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The existence of covalent heme to protein bonds is the most striking structural feature of mammalian peroxidases, including myeloperoxidase and lactoperoxidase (LPO). These autocatalytic posttranslational modifications (PTMs) were shown to strongly influence the biophysical and biochemical properties of these oxidoreductases. Recently, we reported the occurrence of stable LPO-like counterparts with two heme to protein ester linkages in bacteria. This study focuses on the model wild-type peroxidase from the cyanobacterium Lyngbya sp. PCC 8106 (LspPOX) and the mutants D109A, E238A, and D109A/E238A that could be recombinantly produced as apoproteins in Escherichia coli, fully reconstituted to the respective heme b proteins, and posttranslationally modified by hydrogen peroxide. This for the first time allows not only a direct comparison of the catalytic properties of the heme b and PTM forms but also a study of the impact of D109 and E238 on PTM and catalysis, including Compound I formation and the two-electron reduction of Compound I by bromide, iodide, and thiocyanate. It is demonstrated that both heme to protein ester bonds can form independently and that elimination of E238, in contrast to exchange of D109, does not cause significant structural rearrangements or changes in the catalytic properties neither in heme b nor in the PTM form. The obtained findings are discussed with respect to published structural and functional data of human peroxidases.

Chlorite dismutases (Clds) are heme b containing oxidoreductases that convert chlorite to chloride and molecular oxygen. In order to elucidate the role of conserved heme cavity residues in the catalysis of this reaction comprehensive mutational and biochemical analyses of Cld from "Candidatus Nitrospira defluvii" (NdCld) were performed. Particularly, point mutations of the cavity-forming residues R173, K141, W145, W146, and E210 were performed. The effect of manipulation in 12 single and double mutants was probed by UV-vis spectroscopy, spectroelectrochemistry, pre-steady-state and steady-state kinetics, and X-ray crystallography. Resulting biochemical data are discussed with respect to the known crystal structure of wild-type NdCld and the variants R173A and R173K as well as the structures of R173E, W145V, W145F, and the R173Q/W146Y solved in this work. The findings allow a critical analysis of the role of these heme cavity residues in the reaction mechanism of chlorite degradation that is proposed to involve hypohalous acid as transient intermediate and formation of an O═O bond. The distal R173 is shown to be important (but not fully essential) for the reaction with chlorite, and, upon addition of cyanide, it acts as a proton acceptor in the formation of the resulting low-spin complex. The proximal H-bonding network including K141-E210-H160 keeps the enzyme in its ferric (E°' = -113 mV) and mainly five-coordinated high-spin state and is very susceptible to perturbation.

Reconstructing the phylogenetic relationships of the main evolutionary lines of the mammalian peroxidases lactoperoxidase and myeloperoxidase revealed the presence of novel bacterial heme peroxidase subfamilies. Here, for the first time, an ancestral bacterial heme peroxidase is shown to possess a very high bromide oxidation activity (besides conventional peroxidase activity). The recombinant protein allowed monitoring of the autocatalytic peroxide-driven formation of covalent heme to protein bonds. Thereby, the high spin ferric rhombic heme spectrum became similar to lactoperoxidase, the standard reduction potential of the Fe(III)/Fe(II) couple shifted to more positive values (-145 ± 10 mV at pH 7), and the conformational and thermal stability of the protein increased significantly. We discuss structure-function relationships of this new peroxidase in relation to its mammalian counterparts and ask for its putative physiological role.

In mammalian peroxidases the proximal histidine is in close interaction with a fully conserved asparagine which in turn is hydrogen bonded with an arginine that stabilizes the propionate substituent of pyrrol ring D in bent conformation. In order to probe the role of this rigid proximal architecture for structural integrity and catalysis of human myeloperoxidase (MPO), the variants Asn421Asp, Arg333Ala and Arg333Lys have been recombinantly expressed in HEK cell lines. The standard reduction potential of the Fe(III)/Fe(II) couple of Asn421Asp was still wild-type-like (-50mV at pH 7.0) but the spectral properties of the ferric and ferrous forms as well as of higher oxidation states showed significant differences. Additionally, rates of ligand binding and oxidation of both one- and two-electron donors were diminished. The effect of exchange of Arg333 was even more dramatic. We did not succeed in production of mutant proteins that could bind heme at the active site. The importance of this His-Asn-Arg triad in linking the heme iron with the propionate at pyrrol ring D for heme insertion and binding as well as in maintenance of the architecture of the substrate binding site(s) at the entrance to the heme cavity is discussed., (Copyright © 2011 Elsevier Inc. All rights reserved.)

