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. Highest expression in the prostate, small intestine, colon, kidney and liver. The disease is caused by variants affecting the gene represented in this entry. Belongs to the homogentisate dioxygenase family.