Table 1.

Roles of important amino acid residues (physiologically important residues) in the S1-RBD: ACE2 interactions.

Parameter	Important interacting residues
N terminus of α1 of ACE2	The ACE2: α1 (N terminal) residues Tyr 41, Gln 42, Lys 353 and Arg 357 form hydrogen bond with Gln498, Thr500 and Asn501 of S1-RBD region [22].
Middle of the “bridge” region of interacting surface of ACE2	S1-RBD residues Lys417 and Tyr453 interact with Asp 30 and His 34 of ACE2 [22].
C terminus of α1 of ACE2	The ACE2: α1 (C terminal) residues Gln 24 and Met 82 interact with Gln 474 and Phe 486 via H-bond and van-der waals forces [22].
Hotspot residues for recognition of host receptor ACE2	In SARS-CoV, two prime residues 479 and 487 of RBD- CoV are involved in the recognition of ACE2 [24,25]. In the case of SARS-CoV-2, the residues corresponding to N479 is Q493 and T487 is N501.
Physiologically important changes in SARS-CoV-2 when compared to SARS-CoV	The residues val 404 (in SARS-CoV) change into Lys417 in SARS-CoV-2, which may result in stronger association with ACE2. This stronger affinity may be attributed to formation of salt bridge between Lys417 of SARS-CoV-2 and ASP30 of ACE2 [22].
	The residues Leu 472 change into Phe486 may shown the stronger van der Waals interaction at met82. How-ever, replacement of Arg426 to Asn439 appears to weaken the interaction by losing one important salt bridge with Asp 329 on ACE2 [22].
Capping loop residues [6] (Two capping loops in the binding domain, which stabilized the interaction with ACE2 by producing an increased amount of electrostatic interactions)	The capping loop in case of SARS-CoV-2 comprises of residues N 487, G 485,V 445, F 486, Y 449, E 484, A 475, Q 474 and Y 473 [6].