Virtual Probing on the Influence of Ca²⁺ and Zn²⁺ Bound S100A8 and S100A9 Proteins Towards their Interaction Against Pattern Recognition Receptors Aggravating Rheumatoid Arthritis

Danger-associated molecular patterns (DAMPs) are released on the onset of tissue injury or death, which tend to trigger innate immunity and regulate various immune pathways. Among the various DAMP molecules, S100A8 and S100A9 belonging to Ca²⁺ binding proteins with EF-hands and Zn²⁺ ion binding sites have been implicated in aggravating the pathogenesis of rheumatoid arthritis (RA), upon interaction with pattern recognition receptors (PRR) such as TLR4, RAGE and CD36 receptors. Thus, the present study aims to assess the effect of Ca²⁺ or Zn²⁺ ions on the interaction of S100A8 and S100A9 proteins towards the PRRs. Protein-protein interaction analysis showed that the TLR4-S100A8Ca²⁺Zn²⁺, TLR4-S100A8 Zn²⁺, RAGE-S100A8/A8Zn²⁺, RAGE-S100A8/A8Ca²⁺, CD36-S100A8Ca²⁺, and CD36-S100A9/A9Ca²⁺ showed higher affinity against each other. The 100 ns molecular dynamics simulation showed that the TLR4-S100A8Ca²⁺, RAGE-S100A8/A8Ca²⁺ and CD36-S100A8Ca²⁺ complexes showed minimal fluctuations in their trajectory indicating that Ca²⁺ bound complexes were more stable than the other complexes. Furthermore, SPR analysis showed that S100A9 exhibited higher binding affinity towards PRRs in the presence of Ca²⁺ and Zn²⁺ ions. Considering the fact that physiological levels of both Ca²⁺ and Zn²⁺ ions play a critical role in the binding of S100A8 and S100A9 proteins against the PRRs, it can be emphasized that the S100A9 and RAGE receptors could be the critical players in the RA pathogenesis due to its impeccable binding towards the PRRs in the presence of both Ca²⁺ and Zn²⁺ ions. Nonetheless, further in vivo, and in vitro studies are imperative to validate these findings and identify potential targets for RA treatment.