Flavin-Mediated\nReductive Deiodination: Conformational\nEvents and Reactivity Pattern in the Active Site of Human Iodotyrosine\nDeiodinase

Human iodotyrosine deiodinase (hIYD) catalyzes the reductive\ndeiodination\nof iodotyrosine using a flavin mononucleotide cofactor to maintain\nthe iodine concentration in the body. Mutations in the hIYD gene are\nlinked to human hypothyroidism, emphasizing its role in thyroid function\nregulation. The present work employs microsecond-scale molecular dynamics\nsimulations and quantum chemical calculations to elucidate the conformational\ndynamics and reactivity in the active site at various stages of hIYD\nenzymatic\ncycle. The flavin is found to employ a unique butterfly motion of\nits isoalloxazine ring accompanied by a novel active-and-resting state\nof its ribose 2′-OH group at different stages of the enzymatic\ncycle. The flavin dynamics are found to control substrate binding\naffinity, the active site lid closure, and NADPH recognition. The\npredicted hIYD model shows enhanced stabilization of NADPH due to\nadditional interactions with the N-terminal and intermediate domains.\nThe enzyme uses a group of basic residues (R100, R101, R104, K182,\nand R279) to stabilize flavin in different stages of catalysis, suggesting\npotential mutations to control enzyme activity. The reactivity descriptors\nand stereoelectronic analysis predict the N5 nitrogen of flavin as\na proton source during the reductive deiodination, while the anisotropic\ncharge distribution on the halogen atom has negligible structural\nand electronic effects. The present findings provide key insights\ninto the molecular basis of hIYD activity and lay the groundwork for\nfuture research aimed at therapeutic interventions and industrial\napplications.