Peroxidases are heme enzymes found in bacteria, fungi, plants and animals, which exploit the reduction of hydrogen peroxide to catalyze a number of oxidative reactions, involving a wide variety of organic and inorganic substrates. The catalytic cycle of heme peroxidases is based on three consecutive redox steps, involving two high-valent intermediates (Compound I and Compound II), which perform the oxidation of the substrates. Therefore, the thermodynamics and the kinetics of the catalytic cycle are influenced by the reduction potentials of three redox couples, namely Compound I/Fe3+, Compound I/Compound II and Compound II/Fe3+. In particular, the oxidative power of heme peroxidases is controlled by the (high) reduction potential of the latter two couples. Moreover, the rapid H2O2-mediated two-electron oxidation of peroxidases to Compound I requires a stable ferric state in physiological conditions, which depends on the reduction potential of the Fe3+/Fe2+ couple. The understanding of the molecular determinants of the reduction potentials of the above redox couples is crucial for the comprehension of the molecular determinants of the catalytic properties of heme peroxidases. This review provides an overview of the data available on the redox properties of Fe3+/Fe2+, Compound I/Fe3+, Compound I/Compound II and Compound II/Fe3+ couples in native and mutated heme peroxidases. The influence of the electron donor properties of the axial histidine and of the polarity of the heme environment is analyzed and the correlation between the redox properties of the heme group with the catalytic activity of this important class of metallo-enzymes is discussed., (2010 Elsevier Inc. All rights reserved.)

The reaction enthalpy and entropy for the one-electron reduction of the ferric heme in horse heart and sperm whale aquometmyoglobins (Mb) have been determined exploiting a spectroelectrochemical approach. Also investigated were the T67R, T67K, T67R/S92D and T67R/S92D Mb-H variants (the latter containing a protoheme-L: -histidine methyl ester) of sperm whale Mb, which feature peroxidase-like activity. The reduction potential (E degrees ') in all species consists of an enthalpic term which disfavors Fe(3+) reduction and a larger entropic contribution which instead selectively stabilizes the reduced form. This behavior differs from that of the heme redox enzymes and electron transport proteins investigated so far. The reduction thermodynamics in the series of sperm whale Mb variants show an almost perfect enthalpy-entropy compensation, indicating that the mutation-induced changes in DeltaH(o')(rc) and DeltaS(o')(rc) are dominated by reduction-induced solvent reorganization effects. The modest changes in E degrees ' originate from the enthalpic effects of the electrostatic interactions of the heme with the engineered charged residues. The small influence that the mutations exert on the reduction potential of myoglobin suggests that the increased peroxidase activity of the variants is not related to changes in the redox reactivity of the heme iron, but are likely related to a more favored substrate orientation within the distal heme cavity.

In human heme peroxidases the prosthetic group is covalently attached to the protein via two ester linkages between conserved glutamate and aspartate residues and modified methyl groups on pyrrole rings A and C. Here, monomeric recombinant myeloperoxidase (MPO) and the variants D94V and D94N were produced in Chinese hamster ovary cell lines. Disruption of the Asp(94) to heme ester bond decreased the one-electron reduction potential E'(0) [Fe(III)/Fe(II)] from 1 to -55 mV at pH 7.0 and 25 degrees C, whereas the kinetics of binding of low spin ligands and of compound I formation was unaffected. By contrast, in both variants rates of compound I reduction by chloride and bromide (but not iodide and thiocyanate) were substantially decreased compared with the wild-type protein. Bimolecular rates of compound II (but not compound I) reduction by ascorbate and tyrosine were slightly diminished in D94V and D94N. The presented biochemical and biophysical data suggest that the Asp(94) to heme linkage is no precondition for the autocatalytic formation of the other two covalent links found in MPO. The findings are discussed with respect to the known active site structure of MPO and its complexes with ligands.

A basic heme peroxidase has been isolated from cucumber (Cucumis sativus) peelings and characterized through electronic and (1)H NMR spectra from pH 3 to 11. The protein, as isolated, contains a high-spin ferriheme which in the low pH region is sensitive to two acid-base equilibria with apparent pK(a) values of approximately 5 and 3.6, assigned to the distal histidine and to a heme propionate, respectively. At high pH, a new low-spin species develops with an apparent pK(a) of 11, likely due to the binding of an hydroxide ion to the sixth (axial) coordination position of the Fe(III). A number of acid-base equilibria involving heme propionates and residues in the distal cavity also affect the binding of inorganic anions such as cyanide, azide, and fluoride to the ferriheme, as well as the catalytic activity. The reduction potentials of the native protein and of its cyanide derivative, determined through UV-Vis spectroelectrochemistry, result to be -0.320+/-0.015 and -0.412+/-0.010V, respectively. Overall, the reactivity of this protein parallels those of other plant peroxidases, especially horseradish peroxidase. However, some differences exist in the acid-base equilibria affecting its reactivity and in the reduction potential, likely as a result of small structural differences in the heme distal and proximal cavities.