Structural basis of kynurenine 3-monooxygenase inhibition

Inhibition of kynurenine 3-monooxygenase (KMO) leads to amelioration of Huntington’s-disease-relevant phenotypes in yeast, fruitfly and mouse models; here the crystal structures of free and inhibitor-bound yeast KMO are presented, which could aid the development of targeted therapies for human neurodegenerative diseases. KMO enzyme structure The kynurenine pathway is the major route of tryptophan degradation in mammals, and some metabolites generated by tryptophan degradation have an important role in neurodegenerative disorders. Inhibition of the kynurenine 3-monooxygenase (KMO) enzyme leads to the amelioration of Huntington's-disease-relevant phenotypes in various animal models. This paper presents the structure of KMO in the free form and in complex with an inhibitor. This study will provide the basis for the development of inhibitors for possible therapeutic use in neurodegenerative diseases. Inhibition of kynurenine 3-monooxygenase (KMO), an enzyme in the eukaryotic tryptophan catabolic pathway (that is, kynurenine pathway), leads to amelioration of Huntington’s-disease-relevant phenotypes in yeast, fruitfly and mouse models 1 , 2 , 3 , 4 , 5 , as well as in a mouse model of Alzheimer’s disease 3 . KMO is a flavin adenine dinucleotide (FAD)-dependent monooxygenase and is located in the outer mitochondrial membrane where it converts l -kynurenine to 3-hydroxykynurenine. Perturbations in the levels of kynurenine pathway metabolites have been linked to the pathogenesis of a spectrum of brain disorders 6 , as well as cancer 7 , 8 and several peripheral inflammatory conditions 9 . Despite the importance of KMO as a target for neurodegenerative disease, the molecular basis of KMO inhibition by available lead compounds has remained unknown. Here we report the first crystal structure of Saccharomyces cerevisiae KMO, in the free form and in complex with the tight-binding inhibitor UPF 648. UPF 648 binds close to the FAD cofactor and perturbs the local active-site structure, preventing productive binding of the substrate l -kynurenine. Functional assays and targeted mutagenesis reveal that the active-site architecture and UPF 648 binding are essentially identical in human KMO, validating the yeast KMO–UPF 648 structure as a template for structure-based drug design. This will inform the search for new KMO inhibitors that are able to cross the blood–brain barrier in targeted therapies against neurodegenerative diseases such as Huntington’s, Alzheimer’s and Parkin