Hemes and hemoproteins. 3. The reaction of microperoxidase-8 with cyanide: comparison with aquocobalamin and hemoproteins

Experimental and Computational Insight into the Mechanism of NO Binding to Ferric Microperoxidase. The Likely Role of Tautomerization to Account for the pH Dependence

According to the current paradigm, the metal-hydroxo bond in a six-coordinate porphyrin complex is believed to be significantly less reactive in ligand substitution than the analogous metal-aqua bond, due to a much higher strength of the former bond. Here, we report kinetic studies for nitric oxide (NO) binding to a heme-protein model, acetylated microperoxidase-11 (AcMP-11), that challenge this paradigm. In the studied pH range 7.4-12.6, ferric AcMP-11 exists in three acid-base forms, assigned in the literature as [(AcMP-11)Fe^III(H₂O)(HisH)] (1), [(AcMP-11)Fe^III(OH)(HisH)] (2), and [(AcMP-11)Fe^III(OH)(His^-)] (3). From the pH dependence of the second-order rate constant for NO binding (k_on), we determined individual rate constants characterizing forms 1-3, revealing only a ca. 10-fold decrease in the NO binding rate on going from 1 (k_on⁽¹⁾ = 3.8 × 10⁶ M^-1 s^-1) to 2 (k_on⁽²⁾ = 4.0 × 10⁵ M^-1 s^-1) and the inertness of 3. These findings lead to the abandonment of the dissociatively activated mechanism, in which the reaction rate can be directly correlated with the Fe-OH bond energy, as the mechanistic explanation for the process with regard to 2. The reactivity of 2 is accounted for through the existence of a tautomeric equilibrium between the major [(AcMP-11)Fe^III(OH)(HisH)] (2a) and minor [(AcMP-11)Fe^III(H₂O)(His^-)] (2b) species, of which the second one is assigned as the NO binding target due to its labile Fe-OH₂ bond. The proposed mechanism is further substantiated by quantum-chemical calculations, which confirmed both the significant labilization of the Fe-OH₂ bond in the [(AcMP-11)Fe^III(H₂O)(His^-)] tautomer and the feasibility of the tautomer formation, especially after introducing empirical corrections to the computed relative acidities of the H₂O and HisH ligands based on the experimental pK_a values. It is shown that the "effective lability" of the axial ligand (OH^-/H₂O) in 2 may be comparable to the lability of the H₂O ligand in 1.

Enhancing microperoxidase activity and selectivity: immobilization in metal-organic frameworks

Microperoxidases 8 (MP8) and 11 (MP11) are heme-containing peptides obtained by the proteolytic digestion of Cytochrome c. They act as mini-enzymes that combine both peroxidase-like and Cytochrome P450-like activities that may be useful in the synthesis of fine chemicals or in the degradation of environmental pollutants. However, their use is limited by their instability in solution due to (i) the bleaching of the heme in the presence of an excess of H2O2, (ii) the decoordination of the distal histidine ligand of the iron under acidic conditions and, (iii) their tendency to aggregate in aqueous alkaline solutions, even at low concentrations. Additionally, both MP8 and MP11 show relatively low selectivity, due to the lack of control of the substrates by a specific catalytic pocket on the distal face of the heme. Both stability and selectivity issues can be effectively addressed by immobilization of microperoxidases in solid matrices, which can also lead to their possible recycling from the reaction medium. Considering their relatively small size, the pore inclusion of MPs into Metal-Organic Frameworks appeared to be more adequate compared to other immobilization methods that have been widely investigated for decades. The present minireview describes the catalytic activities of MP8 and MP11, their limitations, and various results describing their immobilization into MOFs which led to MP11- or MP8@MOF hybrid materials that display good activity in the oxidation of dyes and phenol derivatives, with remarkable recyclability due to the stabilization of the MPs inside the MOF cavities. An example of selective oxidation of dyes according to their charge by MP8@MOF hybrid materials is also highlighted.

