Involved in the catabolism of homogentisate (2,5-dihydroxyphenylacetate or 2,5-OH-PhAc), a central intermediate in the degradation of phenylalanine and tyrosine. Catalyzes the oxidative ring cleavage of the aromatic ring of homogentisate to yield maleylacetoacetate. homogentisate + O2 = 4-maleylacetoacetate + H(+) Amino-acid degradation; L-phenylalanine degradation; acetoacetate and fumarate from L-phenylalanine: step 4/6. Hexamer; dimer of trimers. Belongs to the homogentisate dioxygenase family. Truncated N-terminus.