Shirley, J. D.; Nauta, K. M.; Gillingham, J. R.; Diwakar, S.; Carlson, E. E.

Penicillin-binding proteins (PBPs) are an essential family of bacterial enzymes that are inhibited by the {beta}-lactam class of antibiotics. PBP inhibition disrupts cell wall 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, irreversible inhibitors. Due to this disconnect, the second order rate constant kinact/KI is a more appropriate metric of covalent inhibitor potency. Despite being the gold standard measurement of potency, kinact/KI values are typically obtained from in vitro assays, which limits assay throughput if investigating an enzyme family with multiple homologs (such as the PBPs). Therefore, we developed a whole-cell kinact/KI 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 kinact/KI data and show a comparable relationship to previously established IC50 values. These results support the validity of our in vivo kinact/KI method as a means of obtaining a full picture of {beta}-lactam potency for a suite of PBPs.