Role of Non-Binding T63 Alteration in IL-18 Binding

Engineered interleukin-18 (IL-18) has attracted interest as a cytokine-based treatment. However, knowledge-based mutagenesis of IL-18 has been reported for only a few regions of the protein structures, including binding sites I and II. When coupled with the binding region mutant (E6K), the non-binding residue of IL-18, Thr63 (T63), has been shown to increase the flexibility of the binding loop. Nevertheless, the function of Thr63 in conformational regulation is still unknown. Using homology modeling, molecular dynamics simulation, and structural analysis, we investigated the effects of Thr63 alteration coupling with E6K on conformational change pattern, binding loop flexibility, and the hydrogen bond network. The results indicate that the 63rd residue was significantly associated with hydrogen-bond relaxation at the core β-barrel binding sites I and II Glu85-Ile100 loop. This result provided conformational and flexible effects to binding sites I and III by switching their binding loops and stabilizing the 63rd residue cavity. These findings may pave the way for the conceptualization of a new design for IL-18 proteins by modifying non-binding residues for structure-based drug development.