<p>The
spike protein of SARS-CoV-2 binds to ACE2 receptor <i>via</i> its receptor-binding domain (RBD), with the RBD-ACE2 complex presenting
an essential molecular target for vaccine development to stall the virus
infection proliferation. The computational analysis at molecular, amino acid
(AA) and atomic levels have been performed systematically to identify the key
interacting AAs in the formation of the RBD-ACE2 complex, including the MD
simulations with molecular mechanics generalized Born surface area (MM-GBSA)
method to predict binding free energy (BFE) and to determine the actual
interacting AAs, as well as two <i>ab initio</i> quantum chemical protocols
based on the density functional theory (DFT) implementation. Based on MD
results, Q<sup>493</sup>, Y<sup>505</sup>, Q<sup>498</sup>, N<sup>501</sup>,
T<sup>500</sup>, N<sup>487</sup>, Y<sup>449</sup>, F<sup>486</sup>, K<sup>417</sup>,
Y<sup>489</sup>, F<sup>456</sup>, Y<sup>495</sup>, and L<sup>455</sup> have
been identified as hotspots in RBD, while those in ACE2 are K<sup>353</sup>, K<sup>31</sup>,
D<sup>30</sup>, D<sup>355</sup>, H<sup>34</sup>, D<sup>38</sup>, Q<sup>24</sup>,
T<sup>27</sup>, Y<sup>83</sup>, Y<sup>41</sup>, E<sup>35</sup>, and E<sup>37</sup>.
Both
the electrostatic and hydrophobic interactions are the main driving force to
form the AA-AA binding pairs. We confirm that Q<sup>493</sup>,
N<sup>501</sup>, F<sup>486</sup>, K<sup>417</sup>, and
F<sup>456</sup> in
RBD are the key residues responsible for the tight binding of SARS-CoV-2 with
ACE2 compared to SARS-CoV. The DFT results reveal that N<sup>487</sup>, Q<sup>493</sup>, Y<sup>449</sup>, T<sup>500</sup>, G<sup>496</sup>, G<sup>446</sup> and G<sup>502</sup> in
RBD form pairs <i>via</i> specific hydrogen bonding with Q<sup>24</sup>, H<sup>34</sup>,
E<sup>35</sup>, D<sup>38</sup>, Y<sup>41</sup>, Q<sup>42</sup> and K<sup>353</sup>
in ACE2. </p>