Conformational Dynamics of Carbapenem:OXA-48 Acylenzymes AC dynamics for both imipenem and meropenem complexed with OXA-48, each in the 2 (enamine) configuration, were explored by running five 200 ns MM MD simulations for each complex. The first 50 ns were excluded from trajectory analysis to allow time for equilibration. For both carbapenems, the salt bridge between the C3 carboxylate and Arg250 was preserved during simulations, and the C7 carbonyl stayed in the oxyanion hole formed by the backbone amides of Ser70 (nucleophile) and Tyr211. The carbapenem C2 (tail) substituents sampled a range of conformations during the simulations, consistent with previous suggestions based on structural analysis. Clustering the substrate poses based on their heavy atom RMSD yielded four distinct clusters per substrate, which differ by 0.8-1.8 Å and 1.7-2.5 Å for imipenem and meropenem, respectively, from the poses in the corresponding crystal structures (Figure , Table and SI section Acylenzyme Clustering). The main deviations between cluster centroids and the crystal structure coordinates are due to the positions of the C2 tail groups, as the pyrroline ring and its substituents are anchored in place by hydrogen bonds to the oxyanion hole and the salt bridge with Arg250. However, for the crystal structures 6P97 and 6P98 there is only limited electron density beyond the sulfur atom for both imipenem and meropenem, so the deposited coordinates may not completely reliably depict the actual substrate binding poses. Additional clustering on the active site residues (explained in further detail in the SI) implies that there may be slight differences also in the positions of active site residues Lys73, Tyr157, as well as of the substrate (Figure and Table ).