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PubMed1985

Anti-thyroid peroxidase antibody in patients with autoimmune thyroid disease: possible identity with anti-microsomal antibody.

Thyroid microsomal antigen and peroxidase (TPO) have a close intracellular anatomical relationship, especially in exocytotic vesicles. We considered that antibodies to microsomal antigen might react with TPO and therefore looked for the presence of antibodies against TPO in the serum of patients with autoimmune thyroid disease (AITD). TPO was prepared from Graves' thyroid glands, solubilized by n-octyl glucoside, and its activity was assayed by the guaiacol method. Control sera and sera with a positive microsomal hemagglutination test (MCHA(+) ) were assayed for their ability to precipitate TPO activity by incubation of sera with TPO and protein A. We identified MCHA(+) sera which caused precipitation of TPO activity, and the extent of precipitation was related to the amount of serum added. A significant correlation was present between this anti-peroxidase activity and microsomal antibodies titers, measured by a micro-ELISA method. Affinity columns prepared from immunoglobulins of MCHA(+) sera, coupled to Reacti-Gel (6X), bound TPO activity, whereas using control IgG the recovery in the unbound fraction was high. These data provide evidence of antibodies against thyroid peroxidase in the serum of patients with AITD and suggest a close link between microsomal antigen and thyroid peroxidase.

Antibodies↗
PubMed1984

Oestrogen dependence of lactate dehydrogenase and peroxidase in cell subpopulations of 7,12-dimethylbenz[alpha]anthracene-induced mammary tumours in the rat.

Recent investigations of certain enzymes as markers for predicting the response of breast tumours to hormonal therapy have neglected the possible differential contribution of cell subpopulation(s) within a solid tumour to enzyme activity. In this investigation, lactate dehydrogenase (LDH) and peroxidase activities in density-defined cell subpopulations from autotransplanted 7,12-dimethylbenz[alpha]anthracene-induced mammary tumours were determined. The effects of ovariectomy and subsequent oestrogen administration on the activity of these enzymes were also examined. Five cell subpopulations (cell bands) were routinely obtained from each mammary tumour. The highest LDH activity was found in cell band 4. The highest level of peroxidase activity was found in cell band 5. These two cell bands with high levels of enzyme activity consisted mainly of poorly differentiated cells. After bilateral ovariectomy, a significant (P less than 0.001) decrease in the level of LDH activity in cell bands 3, 4 and 5 was observed. The enzyme activity was reduced to 20, 2.1 and 12% of the preovariectomy levels respectively. Significant (P less than 0.05) decreases between baseline and postovariectomy peroxidase values were evident in each cell band. In the presence of oestradiol-17 beta, significant increases in the LDH activity of band 4 (P less than 0.001) and the peroxidase activity of band 5 (P less than 0.05) were observed. Our data suggest that, given the existence of multiple cell types in hormone-responsive tumour tissue, the actual cell subpopulation(s) responsible for any enzyme response may be a more precise indicator of hormone dependence.

9,10-Dimethyl-1,2-benzanthracene↗
PubMed1994

Control of superoxide dismutase, catalase and glutathione peroxidase activities in rat lymphoid organs by thyroid hormones.

This study examined the effect of experimental hyper- and hypothyroidism on the superoxide dismutase, catalase and glutathione peroxidase activities of rat lymphoid organs (mesenteric lymph nodes, spleen and thymus) and muscles (soleus and gastrocnemius-white portion) for comparison. The capacity for the generation of reducing equivalents was also investigated: activities of glucose-6-phosphate dehydrogenase (pentose-phosphate pathway) and citrate synthase (Krebs cycle). Hyperthyroidism tended to enhance lipid peroxide content in all tissues. This effect may result from (1) a high capacity for the generation of reducing equivalents in cytosol and mitochondria and (2) a reduced activity of catalase in the lymphoid organs and of glutathione peroxidase in the muscles. The process of lipid peroxidation in these tissues caused by hyperthyroidism was probably slowed down by the augmentation of CuZn- and Mn-superoxide dismutase (Mn-SOD) activities observed under this condition. Hypothyroidism tended to diminish lipid peroxidation and did not affect citrate synthase and glucose-6-phosphate dehydrogenase activities in the lymphoid organs and muscles. Low levels of thyroid hormones tended to diminish Mn-SOD and glutathione peroxidase activities. These findings show that the thyroid hormones might be able to regulate the activities of CuZn- and Mn-SOD, and catalase and glutathione peroxidase in the lymphoid organs and skeletal muscles.

