[5] However, HPPD differs from most molecules in this class due to the fact that it does not use α-ketoglutarate, and it only utilizes two substrates while adding both atoms of diatomic oxygen into the product, homogentisate.

[6] The HPPD reaction occurs through a NIH shift and involves the oxidative decarboxylation of an α-oxo acid as well as aromatic ring hydroxylation.

While even less is known about the function of the N-terminus of the enzyme, scientists have discovered that a single amino acid change in the N-terminal region can cause the disease known as hawkinsinuria.

[15] HPPD can be linked to one of the oldest known inherited metabolic disorders known as alkaptonuria, which is caused by high levels of homogentisate in the blood stream.

[18] In Type I tyrosinemia, a different enzyme, fumarylacetoacetate hydrolase is mutated and doesn't work, leading to very harmful products building up in the body.