2 research outputs found
Kinetics and thermodynamics of halide and nitrite oxidation by mammalian heme peroxidases
No full textThe human heme peroxidases myeloperoxidase (MPO),
eosinophil peroxidase (EPO) and lactoperoxidase (LPO) are
able to oxidise (pseudo)halides and nitrite to reactive species
that participate in host defence against foreign microorganisms
as well as in immunomodulation and tissue degradation
in certain pathologies. The heme in EPO and LPO is covalently
linked to the apoprotein by two ester bonds,
whereas in MPO it is additionally linked by a unique sulfonium
ion bond to a methionine residue. As a consequence,
the prosthetic group in MPO is significantly distorted from a
planar conformation. These structural differences are reflected
by distinct spectral and redox properties as well as
reactivities toward chloride, bromide, iodide, thiocyanate and nitrite, which function as endogenous two- and one-electron
donors for these enzymes in vivo. Standard reduction
potentials at pH 7 have been determined for all redox couples
involved in the halogenation and peroxidase cycle of
MPO and LPO and partially of EPO. A detailed thermodynamic
analysis of the formation of reactive halide species
by MPO and EPO was also performed. Thus, for the first
time, a comprehensive analysis of reactions catalysed by human
heme peroxidases is presented that allows a better understanding
of their role in physiological and pathophysiological
processes
Disruption of the aspartate to heme ester linkage in human myeloperoxidase: Impact on ligand binding, redox chemistry and interconversion of redox intermediates
No full textIn human heme peroxidases the prosthetic group is covalentlyattached to the protein via two ester linkages between conservedglutamate and aspartate residues and modified methyl groupson pyrrole rings A and C. Here, monomeric recombinantmyeloperoxidase (MPO) and the variants D94V and D94N wereproduced in Chinese hamster ovary cell lines. Disruption of theAsp94 to heme ester bond decreased the one-electron reductionpotential E0 [Fe(III)/Fe(II)] from 1 to 55 mV at pH 7.0 and25 °C, whereas the kinetics of binding of low spin ligands and ofcompound I formation was unaffected. By contrast, in both variantsrates of compound I reduction by chloride and bromide(but not iodide and thiocyanate) were substantially decreasedcompared with the wild-type protein. Bimolecular rates of compoundII (but not compound I) reduction by ascorbate and tyrosinewere slightly diminished in D94V and D94N. The presentedbiochemical and biophysical data suggest that the Asp94 to hemelinkage is no precondition for the autocatalytic formation of theother two covalent links found in MPO. The findings are discussedwith respect to the known active site structure of MPOand its complexes with ligands
