Steric Control of Competitive Versus Noncompetitive Lactoperoxidase Inhibition by Amide‐Linked Iridium(III) Complexes

ABSTRACT

Lactoperoxidase (LPO, EC 1.11.1.7), a heme metalloenzyme central to innate antimicrobial defence and implicated in mammary carcinogenesis, remains an underexplored target for metal‐based inhibitors. We evaluated six cyclometalated iridium(III) complexes bearing 2,2′‐bipyridine‐4,4′‐dicarboxamide ancillary ligands with varied N‐substituents as LPO inhibitors. Kinetic analysis using LPO purified from bovine milk revealed nanomolar inhibition (IC ₅₀  = 0.64–3.12 nM; Kᵢ  = 0.05 ± 0.01–5.36 ± 1.88 nM), representing an approximately 10 ⁷ ‐fold potency advantage over the benchmark sulfonamide acetazolamide. Lineweaver–Burk analysis showed substituent‐dependent inhibition: complexes 1, 2, and 4 were competitive, whereas 3, 5, and 6 were non‐competitive, indicating that peripheral steric features influence the inhibition mechanism. Density functional theory calculations showed smaller HOMO–LUMO gaps and higher electrophilicity indices than acetazolamide, although no single descriptor predicted activity within the series. Molecular docking supported binding within the distal heme cavity, with representative interactions including π–π stacking and hydrogen bonding involving Gln‐105 and His‐109. All complexes reduced MDA‐MB‐231 cell viability; among those with reliably determined IC ₅₀ values, complex 4 showed the greatest cytotoxicity (IC ₅₀  = 18.28 µM). These findings identify amide‐linked Ir(III) complexes as highly potent LPO inhibitors with substituent‐dependent inhibition modes.