title: Redox thermodynamics of the Fe(III)/Fe(II) couple of human myeloperoxidase in its high-spin and low-spin forms.
abstract: Myeloperoxidase (MPO) (donor, hydrogen peroxide oxidoreductase, EC 1.11.1.7) is the most abundant neutrophil enzyme and catalyzes predominantly the two-electron oxidation of ubiquitous chloride (Cl-), to generate the potent bleaching oxidant hypochlorous acid (HOCl), thus contributing to bacterial killing and inflammatory reactions of neutrophils. Here, the thermodynamics of the one-electron reduction of the ferric heme in its ferric high-spin and cyanide-bound low-spin forms were determined through spectroelectrochemical experiments. The E(o)' values for free and cyanide-bound MPO (5 and -37 mV, respectively, at 25 degrees C and pH 7.0) are significantly higher than those of other heme peroxidases. Variable-temperature experiments revealed that the enthalpic stabilization of ferric high-spin MPO is much weaker than in other heme peroxidases and is exactly compensated by the entropic change upon reduction. In contrast to those of other heme peroxidases, the stabilization of the ferric cyanide-bound MPO is also very weak and fully entropic. This peculiar behavior is discussed with respect to the MPO-typical covalent heme to protein linkages as well as to the published structures of ferric MPO and its cyanide complex and the recently published structure of lactoperoxidase as well as the physiological role of MPO in bacterial killing.
authors.affiliation: Department of Chemistry, University of Modena and Reggio Emilia, via Campi 183, 41100 Modena, Italy. gianantonio.battistuzzi@unimore.it
authors.forename: Gianantonio
authors.initials: G
authors.lastname: Battistuzzi
authors.forename: Marzia
authors.initials: M
authors.lastname: Bellei
authors.forename: Martina
authors.initials: M
authors.lastname: Zederbauer
authors.forename: Paul G
authors.initials: PG
authors.lastname: Furtmüller
authors.forename: Marco
authors.initials: M
authors.lastname: Sola
authors.forename: Christian
authors.initials: C
authors.lastname: Obinger
mesh_terms: D003486:Cyanides / Q000737:chemistry; D004578:Electron Spin Resonance Spectroscopy; D019277:Entropy; D005290:Ferric Compounds / Q000737:chemistry; D005296:Ferrous Compounds / Q000737:chemistry; D006801:Humans; D007501:Iron / Q000378:metabolism*; D007700:Kinetics; D008958:Models, Molecular; D009504:Neutrophils / Q000201:enzymology; D010084:Oxidation-Reduction; D009195:Peroxidase / Q000097:blood / Q000737:chemistry* / Q000378:metabolism*; D011487:Protein Conformation; D013053:Spectrophotometry; D013816:Thermodynamics
chemical_list: D003486:Cyanides; D005290:Ferric Compounds; D005296:Ferrous Compounds; D007501:Iron; D009195:Peroxidase