Catalyzes the conversion of 4-hydroxyphenylpyruvic acid to homogentisic acid, one of the steps in tyrosine catabolism. 3-(4-hydroxyphenyl)pyruvate + O2 = CO2 + homogentisate Binds 1 Fe cation per subunit. Amino-acid degradation; L-phenylalanine degradation; acetoacetate and fumarate from L-phenylalanine: step 3/6. Homodimer. There are two alleles (F1 and F2), F2 has Asp-104 and F1 has Asn-104. Mice are completely tolerant to the self form of the protein, but make a good antibody response to immunization with the non-self form. Defects in Hpd are the cause of tyrosinemia type III. Belongs to the 4HPPD family.