ACE2 and COVID-19: The experimental affinity measurements (e.g., surface plasmon resonance (SPR)) confirm the high affinity of SARS-2002 RBD–ACE2 binding, with an equilibrium dissociation constant (KD) of ~1.5–10.0 nM [25,26,27], comparable to the binding affinity of ACE2 and the COVID-19 RBD (KD = ~1.2–14.7 nM [27,28]) and consistent with our MD-based calculation (Figure 1A, Figure S3, and Table S2).