Dissolving Silver Nanoparticles Modulate the Endothelial Monocyte-Activating Polypeptide II (EMAP II) by Partially Unfolding the Protein Leading to tRNA Binding Enhancement

Metal nanoparticles (NP) are increasingly used in biomedical applications. Among them, silver NPs (AgNPs) are used as active components in antibacterial coatings for wound dressings, medical devices, implants, cosmetics, textiles, and food packaging. On the other hand, AgNPs can be toxic to humans, depending on the dose and route of exposure, as agents delivering silver to cells. The cysteine residues are the primary molecular targets in such exposures, due to the high affinity of Ag⁺ ions to thiol groups. The Endothelial monocyte-activating polypeptide II (EMAP II), a cleaved C-terminal peptide of the intracellular aminoacyl-tRNA synthetase multifunctional protein AIMP1, contains five cysteines exposed at its surface. This prompted the question of whether they can be targeted by Ag⁺ ions present at the AgNPs surface or released from AgNPs in the course of oxidative metabolism of the cell. We explored the interactions between recombinant EMAP II, tRNA, and AgNPs using UV-Vis and fluorescence spectroscopy, providing insight into the effects of AgNPs dissolution kinetics on interaction EMAP II with tRNA. In addition, the EMAP II fragments binding to intact AgNPs were established by heteronuclear ¹H-¹⁵N HSQC spectra utilizing a paramagnetic probe. Structural analysis of the EMAP II reveal that the 3D structure of protein was destabilized (partially denatured) by the binding of Ag⁺ ions released from AgNPs at the most exposed cysteines. Surprisingly, this effect enhanced tRNA affinity to EMAP II, lowering its Kd. The course of the EMAP II/tRNA/AgNP reaction was also modulated by other factors, such as the presence of Mg²⁺ ions and TCEP, a thiol-group protector used to mimic the reducing conditions of the cell.