63 (Hofmann et al., 2005; Li, 2015), SARS-CoV recognizes, through the RBD in the CTD region of its S1 subunit, angiotensin-converting enzyme 2 (ACE2) as the receptor on the target cell (Li et al., 2003). The RBD (CTD) in two states (standing or lying) has been observed in the trimeric S protein. ACE2 binds to standing RBD, specifically in the receptor-binding motif (RBM), keeping the RBD in the “standing” state (Yuan et al., 2017). In addit