Influence of the covalent heme-protein bonds on the redox thermodynamics of human myeloperoxidase.

Myeloperoxidase (MPO) is the most abundant neutrophil enzyme and catalyzes predominantly the two-electron oxidation of ubiquitous chloride to generate the potent bleaching hypochlorous acid, thus contributing to pathogen killing as well as inflammatory diseases. Its catalytic properties are closely related with unique posttranslational modifications of its prosthetic group. In MPO, modified heme b is covalently bound to the protein via two ester linkages and one sulfonium ion linkage with a strong impact on its (electronic) structure and biophysical and chemical properties. Here, the thermodynamics of the one-electron reduction of the ferric heme in wild-type recombinant MPO and variants with disrupted heme–protein bonds (M243V, E242Q, and D94V) have been investigated by thin-layer spectroelectrochemistry. It turns out that neither the oligomeric structure nor the N-terminal extension in recombinant MPO modifies the peculiar positive reduction potential (E°′ = 0.001 V at 25 °C and pH 7.0) or the enthalpy ...