  COVID-19, since emerged in Wuhan, China, has been a major concern due to its
high infection rate, leaving more than one million infected people around the
world. Huge number of studies tried to reveal the structure of the SARS-CoV-2
compared to the SARS-CoV-1, in order to suppress this high infection rate. Some
of these studies showed that the mutations in the SARS-CoV-1 Spike protein
might be responsible for its higher affinity to the ACE2 human cell receptor.
In this work, we used molecular dynamics simulations and Monte Carlo sampling
to compare the binding affinities of the spike proteins of SARS-CoV and
SARS-CoV-2 to the ACE2. We found that the SARS-CoV-2 binds to ACE2 stronger
than SARS-CoV by 7 kcal/mol, due to enhanced electrostatic interactions. The
major contributions to the electrostatic binding energies are resulting from
the salt-bridges formed between R426 and ACE2-E329 in case of SARS-CoV and K417
and ACE2-D30 for SARS-CoV2. In addition, there is no significant contribution
from a single mutant to the binding energies. However, these mutations induce
sophisticated structural changes that enhance the binding energies. Our results
also indicate that the SARS-CoV-2 is unlikely a lab engineered virus.
