We performed computer simulations to investigate the effect of oxidation on the extracellular cystine (CYC) uptake by the xCantiporter. The latter is important for killing of cancer cells. Specifically, applying molecular dynamics (MD) simulations we studied the transport of CYC across xCT, i.e., the light subunit of the xCantiporter, in charge of bidirectional transport of CYC and glutamate. We considered the outward facing (OF) configuration of xCT, and to study the effect of oxidation, we modified the Cys 327 residue, located in the vicinity of the extracellular milieu, to cysteic acid (CYO 327 ).Our computational results showed that oxidation of Cys 327 results in a free energy barrier for CYC translocation, thereby blocking the access of CYC to the substrate binding site of the OF system. The formation of the energy barrier was found to be due to the conformational changes in the channel. Analysis of the MD trajectories revealed that the reorganization of the side chains of the Tyr 244 and CYO 327 residues play a critical role in the OF channel blocking. Indeed, the calculated distance between Tyr 244 and either Cys 327 or CYO 327 showed a narrowing of the channel after oxidation. The obtained free energy barrier for CYC translocation was found to be 33.9 kJmol −1 , indicating that oxidation of Cys 327 , by e.g., cold atmospheric plasma, is more effective in inhibiting the xCantiporter than in the mutation of this amino acid to Ala (yielding a barrier of 32.4 kJmol −1 ). The inhibition of the xCantiporter may lead to Cys starvation in some cancer cells, eventually resulting in cancer cell death.

Binding of the SARS-CoV-2 S-glycoprotein to cell receptors is vital for the entry of the virus into cells and subsequent infection. ACE2 is the main cell receptor for SARS-CoV-2, which can attach to the C-terminal receptor-binding domain (RBD) of the SARS-CoV-2 S-glycoprotein. The GRP78 receptor plays an anchoring role, which attaches to the RBD and increases the chance of other RBDs binding to ACE2. Although high levels of reactive oxygen and nitrogen species (RONS) are produced during viral infections, it is not clear how they affect the RBD structure and its binding to ACE2 and GRP78. In this research, we apply molecular dynamics simulations to study the effect of oxidation of the highly reactive cysteine (Cys) amino acids of the RBD on its binding to ACE2 and GRP78. The interaction energy of both ACE2 and GRP78 with the whole RBD, as well as with the RBD main regions, is compared in both the native and oxidized RBDs. Our results show that the interaction energy between the oxidized RBD and ACE2 is strengthened by 155 kJ/mol, increasing the binding of the RBD to ACE2 after oxidation. In addition, the interaction energy between the RBD and GRP78 is slightly increased by 8 kJ/mol after oxidation, but this difference is not significant. Overall, these findings highlight the role of RONS in the binding of the SARS-CoV-2 S-glycoprotein to host cell receptors and suggest an alternative mechanism by which RONS could modulate the entrance of viral particles into the cells.

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