Our investigation provides useful information regarding both the local dynamics of their active site. In particular, we have focused our attention on the flexibility of the active site and on the conformational plasticity of the side chain of a particularly important and highly conserved Wloop tryptophan, whose roles in the activity of these hydrolases have been previously demonstrated. Our conventional MD simulations illustrate that the S/I mutation results in enhanced flexibility of the active site loops (as demonstrated by their higher fluctuations), in particular loop D that is connected to the helix that harbors the mutated residues. In turn, this flexibility allows the W-loop tryptophan and the tyrosine residue with whom it forms an active site aromatic clamp to occupy distances compatible with conformers for substrate binding in the two mesophilic enzymes studied. Even though the volume of the active site and its fluctuations over time are essentially unaltered by the mutations, we can speculate that the shape of the external part of the open cleft is, in the case of the S/I mutants, amenable to accommodate the binding of a PET chain in the early stages of the overall degradation process. It is also clear that at the level of the global scaffold, the communication pathways differ by both organism and variant, which affects the flexibility of key loops and thus in turn the conformational sampling of the conserved tryptophan.