The sesquiterpene lactone Calein C irreversibly inhibits AhpC: a 2-Cys peroxiredoxin from E. coli

Abstract The increasing threat of bacterial strains resistant to antibiotics highlights the urgent need for new therapeutic strategies. One promising approach involves targeting bacterial antioxidant enzymes, among them AhpC, a 2-Cys peroxiredoxin determined as virulence factor in different bacterial species. This enzyme uses a reactive cysteine residue to neutralize harmful hydroperoxides produced by immune cells or antibiotics, protecting bacteria from oxidative stress. Despite their importance to bacterial survival, the characterization of AhpC inhibitors remains limited. In this study, we investigated calein C (CalC), a sesquiterpene lactone isolated from the Brazilian plant Calea pinnatifida. Molecular docking simulations indicated that CalC binds stably to the active site of Escherichia coli AhpC (EcAhpC). Biochemical assays confirmed that CalC is a potent inhibitor of EcAhpC (IC50 ~ 1.5 μM). In cell-based approaches, CalC alone effectively inhibited the growth of Gram-positive bacteria but showed no intrinsic activity against Gram-negative strains. However, when combined with other antibacterial agents, gentamicin for Gram-positive bacteria and polymyxin B nonapeptide (PMBN) for Gram-negative strains, CalC potentiated antibacterials action, enhancing the growth inhibition. Our findings identify CalC as the first sesquiterpene lactone known to inhibit bacterial peroxiredoxin AhpC, highlighting its potential as an antibacterial agent or adjuvant to enhance the efficacy of existing antibacterials. Key points • First characterization of a sesquiterpene lactone capable of inhibiting a bacterial 2-Cys Prx • CalC enhances the growth inhibition activity of antibacterial agents • AhpC represents a promising target for overcoming antibiotic resistance Graphical abstract