Can Molecular Dynamics and QM/MM Solve the Penicillin Binding Protein Protonation Puzzle?

Benzylpenicillin, a member of the β-lactam antibiotic class, has been widely used to combat bacterial infections since 1947. The general mechanism is well-known: a serine protease enzyme (i.e., DD-peptidase) forms a long lasting intermediate with the lactam ring of the antibiotic known as acylation, effectively preventing biosynthesis of the bacterial cell wall. Despite this overall mechanistic understanding, many details of binding and catalysis are unclear. Specifically, there is ongoing debate about active site protonation states and the role of general acids/bases in the reaction. Herein, a unique combination of MD simulations, QM/MM minimizations, and QM/MM orbital analyses is combined with systematic variation of active site residue protonation states. Critical interactions that maximize the stability of the bound inhibitor are examined and used as metrics. This approach was validated by examining cefoxitin interactions in the CTX-M β-lactamase from E. coli and compared to an ultra high-resolution (...