Plays an essential role in the utilization of numerous aromatic and hydroaromatic compounds via the beta-ketoadipate pathway. 3,4-dihydroxybenzoate + O2 = 3-carboxy-cis,cis-muconate + 2 H(+) Binds Fe(3+) ion per subunit. Aromatic compound metabolism; beta-ketoadipate pathway; 3-carboxy-cis,cis-muconate from 3,4-dihydroxybenzoate: step 1/1. The enzyme is an oligomer of 12 copies of the alpha and beta chains. Belongs to the intradiol ring-cleavage dioxygenase family.