SARS-CoV
and SARS-CoV-2 bind to the human ACE2 receptor in practically
identical conformations, although several residues of the receptor-binding
domain (RBD) differ between them. Herein, we have used molecular dynamics
(MD) simulations, machine learning (ML), and free-energy perturbation
(FEP) calculations to elucidate the differences in binding by the
two viruses. Although only subtle differences were observed from the
initial MD simulations of the two RBD–ACE2 complexes, ML identified
the individual residues with the most distinctive ACE2 interactions,
many of which have been highlighted in previous experimental studies.
FEP calculations quantified the corresponding differences in binding
free energies to ACE2, and examination of MD trajectories provided
structural explanations for these differences. Lastly, the energetics
of emerging SARS-CoV-2 mutations were studied, showing that the affinity
of the RBD for ACE2 is increased by N501Y and E484K mutations but
is slightly decreased by K417N.