Reactivity of inorganic sulfide species toward a heme protein model

The reactivity of inorganic sulfide species toward heme peptides was explored under biorelevant conditions in order to unravel the molecular details of the reactivity of the endogenous hydrogen sulfide toward heme proteins. Unlike ferric porphyrinates, which are reduced by inorganic sulfide, some heme proteins can form stable Fe(III)-sulfide adducts. To isolate the protein factors ruling the redox chemistry, we used as a system model, the undecapeptide microperoxidase (MP11), a heme peptide derived from cytochrome c proteolysis that retains the proximal histidine bound to the Fe(III) atom. Upon addition of gaseous hydrogen sulfide (H2S) at pH 6.8, the UV-vis spectra of MP11 closely resembled those of the low-spin ferric hydroxo complex (only attained at an alkaline pH) and cysteine or alkylthiol derivatives, suggesting that the Fe(III) reduction was prevented. The low-frequency region of the resonance Raman spectrum revealed the presence of an Fe(III)-S band at 366 cm(-1) and the general features of a low-spin hexacoordinated heme. Anhydrous sodium sulfide (Na2S) was the source of sulfide of choice for the kinetic evaluation of the process. Theoretical calculations showed no distal stabilization mechanisms for bound sulfide species in MP11, highlighting a key role of the proximal histidine for the stabilization of the Fe(III)-S adducts of heme compounds devoid of distal counterparts, which is significant with regard to the biochemical reactivity of endogenous hydrogen sulfide.

Metalloporphyrin Co(III)TMPyP ameliorates acute, sublethal cyanide toxicity in mice

The formation of Co(III)TMPyP(CN)(2) at pH 7.4 has been shown to be completely cooperative (α(H) = 2) with an association constant of 2.1 (±0.2) × 10(11). The kinetics were investigated by stopped-flow spectrophotometry and revealed a complicated net reaction exhibiting 4 phases at pH 7.4 under conditions where cyanide was in excess. The data suggest molecular HCN (rather than CN(-)) to be the attacking nucleophile around neutrality. The two slower phases do not seem to be present when cyanide is not in excess, and the other two phases have rates comparable to that observed for cobalamin, a known effective cyanide scavenger. Addition of bovine serum albumin (BSA) did not affect the cooperativity of cyanide binding to Co(III)TMPyP, only lowered the equilibrium constant slightly to 1.2 (±0.2) × 10(11) and had an insignificant effect on the observed rate. A sublethal mouse model was used to assess the effectiveness of Co(III)TMPyP as a potential cyanide antidote. The administration of Co(III)TMPyP to sodium cyanide intoxicated mice resulted in the time required for the surviving mice to right themselves from a supine position being significantly decreased (9 ± 2 min) compared to that of the controls (33 ± 2 min). All observations were consistent with the demonstrated antidotal activity of Co(III)TMPyP operating through a cyanide-binding (i.e., scavenging) mechanism.

Insights into porphyrin chemistry provided by the microperoxidases, the haempeptides derived from cytochrome c

The water-soluble haem-containing peptides obtained by proteolytic digestion of cytochrome c, the microperoxidases, have been used to explore aspects of the chemistry of iron porphyrins, and as mimics for some reactions catalysed by the haemoproteins, including the reactions catalysed by the peroxidases and the cytochromes P450. The preparation of the microperoxidases, their physical and chemical properties including their electronic structure, the kinetics and thermodynamics of their reactions with ligands, electrochemical studies and examples of their uses as haemoproteins mimics, is reviewed.

Peroxidase Activity of the Hemeoctapeptide N-Acetylmicroperoxidase-8

The pH dependence of the peroxidase activity (guaiacol assay) of the ferric hemeoctapeptide N-acetylmicroperoxidase-8 (N-AcMP8) was studied under conditions where formation of the Compound I analogue of the peroxidase enzymes is rate limiting. The pH profile of the reaction rate is consistent with a mechanism where both H2O2 and HO2- can displace H2O coordinated trans to neutral His but where the hydroxo complex of the hemepeptide (OH- trans to His) is kinetically inert. At pH > 11, where the proximal His ligand of Fe(III) ionizes to form a histidinate, the hydroxo complex (OH- trans to His-) becomes kinetically labile. A comparison of DeltaH(double dagger) and DeltaS(double dagger) values for the reaction of H2O2 and HO2-, obtained from the temperature dependence of the rate constants, with values for CN- and cysteine reported previously, shows that the activation parameters depend on the identity of the incoming ligand. This suggests that ligand substitution at Fe(III) in N-AcMP8 proceeds through an interchange mechanism.

