Component of the 2-aminophenol 1,6-dioxygenase (APD) complex that catalyzes the ring fission of 2-aminophenol to produce 2-aminomuconic semialdehyde. CnbCa may have a role in the stability of the complex. The complex is also active on other substrates such as 2-amino-5-chlorophenol (68% activity), protocatechuate (33% activity) and catechol (5% activity). Both 2-aminophenol and 2-amino-5-cholorophenol are likely native substrates for this dioxygenase which is involved in the reductive degradation pathway of both nitrobenzene (NB) and 4-chloronitrobenzene (4-CNB), allowing C.testosteroni strain CNB-1 to grow on these compounds as sole source of carbon, nitrogen, and energy. Xenobiotic degradation; nitrobenzene degradation. Xenobiotic degradation; 4-chloronitrobenzene degradation. The APD complex is a heterotetramer of 2 alpha (CnbCa) and 2 beta (CnbCb) subunits. Not active on 4-methylcatechol, 4-chlorocatechol, 2,4-dihydroxybenzoate, o-nitrophenol, p-nitrophenol or 4-nitrocatechol. Belongs to the LigB/MhpB extradiol dioxygenase family. In contrast to other members of the family, lacks the conserved iron-binding sites, suggesting that the alpha subunit has no oxidoreductase activity.