Animals↗
PubMed1988

Ascorbate peroxidase in bovine retinal pigment epithelium and choroid.

Peroxidase, catalyzing hydrogen peroxide reduction concurrent with ascorbate oxidation, was demonstrated in the extract of retinal pigment epithelium and choroid. The peroxidase in the choroid, RPE, and retina are 236.1, 25.1, and 0.5 units/mg protein respectively. Ammonium sulfate fractionation and high pressure liquid chromatography showed that the peroxidase in the RPE-choroid is associated with a group of heme proteins with absorption maxima at 410 nm, and optimal activity at pH 4.5. The high peroxidase activity in the RPE-choroid explains the observation of dehydroascorbate in these tissues and indicates a possible role of this enzyme in the removal of H2O2.

Ammonium Sulfate↗
PubMed1976

Glutathione peroxidase activity and glutathione concentration in genetically dystrophic mice.

The present studies were conducted to determine whether inherited muscular dystrophy in the 129/ReJ-dy mouse was associated with differences in specific activity, substrate availability, or apparent Km of glutathione peroxidase. The results indicate that glutathione peroxidase is elevated in skeletal muscle of mice with genetic muscular dystrophy when the activity is expressed on a protein basis. This elevation precedes the development of severe paralysis since muscles from the fore legs showed increased enzyme activity as early as the more severely affected hind legs. There was no difference in glutathione peroxidase activity in tissues other than skeletal muscle. GSH concentration was elevated in muscle and normal in other tissues of dystrophic mice, showing that adequate substrate was available to the enzyme. The apparent Km for cumene hydroperoxide was also similar for muscle of normal and dystrophic mice. This report provides further evidence that mice with dystrophia muscularis have a functional glutathione peroxidase system in all tissues including skeletal muscle, and that a defect in this in vivo protective system is apparently not a contributing factor in the pathology of the disease.

Aging↗
PubMed1976

[Stabilization of polyacrylamide gel immobilized horseradish peroxidase by its covalent coupling to albumin].

Pretreatment of peroxidase by its covalent coupling to inert proteins and albumin by means of glutaraldehyde considerably increases the thermostability and specific activity of polyacrylamide gel (PAAG) immobilized peroxidase. The effects of PAAG composition on the catalytic properties of the immobilized oligomers: peroxidase-inert proteins-albumin, are studied. The oligomers immobilized in 40% PAAG (10% N,N'-methylenebisacrylamide) possess the maximal specific activity (4.5 nmol/g). The effects of oligomer composition on their catalytic activity and stability in PAAG are studied. The stability of oligomers of optimal composition (ratio of albumin/peroxidase is 2.4), incorporated into 40% PAAG, is 15 times higher as compared to that of the soluble enzyme and 250 times higher as compared to that of the enzyme incorporated into PAAG without pretreatment. A mechanism of stabilizing effect exerted by albumin on peroxidase in PAAG-immobilized oligomers, is discussed.

Acrylamides↗
PubMed2000

Improved operational stability of peroxidases by coimmobilization with glucose oxidase.

The operational stability of peroxidases was considerably enhanced by generating hydrogen peroxide in situ from glucose and oxygen. For example, the total turnover number of microperoxidase-11 in the oxidation of thioanisole was increased sevenfold compared with that obtained with continuous addition of H(2)O(2). Coimmobilization of peroxidases with glucose oxidase into polyurethane foams afforded heterogeneous biocatalysts in which the hydrogen peroxide is formed inside the polymeric matrix from glucose and oxygen. The total turnover number of chloroperoxidase in the oxidation of thioanisole and cis-2-heptene was increased to new maxima of 250. 10(3) and 10. 10(3), respectively, upon coimmobilization with glucose oxidase. Soybean peroxidase, which normally shows only classical peroxidase activity, was transformed into an oxygen-transfer catalyst when coimmobilized with glucose oxidase. The combination catalyst mediated the enantioselective oxidation of thioanisole [50% ee (S)] with 210 catalyst turnovers.

Chloride Peroxidase↗
PubMed2000

Propyl gallate inhibition of iodide and tetramethylbenzidine oxidation catalyzed by human thyroid peroxidase.

