N-terminal domain of 3,4-dihydroxyphenylacetate 2,3-dioxygenase (HPCD)
This subfamily contains the N-terminal, non-catalytic, domain of HPCD. HPCD catalyses the second step in the degradation of 4-hydroxyphenylacetate to succinate and pyruvate. The aromatic ring of 4-hydroxyphenylacetate is opened by this dioxygenase to yield the 3,4-diol product, 2-hydroxy-5-carboxymethylmuconate semialdehyde. HPCD is a homotetramer and each monomer contains two structurally homologous barrel-shaped domains at the N- and C-terminus. The active-site metal is located in the C-terminal barrel and plays an essential role in the catalytic mechanism. Most extradiol dioxygenases contain Fe(II) in their active site, but HPCD can be activated by either Mn(II) or Fe(II). These enzymes belong to the type I class II family of extradiol dioxygenases. The class III 3,4-dihydroxyphenylacetate 2,3-dioxygenases belong to a different superfamily.
Jiyao W et al.(2023). "The conserved domain database in 2023.",