The Role of the Flexible L43-S54 Protein Loop in the CcrA Metallo-β-lactamase in Binding Structurally Dissimilar β-Lactam Antibiotics.

The CcrA di-Zn β-lactamase is a bacterial enzyme capable of efficiently hydrolyzing and thus disabling a diverse set of β-lactam antibiotics. Understanding the factors that contribute to the efficiency of CcrA is essential for the design of new CcrA-resistant antibiotics and enzyme inhibitors. The efficacy of CcrA has been speculated to be partially attributable to the flexible protein loop located above the active site (L43-S54), which would mold around structurally different substrates, for snag binding. Confirmation of this hypothesis about the role of the loop has been a challenge, from both an experimental and a theoretical point of view. We employed our newly developed method that combines extensive sampling of the protein structure via discrete molecular dynamics (DMD) and quantum mechanical (QM) treatment of the active site, QM/DMD, to investigate the structural role of the L43-S54 loop in binding three different β-lactam antibiotics: imipenem, ampicillin, and cephalorodine. QM/DMD sampling was followed by high level ab initio calculations for the assessment of the energy contributions to loop-substrate interactions. We show that upon binding of all three antibiotic molecules, the loop comes in direct contact with the substrates and adopts distinctly different conformations depending on the bound substrate. The loop contributes to the binding affinity of CcrA to antibiotics. The primary component of the loop-substrate interaction is hydrophobic, and nonspecific, except for cephalorodine that is capable of π-stacking with W49 via one of the two competing modes.