Figure 4.

Tricyclic TAN likely cannot bind at the AmpC_EC active site due to a steric clash of the putative tricyclic core and side chain. (a) Structural comparison of tricyclic and bicyclic forms of TAN; (b–g) overlay of tricyclic TAN core (orange, not crystallographically observed at AmpC_EC active site) with bicyclic TAN (yellow) crystallographically observed bound to active site S64 of AmpC_EC (PDB ID: 6YEN) reveals a likely steric clash of the rigid tricycle in the AmpC_EC active site as well as active sites of other β-lactamases (Figure S5). By contrast, both tricyclic (orange) and bicyclic (yellow) forms of TAN have been crystallographically observed at the active site of the B1 MBL NDM-1 (PDB ID: 6RMF) [24]. Note, in both structures, that the terminal amine of the side chain was disordered and therefore excluded from the model; (b) observed conformation of bicyclic TAN at the AmpC_EC active site; (c) alignment of tricyclic TAN core to bicyclic TAN observed at the AmpC_EC active site; (d) putative steric clash of tricyclic TAN in the AmpC_EC active site based on the overlay in c). Note the apparently flexible parts of the side chain are not shown, but would make a clear steric clash with the active site; (e) observed conformation of bicyclic TAN at the NDM-1 active site; (f) overlay of tricyclic TAN and bicyclic TAN, both as observed at the NDM-1 active site; (g) observed conformation of tricyclic TAN at the NDM-1 active site.