Figure 8  Comparison of receptor binding sites in S1-CTDs of HCoV-NL63, SARS-CoV and SARS-CoV2 binding to human ACE2. (a) S1-CTD receptor binding domains (RBDs) of HCoV-NL63 (pdb: 3kbh [232], forest), SARS-CoV (pdb: ajf [234], marine) and SARS-CoV-2 (pdb: 6m0j [235], salmon) in complex with human ACE2 (transparent green). (b) Direct interactions between amino acids of human ACE2 and those of the S1-CTD RBD of the indicated CoV. The bond lengths in angstroms are indicated on the dashed lines. (c) Alignment of aa sequences of the S1-CTD RBDs of the three CoVs binding to human ACE2. Amino acid residues in the S1-CTD RBDs facing the ACE2 receptor are colored in red. Residues in the S1-CTD RBDs of HCoV-NL63, SARS-CoV and SARS-CoV-2 directly interacting with residues in the ACE2 receptor are highlighted in green, blue and purple, respectively.