Converts 2-aminomuconate to 4-oxalocrotonate, an intermediate step in the biodegradation of nitrobenzene. (2Z,4E)-2-aminomuconate + H2O = (3E)-2-oxohex-3-enedioate + NH4(+) Optimum pH is 6.6. Xenobiotic degradation; nitrobenzene degradation. Homohexamer.