A new microperoxidase from Marinobacter hydrocarbonoclasticus

The preparation and characterization of a new microperoxidase obtained from proteinase K-treated cytochrome c(552) from Marinobacter hydrocarbonoclasticus (previously known as Pseudomonas nautica) are presented. This microperoxidase (MMP-5) has novel structural properties relative to previously reported microperoxidases, as the two intervening amino acid (X) residues within the consensual CXXCH c-type heme binding motif are missing, yielding a heme-pentapeptide with increased solubility in aqueous solvents and a 1-2 order of magnitude higher stability of the monomeric state relative to canonical microperoxidases. The electronic spectra in the near-UV and visible regions have been studied as a function of MMP-5 concentration and pH. The spectroscopic properties of MMP-5 are typical of microperoxidases with high-spin hexa- or pentacoordinate heme species dominant in the 1-8 pH range and low-spin states prevailing at higher pH values. In the presence of hydrogen peroxide, MMP-5 displays peroxidatic activities towards several compounds.

New activities of a catalytic antibody with a peroxidase activity

In order to estimate the size of the cavity remaining around the heme of the 3A3-microperoxidase 8 (MP8) hemoabzyme, the formation of 3A3-MP8-Fe(II)-nitrosoalkane complexes upon oxidation of N-monosubstituted hydroxylamines was examined. This constituted a new reaction for hemoabzymes and is the first example of fully characterized Fe(II)-metabolite complexes of antibody-porphyrin. Also, via a comparison of the reactions with N-substituted hydroxylamines of various size and hydrophobicity, antibody 3A3 was confirmed to bring about a partial steric hindrance on the distal face of MP8. Subsequently, the influence of the antibody on the stereoselectivity of the S-oxidation of sulfides was examined. Our results showed that MP8 alone and the antibody-MP8 complex catalyze the oxidation of thioanisole by H(2)O(2) and tert-butyl hydroperoxide, following a peroxidase-like two-step oxygen-transfer mechanism involving a radical-cation intermediate. The best system, associating H(2)O(2) as oxidant and 3A3-MP8 as a catalyst, in the presence of 5% tert-butyl alcohol, led to the stereoselective S-oxidation of thioanisole with a 45% enantiomeric excess in favour of the R isomer. This constitutes the highest enantiomeric excess reported to date for the oxidation of sulfides catalyzed by hemoabzymes.

Regioselective nitration of phenol induced by catalytic antibodies

Catalytic antibodies with a metalloporphyrin cofactor represent a new generation of biocatalysts tailored for selective oxidations. Thus monoclonal antibodies, 3A3, were raised against microperoxidase 8 (MP8), and the corresponding 3A3-MP8 complexes were shown previously to have a high peroxidase activity. This paper shows that those complexes also catalyzed efficiently the nitration of phenol into 2- and 4-nitrophenol by NO2- in the presence of H2O2. pH dependence studies suggested that no amino acid from the antibody protein participated in the heterolytic cleavage of the O-O bond of H2O2. The inhibition of the reaction by cyanide and radical scavengers suggested a MP8-mediated peroxidase-like mechanism, involving the reduction of high-valent iron-oxo species by NO2- and phenol producing, respectively, NO2* and phenoxy radicals, which then reacted to give nitrophenols. Finally, the antibody protein appears to have two major roles: (i) it protects MP8 toward oxidative degradations and (ii) it induces a regioselectivity of the reaction toward the formation of 2-nitrophenol.

Microperoxidase 8 catalyzed nitration of phenol by nitrogen dioxide radicals

Microperoxidase 8 (MP8) is a heme octapeptide obtained by hydrolytic digestion of horse heart cytochrome c. At pH below 9, the heme iron is axially coordinated to the imidazole side chain of His18 and to a water molecule. Replacement of this weak ligand by H2O2 allows the formation of high-valent iron-oxo species which are responsible for both peroxidase-like and cytochrome P450-like activities of MP8. This paper shows that MP8 is able to catalyze the nitration of phenol by nitrite. The reaction requires H2O2 and is inhibited by ligands having a high affinity for the iron, catalase and radical scavengers. This suggests that the nitrating species could be NO2* radicals formed by the oxidation of nitrite by high-valent iron-oxo species. This new activity of MP8 opens a new access to nitro-aromatic compounds under mild conditions and validates the use of this minienzyme to mimick heme peroxidases, especially in the reactions of NO-derived species with biomolecules under oxidative stress conditions.

