3 - Hydroxy-anthranilic Acid 3,4 - Dioxygenase (3hao) Structural And Functional Studies

3-Hydroxyanthranilic acid 3,4-dioxygenase (3HA0) is a non-heme ferrousextradiol dioxygenase in the kynurenine pathway from tryptophan. It catalyzesthe conversion of 3-hydroxyanthranilate (HAA) to quinolinic acid (QUIN), anendogenous neurotoxin, via the activation of N-methyl-D-aspartate (NMDA)receptors and the precursor of NAD+ biosynthesis. The crystal structure of3HA0 from S. cerevisiae at 2.4 (?) resolution shows it to be a member of thefunctionally diverse cupin superfamily. The structure represents the firsteukaryotic 3HA0 to be resolved. The enzyme forms homodimers, with twonickel binding sites per molecule. One of the bound nickel atoms occupies theproposed ferrous-coordinated active site, which is located in a conserveddouble-strand β-helix domain. Examination of the structure reveals theparticipation of a series of residues in catalysis different from other extradioldioxygenases. Together with two iron-binding residues (His49 and Glu55),Asp120, Asn51, Glu111, and Arg114 form a hydrogen-bonding network; thishydrogen-bond network is key to the catalysis of 3HAO. Residues Arg101,Gln59, and the substratebinding hydrophobic pocket are crucial for substratespecificity. Structure comparison with 3HA0 from Ralstonia metalliduransreveals similarities at the active site and suggests the same catalyticmechanism in prokaryotic and eukaryotic 3HAO. Based on sequencecomparison, we suggest that bicupin of human 3HA0 is the first example ofevolution from a monocupin dimer to bicupin monomer in the diverse cupinsuperfamilies. A structural stabilization role is proposed for the other metalbinding site. Based on the model of the substrate HAA at the active site ofY3HAO, we propose a mechanism of catalysis for 3HAO. One possibility ofThe Mechanism of Inactivation of 3-Hydroxyanthranilate-3,4-dioxygenase by4-Chloro-3-hydroxyanthranilate is that the chloro substituent perturbs thesubstrate binding in such a way that the addition of oxygen of the C3 carbon ofthe substrate is blocked.