4-Hydroxyphenylpyruvate Dioxygenase (HPD)

4-Hydroxyphenylpyruvate dioxygenase HPPD is an Fe-containing enzyme, that catalyzes the second reaction in the catabolism of tyrosine - the conversion of 4-hydroxyphenylpyruvate to homogentisate.The mechanism of action of 4-hydroxyphenylpyruvate dioxygenase involves a decarboxylation step and an oxidation step, followed by a rearrangement step to form homogentisic acid. The HPPD reaction occurs through a NIH shift and involves the oxidative decarboxylation of an α-oxo acid as well as aromatic ring hydroxylation. The NIH-shift, which has been demonstrated through isotope-labeling studies, involves migration of an alkyl group to form a more stable carbocation. The shift, accounts for the observation that C3 is bonded to C4 in 4-hydroxyphenylpyruvate but to C5 in homogentisate.