Molecular mechanisms underlying the modulation of Co(II)-catalase interactions by polystyrene nanoplastics.

Polystyrene nanoplastics (PS-NPs) can interact with heavy metals and alter metal-protein interactions. However, their effects on Co(II)-catalase (CAT) interactions remain unclear. We integrated enzyme activity assays, isothermal titration calorimetry (ITC), multispectroscopic analyses, molecular docking, and molecular dynamics simulations to compare Co(II) alone with the PS-NPs-Co(II) system. Co(II) increased background-corrected apparent CAT activity at 0.5-0.8 mM, whereas PS-NPs generally attenuated this increase, significantly at 0.6 and 0.8 mM (p = 0.024 and 0.009). ITC yielded distinct apparent responses for Co(II) into CAT titration (fitted ΔH = -335 kJ mol-1) and titration of the filtrate from a directly prepared PS-NPs-Co(II) mixture into CAT (-14.9 kJ mol-1). These values reflect different titrants in 20 mM HEPES and do not isolate a single binding event. Spectroscopy showed changes in CAT secondary structure, aromatic-residue environments, and heme Soret responses, with a distinct heme response in the combined system. Simulations indicated fewer direct CAT-Co(II) contacts and greater heme-pocket solvent exposure in the PS-containing model. These findings suggest that PS-NPs may modify the local CAT-Co(II) environment and contribute to the observed activity differences under these cell-free conditions.