– 4-Hydroxyphenylpyruvate Dioxygenase (4HPPD) belongs to the 4HPPD family. 4HPPD is a key enzyme in the degradation of tyrosine, which catalyzes the second reaction in the catabolism of tyrosine the conversation of 4-hydroxyphenylpyruvate to homogentisate. 4HPPD exists in homodimer forms, which uses zinc as a cofactor to catalyze the third step in the conversion of L-phenylalanine to fumarate and acetoacetic acid. When the active 4HPPD enzyme concentration is low in the human body, it results in high levels of tyrosine concentration in the blood, which can cause mild mental retardation at birth, and degradation in vision as a patient grows older.