CONICET | Buscador de Institutos y Recursos Humanos

Human indoleamine 2,3-dioxygenase catalyzesthe oxidative cleavage of tryptophan to N-formyl kynurenine,the initial and rate-limiting step in the kynurenine pathway.Additionally, this enzyme has been identified as a possibletarget for cancer therapy. A 20-amino acid protein segment(the JK loop), which connects the J and K helices, was notresolved in the reported hIDO crystal structure. Previousstudies have shown that this loop undergoes structuralrearrangement upon substrate binding. In this work, weapply a combination of replica exchange molecular dynamicssimulations and site-directed mutagenesis experiments tocharacterize the structure and dynamics of this protein region. Our simulations show that the JK loop can be divided intotwo regions: the first region (JK loopC) displays specific and well-defined conformations and is within hydrogen bonding distanceof the substrate, while the second region (JK loopN) is highly disordered and exposed to the solvent. The peculiar flexible natureof JK loopN suggests that it may function as a target for post-translational modifications and/or a mediator for protein−proteininteractions. In contrast, hydrogen bonding interactions are observed between the substrate and Thr379 in the highly conserved?GTGG? motif of JK loopC, thereby anchoring JK loopC in a closed conformation, which secures the appropriate substratebinding mode for catalysis. Site-directed mutagenesis experiments confirm the key role of this residue, highlighting theimportance of the JK loopC conformation in regulating the enzymatic activity. Furthermore, the existence of the partially andtotally open conformations in the substrate-free form suggests a role of JK loopC in controlling substrate and product dynamics.