The kinetic characteristics (kcat, Km, and their ratio) for oxidation of iodide (I-) at 25 degrees C in 0.2 M acetate buffer, pH 5.2, and tetramethylbenzidine (TMB) at 20 degrees C in 0.05 M phosphate buffer, pH 6.0, with 10% DMF catalyzed by human thyroid peroxidase (HTP) and horseradish peroxidase (HRP) were determined. The catalytic activity of HRP in I- oxidation was about 20-fold higher than that of HTP. The kcat/Km ratio reflecting HTP efficiency was 35-fold higher in TMB oxidation than that in I- oxidation. Propyl gallate (PG) effectively inhibited all four peroxidase processes and its effects were characterized in terms of inhibition constants Ki and the inhibitor stoichiometric coefficient f. For both peroxidases, inhibition of I- oxidation by PG was characterized by mixed-type inhibition; Ki for HTP was 0.93 microM at 25 degrees C. However, in the case of TMB oxidation the mixed-type inhibition by PG was observed only with HTP (Ki = 3.9 microM at 20 degrees C), whereas for HRP it acted as a competitive inhibitor (Ki = 42 microM at 20 degrees C). A general scheme of inhibition of iodide peroxidation containing both enzymatic and non-enzymatic stages is proposed and discussed.

Antioxidants↗
PubMed2000

Mud bath therapy influences nitric oxide, myeloperoxidase and glutathione peroxidase serum levels in arthritic patients.

Nitric oxide (NO) has recently been proposed as an important mediator in inflammatory phases and in loss of cartilage. In inflammatory arthritis NO levels are correlated with disease activity and articular cartilage is able to produce large amounts of NO with the appropriate inducing factors such as cytokines and/or endotoxin. Neutrophils also play an important role in inflammatory reactions and the level of myeloperoxidase, a constituent of neutrophil granules, is related to the intensity of the inflammation. Because there is evidence that suggests that mud packs influence the main cytokines involved in cartilage damage, we tried to determine whether NO and myeloperoxidase are involved in the mechanisms of action of mud bath treatment. We enrolled 37 subjects and randomly assigned them to two groups: 19 patients underwent mud bath treatment (group A) while 18 patients underwent bath treatment alone. Blood samples were obtained before and after the treatment cycles to assay serum levels of NO, myeloperoxidase (MPO) and glutathione (GSH)-peroxidase. The results showed a statistically significant decrease in NO and myeloperoxidase serum values in groups A and B, while GSH-peroxidase was not significantly increase in either of the groups; no correlation was found between NO, myeloperoxidase and GSH-peroxidase serum values. Mud bath treatment can exert beneficial effects on cartilage homeostasis and inflammatory reactions, influencing NO and decreasing myeloperoxidase serum values. The increase in GSH-peroxidase was not correlated with the reduction of other biochemical markers, suggesting that mud bath treatment has different mechanisms of action.

Aged↗
PubMed1992

Molecular cloning and characterization of a gene encoding pea cytosolic ascorbate peroxidase.

A gene encoding cytosolic ascorbate peroxidase (ApxI) from pea (Pisum sativum L.) was isolated and its nucleotide sequence determined. By homologous alignment between the ApxI cDNA (Mittler, R., and Zilinskas, B. (1991) FEBS Lett. 289, 257-259) and the genomic clone, positions of introns and exons were determined. The isolated ApxI gene was found to contain 9 introns, the first of which was located within the 5'-untranslated region of the mRNA. Southern blot analysis of pea genomic DNA suggests that in pea cytosolic ascorbate peroxidase is encoded by a single copy gene. Steady state ApxI transcript levels were found to increase in response to several stresses imposed by drought, heat, and application of ethephon, abscisic acid, and the superoxide-generating agent paraquat. Increases in ascorbate peroxidase activity in response to stresses were less marked than changes observed in transcript levels; cytosolic ascorbate peroxidase protein levels measured by immunoblot analysis remained unchanged.

Amino Acid Sequence↗
PubMed1977

Steady state kinetics and binding of eukaryotic cytochromes c with yeast cytochrome c peroxidase.

