Catalyzes the conversion of homogentisate to maleylacetoacetate. homogentisate + O2 = 4-maleylacetoacetate + H(+) Optimum pH is 6.1. Amino-acid degradation; L-phenylalanine degradation; acetoacetate and fumarate from L-phenylalanine: step 4/6. Homohexamer arranged as a dimer of trimers. Defects in Hgd are the cause of alkaptonuria (aku). Aku is an autosomal recessive error of metabolism which is characterized by an increase in the level of homogentisic acid. Belongs to the homogentisate dioxygenase family.