4-Hydroxyphenylpyruvate Dioxygenase Isoform-1 is an Fe-containing enzyme， which catalyzes the second reaction in the catabolism of tyrosine the conversion of 4-hydroxyphenylpyruvate to homogentisate. Present as a homodimer， HPD uses zinc as a cofactor to catalyze the third step in the conversion of L-phenylalanine to fumarate and acetoacetic acid. Flaws in the gene encoding HPD result in tyrosinemia type 3 and hawkinsinuria， two inborn defects of metabolism which are related to a number of symptoms， like mental retardation and seizures and hair and urine abnormalities.
