [ ].PCA dioxygenases fall into the broader category of catechol dioxygenases. These are metalloenzymes which bind non-haem iron [ , ]. The extradiol dioxygenases use Fe(II) to activate oxygen for nucleophilic attack on the aromatic substrate, while the intradiol dioxygenases use Fe(III) to activate the aromatic substrate for an electrophillic attack by oxygen [].This entry represents the alpha subunit of protocatechuate 3,4-dioxygenase, an enzyme which cleaves the PCA ring by an intradiol mechanism to produce beta-carboxy-cis,cis-muconate. The enzyme is a large aggregate of 12 protomers, each composed of two subunits, alpha and beta ( ) which are highly similar in structure and are thought to share a common ancestor [ , , ]. The core of each subunit is two four-stranded β-sheets that fold upon each other to form a β sandwich. The active site cavity contains the Fe(III)-binding site and is located between the two subunits. All Fe(III) ligands are contributed by the beta subunit. | Name | Protocatechuate 3,4-dioxygenase, alpha subunit |
| Short Name | Protocat_dOase_a | Type | Family |
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