| Step | Enzyme name | EC number | Substrate | Product | Associated disease when deficient |
|---|---|---|---|---|---|
| 1 | Phenylalanine hydroxylase (PAH) | EC 1.14.16.1 | L-Phenylalanine | L-Tyrosine | Phenylketonuria (PKU) |
| 2 | Tyrosine aminotransferase (TAT) | EC 2.6.1.5 | L-Tyrosine | 4-Hydroxyphenylpyruvate | Tyrosinemia type II |
| 3 | 4-Hydroxyphenylpyruvate dioxygenase (HPPD/HPD) | EC 1.13.11.27 | 4-Hydroxyphenylpyruvate | Homogentisate | Hawkinsinuria / Tyrosinemia type III; inhibited pharmacologically by nitisinone (NTBC) (pqac-00000007, pqac-00000008, pqac-00000009) |
| 4 | Homogentisate 1,2-dioxygenase (HGD) | EC 1.13.11.5 | Homogentisate | Maleylacetoacetate | Alkaptonuria (pqac-00000001, pqac-00000007, pqac-00000010) |
| 5 | Maleylacetoacetate isomerase (MAAI/GSTZ1) | EC 5.2.1.2 | Maleylacetoacetate | Fumarylacetoacetate | Maleylacetoacetate isomerase deficiency (rare); pathway context in tyrosine catabolism (pqac-00000007) |
| 6 | Fumarylacetoacetate hydrolase (FAH) | EC 3.7.1.2 | Fumarylacetoacetate | Fumarate + Acetoacetate | Tyrosinemia type I (pqac-00000007) |


*Table: This table summarizes the canonical tyrosine degradation pathway, highlighting where HGD acts and where nitisinone blocks the upstream HPPD step. It is useful for placing HGD within its metabolic context and linking each step to the major inborn error associated with enzyme deficiency.*