1. The steady state kinetics for the oxidation of ferrocytochrome c by yeast cytochrome c peroxidase are biphasic under most conditions. The same biphasic kinetics were observed for yeast iso-1, yeast iso-2, horse, tuna, and cicada cytochromes c. On changing ionic strength, buffer anions, and pH, the apparent Km values for the initial phase (Km1) varied relatively little while the corresponding apparent maximal velocities varied over a much larger range. 2. The highest apparent Vmax1 for horse cytochrome c is attained at relatively low pH (congruent to 6.0) and low ionic strength (congruent to 0.05), while maximal activity for the yeast protein is at higher pH (congruent to 7.0) and higher ionic strength (congruent to 0.2), with some variations depending on the nature of the buffering ions. 3. Direct binding studies showed that cytochrome c binds to two sites on the peroxidase, under conditions that give biphasic kinetics. Under those ionic conditions that yield monophasic kinetics, binding occurred at only one site. At the optimal buffer concentrations for both yeast and horse cytochromes c, the KD1 and KD2 values approximate the Km1 and Km2 values. At ionic strengths below optimal, binding becomes too strong and above optimal, too weak. 4. Under ionic conditions that are optimal and give monophasic kinetics with horse cytochrome c but are suboptimal for the yeast protein, yeast cytochrome c strongly inhibits the reaction of horse cytochrome c with peroxidase, uncompetitively at one site and competitively at a second site. The appearance of the second site under monophasic conditions is interpreted as an allosteric effect of the inhibitor binding to the first site. 5. The simplest model accounting for these observations postulates two kinetically active sites on each molecule of peroxidase, a high affinity and a low affinity site, that may correspond to the free radical and the heme iron (IV) of the oxidized enzyme, respectively. Both oxidizing equivalents may be discharged at either site. Furthermore, the enzyme appears to exist as an equilibrium mixture of a high ionic strength form, EH and a low ionic strength form, EL, the former reacting optimally with yeast cytochrome c, and the latter with horse cytochrome c.

Animals↗
PubMed1989

The peroxidase-dependent activation of butylated hydroxyanisole and butylated hydroxytoluene (BHT) to reactive intermediates. Formation of BHT-quinone methide via a chemical-chemical interaction.

The food antioxidants butylated hydroxyanisole (BHA) and butylated hydroxytoluene (BHT) are shown to be metabolized to covalent binding intermediates and various other metabolites by prostaglandin H synthase and horseradish peroxidase. BHA was extensively metabolized by horseradish peroxidase (80% conversion of parent BHA into metabolites) resulting in the formation of three dimeric products. Only two of these dimers were observed in prostaglandin H synthase-catalyzed reactions. In contrast to BHA, BHT proved to be a relatively poor substrate for prostaglandin synthase and horseradish peroxidase, resulting in the formation of a small amount of polar and aqueous metabolites (23% conversion of parent BHT into metabolites). With arachidonic acid as the substrate, prostaglandin H synthase catalyzed the covalent binding of [14C]BHA and [14C]BHT to microsomal protein which was significantly inhibited by indomethacin and glutathione. The covalent binding of BHA and its metabolism to dimeric products were also inhibited by BHT. In contrast, the addition of BHA enhanced the covalent binding of BHT by 400%. Moreover, in the presence of BHA, the formation of the polar and aqueous metabolites of BHT was increased and two additional metabolites, BHT-quinone methide and stilbenequinone, were detected. The increased peroxidase-dependent oxidation of BHT in the presence of BHA is proposed to occur via the direct chemical interaction of BHA phenoxyl radical with BHT or BHT phenoxyl radical. These results suggest a potential role for phenoxyl radicals in the activation of xenobiotic chemicals to toxic metabolites.

Butylated Hydroxyanisole↗
PubMed1989

The non-flavin redox center of the streptococcal NADH peroxidase. I. Thiol reactivity and redox behavior in the presence of urea.

Unlike the 2-electron-reduced (EH2) forms of the flavoprotein disulfide reductases and mercuric reductase, the native EH2 form of the streptococcal NADH peroxidase is quite refractile toward chemical modification with thiol-specific reagents. In the presence of 1.3 M urea, however, the single thiol of the reduced enzyme reacts with phenylmercuric acetate with a t1/2 of 3 min. This modification abolishes the charge-transfer absorbance band at 540 nm and inactivates the enzyme; the latter effect is shown to be reversed with dithiothreitol. Alkylation of the streptococcal peroxidase with iodo[1-14C]acetamide under reducing conditions in the presence of 8 M guanidine hydrochloride allows the isolation of a single labeled tryptic peptide with the sequence: Gly-Asp-Phe-Ile-Ser-Phe-Leu-Ser-C*ys-Gly-Met-Gln-Leu-Tyr-Leu- Glu-Gly-Lys. This sequence is identical to that previously reported (Poole, L. B., and Claiborne, A. (1988) Biochem. Biophys. Res. Commun. 153, 261-266) for the cysteinyl peptide isolated from the NADH peroxidase labeled metabolically with [35S]cysteine. Careful examination of the physical properties of the streptococcal peroxidase in the presence of 1.3 M urea shows that, while catalytic activity and native structural features are largely retained, the relative potentials of flavin and non-flavin redox centers are dramatically affected. We propose that low concentrations of urea stabilize an intermediate state in the transition between native and denatured forms, which is responsible for the observed changes in both active-site thiol reactivity and in redox properties.

