Characterization of carbon monoxide photodissociation from Fe LPO with photoacoustic calorimetry

Lactoperoxidase belongs to a family of mammalian peroxidases that catalyze the oxidation of halides and small organic molecules in the presence of H O. We have used photoacoustic calorimetry to characterize thermodynamic parameters associated with ligand dissociation from bovine milk lactoperoxidase. Upon CO photorelease, a prompt (τ < 50 ns) exothermic volume contraction (Δ = -20 ± 7 kcal mol and ΔV = -2 ± 1 mL mol) was measured at pH 7.0 and 4.0, whereas an endothermic expansion (ΔH = 30 ± 13 kcal mol and ΔV = 9 ± 2 mL mol) was observed at pH 10.0 and 7.0 in the presence of 500 mM NaCl. We attribute the observed volume and enthalpy changes to electrostriction arising from changes in the charge distribution associated with a reorganization of the heme binding pocket upon ligand dissociation. It is likely that cleavage of the Fe-CO bond is accompanied by distortion of a salt bridge between Arg557 and the heme propionate group, resulting in the observed electrostriction due to changes in charge distribution. © 2006 American Chemical Society.