Characterization of N-Acetylated Heme Undecapeptide and Some of Its Derivatives in Aqueous Media: Monomeric Model Systems for Hemoproteins

A Personal Account on Inorganic Reaction Mechanisms

The presented Review is focused on the latest research in the field of inorganic chemistry performed by the van Eldik group and his collaborators. The first part of the manuscript concentrates on the interaction of nitric oxide and its derivatives with biologically important compounds. We summarized mechanistic information on the interaction between model porphyrin systems (microperoxidase) and NO as well as the recent studies on the formation of nitrosylcobalamin (CblNO). The following sections cover the characterization of the Ru(II)/Ru(III) mixed-valence ion-pair complexes, including Ru(II)/Ru(III)(edta) complexes. The last part concerns the latest mechanistic information on the DFT techniques applications. Each section presents the most important results with the mechanistic interpretations.

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)FeIII(H2O)(HisH)] (1), [(AcMP-11)FeIII(OH)(HisH)] (2), and [(AcMP-11)FeIII(OH)(His-)] (3). From the pH dependence of the second-order rate constant for NO binding (kon), 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 (kon(1) = 3.8 × 106 M-1 s-1) to 2 (kon(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)FeIII(OH)(HisH)] (2a) and minor [(AcMP-11)FeIII(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)FeIII(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 pKa 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.

Reactivity of inorganic sulfide species towards a pentacoordinated heme model system

The reactivity of inorganic sulfide towards ferric bis(N-acetyl)- microperoxidase 11 in sodium dodecyl sulfate has been explored by means of visible absorption and resonance Raman spectroscopies. The reaction has been previously studied in buffered solutions at neutral pH and in the presence of excess sulfide, revealing the formation of a moderately stable hexacoordinated low spin ferric sulfide complex that yields the ferrous form in the hour's timescale. In the surfactant solution, instead, the ferrous form is rapidly formed. The spectroscopic characterization of the heme structure in the surfactant milieu revealed the stabilization of a major ferric mono-histidyl high spin heme, which may be ascribed to out of plane distortions prompting the detachment of the axially ligated water molecule, thus leading to a differential reactivity. The ferric bis(N-acetyl)- microperoxidase 11 in sodium dodecyl sulfate provides a model for pentacoordinated heme platforms with an imidazole-based ligand.

Peroxidase activation of cytoglobin by anionic phospholipids: Mechanisms and consequences

Cytoglobin (Cygb) is a hexa-coordinated hemoprotein with yet to be defined physiological functions. The iron coordination and spin state of the Cygb heme group are sensitive to oxidation of two cysteine residues (Cys38/Cys83) and/or the binding of free fatty acids. However, the roles of redox vs lipid regulators of Cygb's structural rearrangements in the context of the protein peroxidase competence are not known. Searching for physiologically relevant lipid regulators of Cygb, here we report that anionic phospholipids, particularly phosphatidylinositolphosphates, affect structural organization of the protein and modulate its iron state and peroxidase activity both conjointly and/or independently of cysteine oxidation. Thus, different anionic lipids can operate in cysteine-dependent and cysteine-independent ways as inducers of the peroxidase activity. We establish that Cygb's peroxidase activity can be utilized for the catalysis of peroxidation of anionic phospholipids (including phosphatidylinositolphosphates) yielding mono-oxygenated molecular species. Combined with the computational simulations we propose a bipartite lipid binding model that rationalizes the modes of interactions with phospholipids, the effects on structural re-arrangements and the peroxidase activity of the hemoprotein.

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.

Structural re-arrangement and peroxidase activation of cytochrome c by anionic analogues of vitamin E, tocopherol succinate and tocopherol phosphate

Cytochrome c is a multifunctional hemoprotein in the mitochondrial intermembrane space whereby its participation in electron shuttling between respiratory complexes III and IV is alternative to its role in apoptosis as a peroxidase activated by interaction with cardiolipin (CL), and resulting in selective CL peroxidation. The switch from electron transfer to peroxidase function requires partial unfolding of the protein upon binding of CL, whose specific features combine negative charges of the two phosphate groups with four hydrophobic fatty acid residues. Assuming that other endogenous small molecule ligands with a hydrophobic chain and a negatively charged functionality may activate cytochrome c into a peroxidase, we investigated two hydrophobic anionic analogues of vitamin E, α-tocopherol succinate (α-TOS) and α-tocopherol phosphate (α-TOP), as potential inducers of peroxidase activity of cytochrome c. NMR studies and computational modeling indicate that they interact with cytochrome c at similar sites previously proposed for CL. Absorption spectroscopy showed that both analogues effectively disrupt the Fe-S(Met(80)) bond associated with unfolding of cytochrome c. We found that α-TOS and α-TOP stimulate peroxidase activity of cytochrome c. Enhanced peroxidase activity was also observed in isolated rat liver mitochondria incubated with α-TOS and tBOOH. A mitochondria-targeted derivative of TOS, triphenylphosphonium-TOS (mito-VES), was more efficient in inducing H2O2-dependent apoptosis in mouse embryonic cytochrome c(+/+) cells than in cytochrome c(-/-) cells. Essential for execution of the apoptotic program peroxidase activation of cytochrome c by α-TOS may contribute to its known anti-cancer pharmacological activity.

