3.3 Ligand Receptor binding pose depictions
The best docking pose of 2-DG, and its derivatives with SARS-CoV-2 viral receptors was also identified using Pose View tool so as to visualize the interactions of the ligands with that of the residues present in the active sites of the viral receptors. Both 2-deoxy-D-glucose and its derivative were found to form salt bridges with the amino acid residues of the viral receptors, namely, main protease 3CLpro and viral spike glycoprotein, respectively. The orientational binding of the ligands and the viral receptors showing the pose view and residue interactions have been depicted in Fig. 3 . It was observed that the hydroxyl group of 2-deoxy-D-glucose formed a hydrogen bond with the carbonyl residue of Proline amino acid (108th position) found in the viral main protease. In earlier studies it has been found that the proline amino acid residues are found in the conserved domains of HIV viral infectivity factor (Vif) and these proline-rich motifs are therapeutic targets for neutralizing the human immunodeficiency virus (Yang et al., 2003; Ralph et al., 2020). Chemical bridging of 2-deoxy-D-glucose and proline residues of viral main protease 3CLpro present a similar case where proline residues were invariably bound and neutralized, thereby possibly neutralizing the COVID-19 virus. Similarly, the 2-DG derivative (1, 3, 4, 6-Tetra-O-acetyl-2-deoxy-D-glucopyranose) formed a hydrogen bond with the amide group of Glutamine amino acid (804thposition) found in the viral spike glycoprotein. Reynard and Volchkov have also previously highlighted that mutation or any change in the glutamine residues of Ebola virus spike glycoprotein causes viral neutralization (Reynard & Volchkov, 2015). In conclusion, the binding interactions of 2-deoxy-D-glucose with viral main protease and 1, 3, 4, 6-Tetra-O-acetyl-2-deoxy-D-glucopyranose with viral spike glycoprotein is now evident, as analysed by using Pose View tool.
2-deoxy-D-glucose and its derivative can influence several cellular pathways, including glycolysis, glycosylation, endoplasmic stress response (ER), phagocytosis and apoptosis. Both the moieties inhibit the processes of glucose transport and glycolysis by competing with glucose. Competitive uptake of 2-DG or its derivative in the infected cell leads to the formation of 2-deoxy-d-glucose-6-phosphate (2-DG-6-P) by means of hexokinase enzyme. 2-DG-6-P cannot be further metabolized, thereby hampering the bioenergetic process of ATP production by glycolysis (Sharma et al., 1996); inactivating the glycolytic enzymes; inducing cell cycle arrest and ultimately leading to inactivation of nCoV-19 in infected cells (Maher et al., 2004; Pajak et al., 2020). The depletion of ATP levels leads to activation of AMP-activated protein kinase (AMPK). Such activation will lead to phosphorylation of proteins of the mTOR kinase complex (mammalian target of rapamycin kinase, mTORC). As a consequence, expression of p53 is induced which ultimately promotes cell cycle arrest (G1 phase arrest) in virus infected cells. All these factors (glycolysis inhibition, ATP depletion and cell cycle arrest) cause a sensitized response leading to the upregulation of TNF expression, ultimately leading to an apoptotic response. Moreover, both 2-DG and its tetra-acetate glucopyranose derivative escalate the production of reactive oxygen species, ultimately leading to virus infected cell death (Fig. 4 ) (Zhang et al., 2015; Pajak et al., 2020).