k inact/K I Value Determination for Penicillin-Binding Proteins in Live Cells

Penicillin-binding proteins (PBPs) are an essential family of bacterial enzymes that are covalently inhibited by the β-lactam class of antibiotics. PBP inhibition disrupts peptidoglycan biosynthesis, which results in deficient growth and proliferation, and ultimately leads to lysis. IC50 values are often employed as descriptors of enzyme inhibition and inhibitor selectivity, but can be misleading in the study of time-dependent, covalent inhibitors. Due to this disconnect, the second-order rate constant, k inact/K I, is a more appropriate metric of covalent-inhibitor potency. Despite being the gold standard measurement of potency, k inact/K I values are typically obtained from in vitro assays, which limits assay throughput if investigating an enzyme family with multiple homologues (such as the PBPs). Therefore, we developed a whole-cell k inact/K I assay to define inhibitor potency for the PBPs in Streptococcus pneumoniae using the fluorescent, activity-based probe, Bocillin-FL. Our results align with in vitro k inact/K I data and show a comparable relationship to previously established IC50 values. These results support the validity of our in vivo k inact/K I method as a means of obtaining β-lactam potency for a suite of PBPs to enable structure–activity relationship studies.