Crystal structure of Pseudomonas fluorescens 4-hydroxyphenylpyruvate dioxygenase: an enzyme involved in the tyrosine degradation pathway

Background: In plants and photosynthetic bacteria, the tyrosine degradation pathway is crucial because homogentisate, a tyrosine degradation product, is a precursor for the biosynthesis of photosynthetic pigments, such as qui nones or tocophenols. Homogentisate biosynthesis includes a decarboxylation step, a dioxygenation and a rearrangement of the pyruvate sidechain, This complex reaction is carried out by a single enzyme, the 4-hydroxyphenylpyru vate dioxygenase (HPPD), a non-heme iron dependent enzyme that is active as a homotetramer in bacteria and as a homodimer in plants. Moreover, in huma ns, a HPPD deficiency is found to be related to tyrosinemia, a rare heredit ary disorder of tyrosine catabolism, Results: We report here the crystal structure of Pseudomonas fluorescens HP PD refined to 2.4 Angstrom resolution (R-free 27.6%; R factor 21.9%). The g eneral topology of the protein comprises two barrel-shaped domains and is s imilar to the structures of Pseudomonas 2,3-dihydroxybiphenyl dioxygenase ( DHBD) and Pseudomonas putida catechol 2,3-dioxygenase (MPC), Each structura l domain contains two repeated beta alpha beta beta beta alpha modules. The re is one non-heme iron atom per monomer liganded to the sidechains of His1 61, His240, Glu322 and one acetate molecule, Conclusions: The analysis of the HPPD structure and its superposition with the structures of DHBD and MPC highlight some important differences in the active sites of these enzymes, These comparisons also suggest that the pyru vate part of the HPPD substrate (4-hydroxyphenylpyruvate) and the O-2 molec ule would occupy the three free coordination sites of the catalytic iron at om. This substrate-enzyme model will aid the design of new inhibitors of th e homogentisate biosynthesis reaction.