Effects of 2,4-substituents of deuteropheme upon redox potentials of horseradish peroxidases

Formation of porphyrin pi-cation radical in myoglobin. A study on one electron oxidation products of nickel (II)-substituted hemoproteins

Nickel (II)-substituted myoglobin (Mb), hemoglobin (Hb) and horseradish peroxidase (HRP) were oxidized with iridate to examine whether porphyrin pi-cation radical is formed or not in these hemoproteins. It was found that Ni (II)-porphyrin pi-cation radical is formed in all of these hemoproteins as confirmed by UV-visible and ESR spectra, although the porphyrin pi-cation radical in Mb and Hb was less stable than in HRP. These results are discussed in relation to the different features of higher oxidation states of native Mb and HRP.

Heme-linked ionizations of myeloperoxidase detected by Raman difference spectroscopy. A comparison with plant and yeast peroxidases

The pH-dependence of the oxidation state marker line v4 of human leucocyte myeloperoxidase is determined in the absence of chloride using Raman difference spectroscopy (RDS). A transition in the frequency of v4 with pK of 4.2 +/- 0.3 is found. The pK compares favorably with that previously determined by spectrophotometric titration and kinetic studies. The shift in v4 across the transition is -1.3 cm-1. The shift in v4 and other Raman marker lines indicates enhanced pi charge in the chlorin ring below the transition. The low frequencies of the oxidation state marker lines indicate that a structural change occurs near the chromophore, which results in the formation of a more pi-charge donating protein environment for the chlorin ring at low pH. The Raman results are discussed in terms of a proposed catalytic control mechanism based on charge stabilization of the energy of ring charge-depleted ferryl intermediates of the reaction with peroxide. The myeloperoxidase findings are compared with similar RDS results for ferrous horseradish peroxidase and ferric cytochrome c peroxidase.

Structural characterization of lactoperoxidase in the heme environment by proton NMR spectroscopy

The heme environmental structures of lactoperoxidase (LP) have been studied by the use of hyperfine-shifted proton NMR and optical absorption spectra. The NMR spectra of the enzyme in native and cyanide forms in H2O indicated that the fifth ligand of the heme iron is the histidyl imidazole with an anionic character and that the sixth coordination site is possibly vacant. These structural characteristics are quite similar to those of horseradish peroxidase (HRP), suggesting that these may be prerequisite to peroxidase activity. The pH dependences of the spectra of LP in cyanide and azide forms showed the presence of two ionizable groups with pK values of 6 and 7.4 in the heme vicinity, which is consistent with the kinetic results. The group with pK = 7.4 is associated with azide binding to LP in a slow NMR exchange limit, which is in contrast to the fast entry of azide to HRP.

Resonance Raman evidence for oxygen exchange between the FeIV = O heme and bulk water during enzymic catalysis of horseradish peroxidase and its relation with the heme-linked ionization

Raman spectroscopic studies of compound II of horseradish peroxidase show that the oxygen atom in the FeIV = O group of the heme is rapidly exchanged in H2O at pH 7.0 but not in an alkaline solution (pH 11.0). This conclusion is based on studies of shift in the FeIV = O stretching mode of compound II in H2(18)O; further studies show that the FeIV = O heme is hydrogen-bonded to an amino acid residue of the protein in neutral solutions but not in the alkaline solution. Deprotonation of this residue takes place with the midpoint pH at 8.8 and accordingly corresponds to the so-called heme-linked ionization. It is concluded that this hydrogen-bonded proton plays an important part in the oxygen exchange mechanism. From this it seems clear that this hydrogen-bonded proton has an essential role in the acid/base catalysis of this enzyme and that alkaline deactivation of this enzyme can be attributed to the lack of a hydrogen-bonded proton at high pH.

Spin and electron distributions in heme-cyanide models and hemeproteins

Proton NMR spectra of low-spin Fe(III) cyanoprotoheme as prosthetic group in a number of proteins are presented. The diagonally positioned 1-, 5- and 3-, 8-methyl groups obey shifts proportional to the Fe(III)/(II) reduction potential Em7, which indicates a pseudo-contact interaction. The correlation with Em7 is understandable if one postulates an enhanced rhombic distortion, dominating the Fe-methyl dipolar interactions. Hartree-Fock-Slater quantum chemical calculations show no significant changes of spin density as a function of the Fe-L5 distance, except at the iron atom and predominantly in the 3dxz and 3dyz orbitals. 4p orbitals, on the other hand, uphold most of the changes of electron density. We also observe a principal difference in the amino acid sequences in the heme-accommodating pocket of oxygen carriers and two-electron transmitters.

