The receptor binding domain (RBD) is the part the viral S protein that interacts with human ACE2. It can exist in two conformations: open (B) or closed (A). Molecular docking assays have shown that SARS-CoV-2 S protein can interact with the human ACE2 in both of its conformations (D - open; C - closed). However, the open conformation interacts protease domain (PD) at a more favorable binding energy than the closed domain which interacts with a different region of ACE2. This confirms the importance of the open conformation of S protein in SARS-CoV-2. infection.