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.

Multidomain Human Peroxidasin 1 Is a Highly Glycosylated and Stable Homotrimeric High Spin Ferric Peroxidase

“…The reduction potential of the Fe(III)/Fe(II) couple at pH 7.4 and 25°C was calculated to be Ϫ233 Ϯ 5 mV, which is more negative than that reported for bovine LPO (i.e. Ϫ176 to Ϫ190 mV) (28,34) or human MPO (5 mV) (27). In case of the construct hsPxd01-con4, the corresponding EЈ 0 value was determined to be Ϫ215 Ϯ 10 mV.…”

“…The reduction potential of the Fe(III)/Fe(II) couple of Ϫ233 Ϯ 5 mV is more negative compared with LPO (i.e. Ϫ176 to Ϫ190 mM) (28,34), EPO (Ϫ126 mV) (28), or MPO (5 mV) (27).…”

Impact of Two Novel Mutations on the Structure and Function of Human Myeloperoxidase

“…The iron ion in native MPO has six coordination sites as follows: four sites are mediated by pyrrole nitrogen atoms in the heme, and the fifth coordination is the axial His-502-iron bond. The sixth coordination site on the iron ion is vacant and serves to accept electrons and thereby participate in the electrochemistry of the iron-catalyzed oxidation (43) (Fig. 1A).…”

A Stable Bacterial Peroxidase with Novel Halogenating Activity and an Autocatalytically Linked Heme Prosthetic Group

“…This E 0 Ј value is higher than in heme b peroxidases from the peroxidasecatalase superfamily (34) but is significantly lower than in myeloperoxidase (ϩ5 mV) (18). The latter enzyme is unique due to its electron-withdrawing sulfonium ion linkage and its strongly distorted and asymmetric heme group.…”