Flavin-Mediated Reductive Deiodination: Conformational Events and Reactivity Pattern in the Active Site of Human Iodotyrosine Deiodinase

Human iodotyrosine deiodinase (hIYD) catalyzes the reductive deiodination of iodotyrosine using a flavin mononucleotide cofactor to maintain iodine concentration in the body. Mutations in the hIYD gene are linked to human hypothyroidism, emphasizing its role in thyroid function regulation. The present work employs microsecond-scale molecular dynamics simulations and quantum chemical calculations to elucidate the conformational dynamics and reactivity in the active site at various stages of hIYD catalysis. The flavin is found to employ a unique butterfly motion of its isoalloxazine ring accompanied by a novel active-and-resting state of its ribose 2′-OH group during the catalytic cycle. The flavin dynamics is found to control substrate binding affinity, the active site lid closure, and NADPH recognition. The enzyme uses a group of basic residues (R100, R101, R104, K182, and R279) to stabilize flavin at different stages of catalysis, suggesting potential mutations to control enzyme activity. The reactivity descriptor analysis and stereo-electronic analysis predict the N5 nitrogen of flavin as a proton source during the reductive deiodination. The present findings provide key insights into the molecular basis of hIYD activity and lay the groundwork for future research aimed at therapeutic interventions and industrial applications.