Full-length in meso structure and mechanism of rat kynurenine 3-monooxygenase inhibition

The structural mechanisms of single-pass transmembrane enzymes remain elusive. Kynurenine 3-monooxygenase (KMO) is a mitochondrial protein involved in the eukaryotic tryptophan catabolic pathway and is linked to various diseases. Here, we report the mammalian full-length structure of KMO in its membrane-embedded form, complexed with compound 3 (identified internally) and compound 4 (identified via DNA-encoded chemical library screening) at 3.0 Å resolution. Despite predictions suggesting that KMO has two transmembrane domains, we show that KMO is actually a single-pass transmembrane protein, with the other transmembrane domain lying laterally along the membrane, where it forms part of the ligand-binding pocket. Further exploration of compound 3 led to identification of the brain-penetrant compound, 5 . We show that KMO is dimeric, and that mutations at the dimeric interface abolish its activity. These results will provide insight for the drug discovery of additional blood-brain-barrier molecules, and help illuminate the complex biology behind single-pass transmembrane enzymes.