Ligational effects on reduction of myoglobin and horseradish peroxidase by inorganic reagents

Previous studies of the reduction of metmyoglobin and adducts by dithionite have been extended to horseradish peroxidase and its complexes. In addition, the reduction of metmyoglobin, horseradish peroxidase and adducts by a much bulkier reactant, cobalt(II) sepulchrate has been studied. Similar patterns of kinetic behavior were observed, namely, direct reduction of cyanide and imidazole adducts of the iron(III) proteins and indirect (via dissociation) reduction of the fluoride adduct. In the reduction of horseradish ferriperoxidase by cobalt(II) sepulchrate, three steps are observed and the spectral properties of the intermediate(s) and their kinetic behavior delineated. The final product is ferroperoxidase confirmed by spectral properties and its behavior on oxygenation. Reduction of cytochrome c(III) and Hipip by cobalt(II) sepulchrate appears to be a uniphasic reaction and second-order rate constants have been determined.

A kinetic study of the binding of carbon monoxide to ferrous chloroperoxidase

The binding of carbon monoxide to ferrous chloroperoxidase in the pH range 4-6.5 is influenced by a titratable group on the enzyme having a pKA of 5.5 +/- 0.2 at 20 degrees C. The basic form of the enzyme reacts much faster with carbon monoxide than does the protonated form of the enzyme. The delta H degrees for the ionization of the functional group in the enzyme involved in carbon monoxide binding is about 8 kcal mol-1, and the delta S degrees is approximately 1 cal mol-1 K-1. These pKA and delta H degrees values suggest that this functional group is an imidazole ring associated with a histidine residue situated at the active site of the enzyme. The rates of the reaction for the formation and dissociation of the complex suggest that this histidine residue is not directly liganded to the iron atom of the heme prosthetic group. The relatively good agreement between the various kinetic approaches with several methods of experimentation, data collection, and data analysis lends strength to a proposed model in which the histidine occupies a distal site close to the sixth axial ligand position of the heme iron atom.

Kinetic analysis of the acid-alkaline conversion of horseradish peroxidases

The nature of the acid-alkaline conversion of horseradish peroxidases was studied by measuring four rate constants in reactions, E + H+ (k1) in equilibrium (k2) EH+ and E + H2O (k3) in equilibrium (k4) EH+ + OH-, where EH+ and E denote the acid and alkaline forms of the enzymes. The values of k1, (k2 + k3), and k4 were obtained by measuring the relaxation rates of the acid leads to alkaline and alkaline leads to acid conversions by means of th pH jump method with a stopped-flow apparatus. The value of k3 could also be obtained by measuring the rate of reactions between hydrogen peroxide and peroxidases at alkaline pH. The measurements were conducted with four peroxidases having different pKa values: peroxidase A )pKa = 9.3), peroxidase C (pKa = 11.1), diacetyldeuteroperoxidase A (pKa = 7.7), and diacetyldeuteroperoxidase C (pKa = 9.1). The value of k1 was about 10(10) M-1 s-1 in the reaction of the four enzymes while k4 was quite different between the enzymes. The pKa was determined by k3 and k4 for the natural peroxidases and by k1 and k2 for the diacetyldeuteroperoxidases. The mechanism of the acid-alkaline conversion was discussed in comparison with that of metmyoglobin.

Heme-linked proton dissociation of carbon monoxide complexes of myoglobin and peroxidase

It was found from spectrophotometric titration and proton balance measurement that the pKa value of a heme-linked protonation group of horseradish ferro-peroxidase C (donor:H2O2 oxidoreductase, EC 1.11.1.7) shifted from 7.25 to 8.25 upon combination with CO. The spectrophotometric titration experiment with myoglobin also revealed the presence of a heme-linked protonation group, the pKa value being 5.57 in myoglobin and 5.67 in the CO-myoglobin complex. It was concluded that the distinct shift of the pKa value in the case of peroxidase was attributable to the presence of a hydrogen bond between the sixth ligand and the distal base. The difference in the strength of such hydrogen bonding between peroxidase and myoglobin was discussed.

Comparison of function of the distal base between myoglobin and peroxidase

The heme-linked protonation of a ferrous horseradish peroxidase is assigned to a distal amino acid residue. The conclusion is drawn from analyses of reactions that involve the protonation. Unfortunately it is difficult to apply it to the myoglobin case because no reaction has been found which is coupled with the protonation of the distal histidine. It is therefore of special interest to note that the pK value of 5.7 has been assigned to the distal histidine of metmyoglobin from binding kinetics with ligands20). It is well known that the reaction with hydrogen peroxide is quite different for the two types of hemoproteins. The distal base may be associated with the stabilization of the primary compound with hydrogen peroxide and also another amino acid residue may serve as a nucleophile for the stabilization of the pi-cation radical of porphyrin in the case if peroxidases. Numerous papers have dealt with heme substitution, heme linked protonation and reactions of hemoproteins related to the present subject. In this short paper, however, the discussion has mostly centered around the data obtained recently in our laboratory.