Antibiotic Binding to Monozinc CphA β-Lactamase from <i>Aeromonas </i><i>h</i><i>ydropila</i>:  Quantum Mechanical/Molecular Mechanical and Density Functional Theory Studies

The active-site dynamics of apo CphA β-lactamase from <i>Aeromonas </i><i>hydropila</i> and its complex with a β-lactam antibiotic molecule (biapenem) are simulated using a quantum mechanical/molecular mechanical (QM/MM) method and density functional theory (DFT). The quantum region in the QM/MM simulations, which includes the Zn(II) ion and its ligands, the antibiotic molecule, the catalytic water, and an active-site histidine residue, was treated using the self-consistent charge density functional tight binding (SCC-DFTB) model. Biapenem is docked at the active site unambiguously, based on a recent X-ray structure of an enzyme−intermediate complex. The substrate is found to form the fourth ligand of the zinc ion with its 3-carboxylate oxygen and to hydrogen bond with several active-site residues. The stability of the metal−ligand bonds and the hydrogen-bond network is confirmed by 500 ps molecular dynamics simulations of both the apo enzyme and the substrate−enzyme complex. The structure and dynamics of the substrate−enzyme complex provide valuable insights into the mode of catalysis in such enzymes that is central to the bacterial resistance to β-lactam antibiotics.