Before revision:  Heparin can mask both the S1/S2 basic motif, thereby inhibiting furin cleavage and the formation of the prefusion conformation, and the basic residues of the receptor binding domain (RBD), thus acting on the hinge region responsible for the motion of the RBD between inactive closed and active open conformations of the spike.
After revision:  Heparin can mask both the S1/S2 basic motif, thereby inhibiting furin cleavage and the subsequent formation of its post-fusion conformation. Experiments corroborated the simulation results by showing that heparin binds the basic S1/S2 site of spike, inhibiting cleavage by furin. The simulations show that heparin can act on the hinge region responsible for the motion of the RBD between inactive closed and active open conformations of the spike.