Amino Acid Sequence↗
PubMed1989

Structural characterization of cytochrome c peroxidase by resonance Raman scattering.

Resonance Raman scattering studies are reported on freshly prepared and aged ferric, ligand-free ferrous, and CO-bound ferrous cytochrome c peroxidase. The ferric form of the fresh enzyme has a heme which is penta-coordinate high spin, independent of buffer over the pH range 4.3-7, as determined by well established Raman marker lines. The aged enzyme displays a mixture of spin and coordination states, but it can be stabilized in the penta-coordinate high spin form in the presence of phosphate. These results can be accounted for by considering the size of the channel (6 A wide, 11 A long) between the distal side of the heme and the outer surface of the protein. A phosphate ion may be accommodated in this channel resulting in the stabilization of the distal heme pocket. The ferrous cytochrome c peroxidase in both the ligand-free and CO-bound states has an acidic and an alkaline form. The acidic form has the characteristic spectral features of peroxidases: a high frequency iron-histidine stretching mode (248 cm-1), a high frequency Fe-CO stretching mode (537 cm-1), and a low frequency C-O stretching mode (1922 cm-1). At alkaline pH these frequencies become similar to those of hemoglobin and myoglobin, with the corresponding modes located at 227, 510, and 1948 cm-1, respectively. We attribute the acid/alkaline transition in the ferrous forms of cytochrome c peroxidase to a rearrangement mainly of the proximal side of the heme, culminating in a change of steric interactions between the proximal histidine and the heme or of the hydrogen bonding network involving the proximal histidine. The new data presented here reconcile many inconsistencies reported in the past.

Buffers↗
PubMed1989

Stabilized isoporphyrin intermediates in the inactivation of horseradish peroxidase by alkylhydrazines.

The reaction of horseradish peroxidase with alkylhydrazines results in delta-meso-alkylation of the prosthetic heme group and enzyme inactivation (Ator, M. A., David, S. K., and Ortiz de Montellano, P. R. (1987) J. Biol. Chem. 262, 14954-14960). As reported here, enzyme inactivation is associated with the accumulation of intermediates that absorb at approximately 835 nm. The properties of these intermediates, including their collapse to give meso-alkylhemes, identify them as isoporphyrins. The t1/2 values for inactivation and formation of the isoporphyrin intermediate at 25 degrees C are, respectively, 11.6 and 12.5 min for methylhydrazine (2.0 mM), 8.7 and 7.2 min for ethylhydrazine (1.0 mM), and 30 and 25 s for phenylethylhydrazine (50 microM). The isoporphyrin intermediates are surprisingly long-lived, with half-lives (35 degrees C, pH 7.0) of 9, 28, 96, and 450 min for, respectively, the phenylethyl, methyl, n-butyl, and ethyl analogues. pH studies show that protonation of a group with pKa = 5.0-6.5 accelerates isoporphyrin decay and decreases steady state isoporphyrin accumulation. Horseradish peroxidase reconstituted with delta-meso-methylheme, unlike horseradish peroxidase with a heme that has a larger meso-substituent, is catalytically active but is more sensitive to H2O2-mediated degradation of the prosthetic group than is the native enzyme. The delta-meso-methylheme prosthetic group is converted in the reaction with H2O2 to a biliverdin-like product. The results implicate highly stabilized isoporphyrin intermediates in the inactivation of horseradish peroxidase by alkylhydrazines and indicate that inactivation by the meso-alkyl groups is due to steric interference with electron delivery to the heme edge rather than to intrinsic electronic consequences of meso-alkylation. The structural features that stabilize the cationic isoporphyrins may also be involved in stabilization of the Compound I porphyrin radical cation.

Alkylation↗
PubMedDate not supplied

[Various features of peroxidase in human saliva].