Microperoxidase/H2O2-catalyzed aromatic hydroxylation proceeds by a cytochrome-P-450-type oxygen-transfer reaction mechanism

The mechanism of aromatic hydroxylation of aniline and phenol derivatives in a H2O2-driven microperoxidase-8(MP8)-catalyzed reaction was investigated. It was shown that the reaction was not inhibited by the addition of scavengers of superoxide anion or hydroxyl radicals, which demonstrates that the reaction mechanism differs from that of the aromatic hydroxylation catalyzed by a horseradish peroxidase/ dihydroxyfumarate system. Additional experiments with 18O-labelled H2 18O2 demonstrated that the oxygen incorporated into aniline to give 4-aminophenol originates from H2O2. Furthermore, it was found that the addition of ascorbic acid efficiently blocks all peroxidase-type reactions that can be catalyzed by the MP8/H2O2 system, but does not inhibit the aromatic hydroxylation of aniline and phenol derivatives. Together, these observations exclude reaction mechanisms for the aromatic hydroxylation that proceed through peroxidase-type mechanisms in which the oxygen incorporated into the substrate originates from O2 or H2O. The mechanism instead seems to proceed by an initial attack of the high-valent iron-oxo intermediate of MP8 on the pi-electrons of the aromatic ring of the substrate leading to product formation by a cytochrome-P-450-type of sigma-O-addition or oxygen-rebound mechanism. This implies that MP8, which has a histidyl and not a cysteinate fifth axial ligand, is able to react by a cytochrome-P-450-like oxygen-transfer reaction mechanism.

Heme-Peptide Models for Hemoproteins. 1. Solution Chemistry of N-Acetylmicroperoxidase-8

An improved method for the preparation of the heme octapeptide acetyl-MP8, obtained by proteolysis of horse heart cytochrome c, is described. AcMP8 obeys Beer's law at pH 7.0 in aqueous solution up to a concentration of 3 x 10(-)(5) M. The self-association constant measured at 25 degrees C (log K(D) = 4.04) is an order of magnitude lower than that for MP8, reflecting the role of the N-acetyl protecting group in abolishing intermolecular coordination. However, AcMP8 does form pi-stacked dimers in aqueous solution with increasing ionic strength. A more weakly packed pi-pi dimer reaches a maximum abundance at approximately 3 M ionic strength, but a more tightly packed dimer is favored at &mgr; > 3 M. An equilibrium model based on charge neutralization by specific binding of Na(+) ions gives a total molecular charge of 3- for AcMP8 at pH 7.0 and a self-association constant log K(D) = 4.20. AcMP8 exhibits six spectroscopically active pH-dependent transitions. The Glu-21 c-terminal carboxylate binds to the heme iron at low pH (pK(a) = 2.1) but is substituted by His-18 (pK(a) = 3.12) as the pH increases. The two heme propanoic acid substituents ionize with pK(a)'s of 4.95 and 6.1. This is followed by ionization of iron-bound water with a pK(a) = 9.59, DeltaH = 48 +/- 1 kJ mol(-)(1), and DeltaS = -22 +/- 3 J K(-)(1) mol(-)(1). The electronic spectra indicate that AcMP8 is predominantly in the S = (5)/(2) state at pH 7.0, while the hydroxo complex at pH 10.5 corresponds to an equilibrium mixture of S = (5)/(2) and S = (1)/(2) states at 25 degrees C. In the final transition, His-18 ionizes to form the S = (1)/(2) histidinate complex with a pK(a) of 12.71. AcMP8 is relatively stable under alkaline conditions, dimerizing slowly at high pH (k = 2.59 +/- 0.14 M(-)(1) s(-)(1)) to form a high-spin &mgr;-oxo-bridged species. The pH-dependent behavior of AcMP8 in the presence of excess 3-cyanopyridine, however, is markedly different. At low pH, AcMP8 simultaneously binds the exogenous ligand and the Glu-21 c-terminal carboxylate with a pK(a) < 2. His-18 replaces the carboxylate ligand at higher pH (pK(a) = 2.60), and both heme propanoic acid groups ionize with a mean pK(a) = 5.10. Unlike AcMP8.OH(-), the axial histidine of the 3-CNPy complex ionizes at near neutral pH (pK(a) = 7.83), prior to being replaced by OH(-) (pK(a) = 10.13). The sixth transition in the AcMP8/3-CNPy system produces the bis(hydroxo) complex (pK(a) > 13).

