To understand how solvation can impact on the catalytic site, we next define an 'active' geometry: this features the PHE140/HIS163 intra-subunit hydrophobic contact and the inter-subunit interactions between m-shaped loop and the N-finger of the adjacent subunit (Fig. ) . Such hydrophobic contacts of PHE140/HIS163 are analyzed in terms of the distance between the center of these two rings as a function of time (dCM). The empty binding cavity (subunit B of 8P57) becomes inactive after a short simulation time (Fig. ): dCM passes from 0.46 (SD=0.13) nm to 0.84 (SD=0.08) nm. This is not the case for all the other occupied cavities, where dCM is 0.42 nm (SD ≤ 0.03) for the overall 500ns of MD (Fig. ). This suggests that, in the singly occupied protein, the presence of one ligand in one subunit might induce a non active geometry in the empty cavity of the adjacent subunit. Water plays a key role for this distortion: while basically absent in the occupied cavity (total number 0.07 (SD=0.26)), as many as (total number 4.29 (SD=1. 19)) are present in the empty one (Fig. ). As a result, HIS163 and PHE140 pi-pi stacking is broken, leading to the inactive state (Fig. ). A Principal Component Analysis (PCA) on each subunit further shows that the largest scale motion of subunit A is anticorrelated to that of subunit B: the former causes the closing, the latter the opening of the binding cavity (Fig. ). Interestingly, the trend of water occupation is also observed in the fully occupied enzymes. In subunit A they have 0.12 (SD=0.34) and 0.08 (SD=0.26) number of water molecules, respectively, and in subunit B, 0.28 (SD=0.49) and 0.91 (SD=0.83), respectively. Notably, toward the end of the simulations, both subunits A are without water molecules, while both subunits B are with two water molecules on average (Fig. ). This is more clear cut in the asymmetric space group crystals (PDBid 7PHZ).