Optimal conditions are determined for isolation and purification of peroxidase (EC 1.11.1.7) from the mixed human saliva by gel-filtration and ion chromatography. Such properties as pH and temperature optima, molecular weight, Km, amino acid composition, the effect of activators and inhibitors (purified saliva peroxidases and commercial preparation of horse-radish peroxidase "Reanal", Hungary) are studied with the use of different substrates. The data obtained are necessary to optimize production processes of isolation, storage and methods of the experimental and clinical application of peroxidases of different origin.

Amino Acids↗
PubMed1988

Functional differentiation of cyclooxygenase and peroxidase activities of prostaglandin synthase by trypsin treatment. Possible location of a prosthetic heme binding site.

Treatment of prostaglandin (PG)H synthase purified from ram seminal vesicle microsomes with trypsin cleaves the 70-kDa subunits into 33- and 38-kDa fragments (Chen, Y.-N. P., Bienkowski, M. J., and Marnett, L. J. (1987) J. Biol. Chem. 262, 16892-16899). In contrast to a minimal decrease in cyclooxygenase activity, peroxidase activity declines rapidly following trypsin treatment. The time course for loss of guaiacol peroxidase activity corresponds closely to the time course for protein cleavage. The ability of trypsin-treated enzyme to support catalytic reduction of 5-phenyl-4-pentenyl-1-hydroperoxide in the presence of reducing substrates is significantly reduced. The products of metabolism of 10-hydroperoxy-8,12-octadecadienoic acid indicate that trypsin-treated enzyme catalyzes homolytic scission of the hydroperoxide bond in contrast to the heterolytic scission catalyzed by intact enzyme. Spectrophotometric titrations of hematin addition to trypsin-treated PGH synthase indicate approximately a 50% reduction in heme binding. These observations suggest that trypsin treatment of PGH synthase decreases the ability of the protein to bind prosthetic heme at a site that controls peroxidase activity. Comparison of the N-terminal sequence of the 38-kDa fragment of trypsin-treated PGH synthase to the amino acid sequence of the intact protein indicates that cleavage occurs between Arg253 and Gly254. Based on literature precedents and the results of the present investigations, we propose that the heme prosthetic group that controls the peroxidase activity of PGH synthase binds to the His residue of the sequence His250-Tyr251-Pro252-Arg253 located immediately adjacent to the trypsin cleavage site.

Animals↗
PubMed1986

Resonance Raman study on cytochrome c peroxidase and its intermediate. Presence of the Fe(IV) = O bond in compound ES and heme-linked ionization.

Resonance Raman spectra of ferrous and ferric cytochrome c peroxidase and Compound ES and their pH dependences were investigated in resonance with Soret band. The Fe(IV) = O stretching Raman line of Compound ES was assigned to a broad band around 767 cm-1, which was shifted to 727 cm-1 upon 18O substitution. The 18O-isotopic frequency shift was recognized for Compound ES derived in H218O, but not in H216O. This clearly indicated occurrence of an oxygen exchange between the Fe(IV) = O heme and bulk water. The Fe(IV) = O stretching Raman band was definitely more intense and of higher frequency in D2O than in H2O as in Compound II of horseradish peroxidase, but in contrast with this its frequency was unaltered between pH 4 and 11. The Fe(II)-histidine stretching Raman line was assigned on the basis of the frequency shift observed for 54Fe isotopic substitution. From the intensity analysis of this band, the pKa of the heme-linked ionization of ferrocytochrome c peroxidase was determined to be 7.3. The Raman spectrum of ferricytochrome c peroxidase strongly suggested that the heme is placed under an equilibrium between the 5- and 6-coordinate high-spin structures. At neutral pH it is biased to the 5-coordinate structure, but at the acidic side of the transition of pKa = 5.5 the 6-coordinate heme becomes dominant. F- was bound to the heme iron at pH 6, but Cl- was bound only at acidic pH. Acidification by HNO3, H2SO4, CH3COOH, HBr, or HI resulted in somewhat different populations of the 5- and 6-coordinate forms when they were compared at pH 4.3. Accordingly, it is inferred that a water molecule which is suggested to occupy the sixth coordination position of the heme iron is not coordinated to the heme iron at pH 6 but that protonation of the pKa = 5.5 residue induces an appreciable structural change, allowing the coordination of the water molecule to the heme iron.

Computers↗