Catalyzes the conversion of homogentisate to maleylacetoacetate. homogentisate + O2 = 4-maleylacetoacetate + H(+) Amino-acid degradation; L-phenylalanine degradation; acetoacetate and fumarate from L-phenylalanine: step 4/6. Homohexamer arranged as a dimer of trimers. Belongs to the homogentisate dioxygenase family.