Monomeric ferric heme peptide derivatives: model systems for hemoproteins

Electron paramagnetic resonance spectra of a number of ferric heme peptide derivatives, in aqueous-detergent and various aqueous-alcohol solvent mixtures, have been obtained using samples in the concentration range 0.1-1.0 mM. Some of these were clearly monomeric, homogeneous, mixed-ligand adducts, entirely suitable for use as model systems for hemoprotein spectroscopic studies. As anticipated, the measured EPR parameters were largely independent of solvent environment. Surprisingly, micellar preparations of ferric heme undecapeptide in mildly alkaline solution showed no evidence for the formation of a hydroxide adduct, contrary to a previous report [S. Mazumdar, O. K. Medhi and S. Mitra, Inorg. Chem. 30 700 (1991)].

Free radical intermediates formed during the oxidation of cyanide by horseradish peroxidase/H2O2 as detected with nitroso spin traps

Aqueous solutions of cyanide react with hydrogen peroxide/horseradish peroxidase and form the cyanyl radical, which can be trapped by 2-methyl-2-nitrosopropane (t-nitrosobutane, tNB) at pH 9.8. At lower pH a variety of radical adducts are formed; at higher pH, the main product was the spin adduct of the formamide radical with tNB. The use of deuterated tNB and 15N-labeled potassium cyanide allowed the observation of the very small nitrogen coupling of this radical adduct. Experiments using 3,5-dibromo-4-nitrosobenzenesulfonic acid (DBNBS) as the spin trap yielded only the formamide radical adduct, which was identified by an independent synthesis starting from formamide. Both hydrogen splittings of its amino group could be resolved using deuterated DBNBS as the spin trap.

Oxygen activation and ligand binding by pure heme-octapeptide microperoxidase-8 (MP-8)

The rapid, efficient preparation of pure microperoxidase-8 (MP-8) is described. Ligand binding studies confirm that MP-8 is monomeric in alkaline solution. It is shown that the monomeric MP-8 activates oxygen in a similar manner to that already reported for alkaline hemin, establishing the octapeptide as a possible second generation model for the oxygen activation/insertion reactions of the cytochrome P-450.

Cyanide scavengers: kinetics of the reactions of cyanide with a water soluble cobalt(III) porphyrin

The equilibrium and kinetic aspects of the interaction of cyanide with a model anticyanide drug cobalt(III)-tetrakis(4-sulfonatophenyl)porphyrin [Co-P] were studied at 25 degrees C, I = 0.1 (NaNO3). At the physiologic pH of 7.4, 1.9 +/- 0.1 mol of cyanide were rapidly bound per molecule of Co-P. The dissociation constant of cyanide from Co-P(H2O) (CN) was less than 10(-12), and the formation constant of Co-P(CN)2 from Co-P(H2O) (CN) and CN- was 3.5 X 10(6). From pH 4 to 10.5, the kinetics of mono-cyano Co-P formation were first order in cyanide and porphyrin, with the following specific rate constants (units M-1 s-1): Co-P(H2O)2/CN-, 3.1 X 10(2); Co-P(H2O) (OH)/CN-, 2.4 X 10(3); Co-P(OH)2/CN-, 5.1 X 10(1) and Co-P(H2O)/HCN, 3.1 X 10(-3). At pH 7.4, a second cyanide molecule adds more rapidly than the first: Co-P(H2O) (CN)/CN-, 3 X 10(4) M-1 s-1. It is concluded that low molecular weight water soluble cobalt(III) porphyrins might be used as effectively and at lower dose levels than hydroxocobalamin (B12b), a known in vivo anticyanide agent.

Hemes and hemoproteins. 5: Kinetics of the peroxidatic activity of microperoxidase-8: model for the peroxidase enzymes

The peroxidatic activity of the heme octapeptide from cytochrome c, microperoxidase-8 (MP-8), was assayed at 25 degrees C under conditions where formation of Compound I is rate limiting. In the pH range 6-9, the reaction rate increased linearly with a slope close to unity. The active form of the substrate is the hydroperoxide anion, HO2-, and an extrapolated second-order rate constant was obtained for the reaction of aquoMP-8 with HO2- of 3.7 X 10(8) M-1 sec-1, which is close to the second-order rate constants reported for reaction of the peroxidase enzymes with H2O2. Comparison with published data shows that the Fe3+ ion of MP-8 reacts as expected with simple anions, electrons, and HO2-, while the analogous reactions of the enzymes all show a requirement for one H+. We conclude that the peroxidase enzymes activate H2O2 under physiological conditions through a pH-independent, H+-coupled binding of the required H2O2-. The peroxidase activity of MP-8 can be increased more than tenfold by the presence of the guanidinium ion, which is ascribed to formation of the ion-pair GuaH+HO2-; this suggests a role for the invariant distal Arg in the enzymes.