4-hydroxyphenylpyruvate dioxygenase isoform 1, also known as HPD, is an Fe-containing enzyme, that catalyzes the second reaction in the catabolism of tyrosine the conversion of 4-hydroxyphenylpyruvate to homogentisate. Existing 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.