Designing inhibitors of cytochrome c/cardiolipin peroxidase complexes: mitochondria-targeted imidazole-substituted fatty acids

Mitochondria have emerged as the major regulatory platform responsible for the coordination of numerous metabolic reactions as well as cell death processes, whereby the execution of intrinsic apoptosis includes the production of reactive oxygen species fueling oxidation of cardiolipin (CL) catalyzed by cytochrome (Cyt) c. As this oxidation occurs within the peroxidase complex of Cyt c with CL, the latter represents a promising target for the discovery and design of drugs with antiapoptotic mechanisms of action. In this work, we designed and synthesized a new group of mitochondria-targeted imidazole-substituted analogs of stearic acid TPP-n-ISAs with various positions of the attached imidazole group on the fatty acid (n = 6, 8, 10, 13, and 14). By using a combination of absorption spectroscopy and EPR protocols (continuous wave electron paramagnetic resonance and electron spin echo envelope modulation) we demonstrated that TPP-n-ISAs indeed were able to potently suppress CL-induced structural rearrangements in Cyt c, paving the way to its peroxidase competence. TPP-n-ISA analogs preserved the low-spin hexa-coordinated heme-iron state in Cyt c/CL complexes whereby TPP-6-ISA displayed a significantly more effective preservation pattern than TPP-14-ISA. Elucidation of these intermolecular stabilization mechanisms of Cyt c identified TPP-6-ISA as an effective inhibitor of the peroxidase function of Cyt c/CL complexes with a significant antiapoptotic potential realized in mouse embryonic cells exposed to ionizing irradiation. These experimental findings were detailed and supported by all-atom molecular dynamics simulations. Based on the experimental data and computation predictions, we identified TPP-6-ISA as a candidate drug with optimized antiapoptotic potency.

Synthesis and reactivity studies of a manganese 'microperoxidase' containing b-type heme

Mn(III)protoporphyrin IX-6(7)-gly-gly-his methyl ester (MnGGH) has been prepared by condensation of glycyl-glycyl-L-histidine methyl ester with the propionic side chains of Mn(III)protoporphyrin IX. It was characterised by mass spectrometry and UV/VIS spectroscopy. Stopped-flow spectrophotometry was used to study the reaction of the Mn 'microperoxidase' with hydrogen peroxide. The formation of active intermediates analogous to previously described metal-hydroperoxo (compound 0) and metal-oxo (compound I) intermediates of the 'natural' Fe(III) microperoxidase-8 and Mn(III) microperoxidase-8 was observed. The rate of formation of the MnGGH-based compound I analogue was found to increase dramatically with increasing pH. A steady-state kinetic analysis of the catalytic peroxidase activity of MnGGH towards K4[Fe(CN)6], L-tyrosine methyl ester, o-dianisidine, o-methoxyphenol and ascorbic acid showed that the peroxidase reaction proceeds via the formation of a microperoxidase-substrate complex followed by electron transfer from the substrate to the metal. The reactivity of MnGGH depends on the size and hydrophobicity of the substrate, and these properties appear to influence the rate of the electron transfer, which is the rate-limiting step for the whole process. MnGGH showed higher reactivity towards reducing substrates than its Fe(iii) analogue.

Preparation and reactivity studies of synthetic microperoxidases containing b-type heme

In order to create a heme environment that permits biomimicry of heme-containing peroxidases, a number of new hemin-peptide complexes--hemin-2(18)-glycyl-L-histidine methyl ester (HGH), hemin-2(18)-glycyl-glycyl-L-histidine methyl ester (HGGH), and hemin-2,18-bis(glycyl-glycyl-L-histidine methyl ester) (H2GGH)--have been prepared by condensation of glycyl-L-histidine methyl ester or glycyl-glycyl-L-histidine methyl ester with the propionic side chains of hemin. Characterization by means of UV/vis- and 1H NMR spectroscopy as well as cyclic- and differential pulse voltammetry indicates the formation of five-coordinate complexes in the case of HGH and HGGH, with histidine as an axial ligand. In the case of H2GGH, a six-coordinate complex with both imidazoles coordinated to the iron center appears to be formed. However, 1H NMR of H2GGH reveals the existence of an equilibrium between low-spin six-coordinate and high-spin five-coordinate species in solution. The catalytic activity of the hemin-peptide complexes towards several organic substrates, such as p-cresol, L-tyrosine methyl ester, and ABTS, has been investigated. It was found that not only the five-coordinate HGH and HGGH complexes, but also the six-coordinate H2GGH, catalyze the oxidation of substrates by H2O2. The longer and less strained peptide arm provides the HGGH complex with a slightly higher catalytic efficiency, as compared with HGH, due to formation of more stable intermediate complexes.

Effects of the environment on microperoxidase-11 and on its catalytic activity in oxidation of organic sulfides to sulfoxides

Microperoxidase-11 (MP-11, also known as heme undecapeptide of cytochrome c) was immobilized by encapsulation into sol-gel silica glass and by physisorption, chemisorption, and covalent attachment to silica gel. We then compared these species with one another and with dissolved microperoxidase-11 as catalysts for the sulfoxidation of methyl phenyl sulfide by hydrogen peroxide. MP-11 is prone to oligomerization in solution, both via axial ligation and via intermolecular interactions. When the ligation oligomerization is suppressed upon immobilization, heme becomes more accessible, and the sulfoxide yield increases 4-6 times, from 15% up to 95%. When the ligation oligomerization of dissolved MP-11 is suppressed by protonation and acetylation of amino groups and by addition of methanol, sodium dodecyl sulfate (SDS), or trifluoroethanol, the sulfoxide yield increases 3-5 times (up to 76%). The oligomerization via intermolecular interactions is important for preserving enantioselectivity in immobilized and dissolved MP-11. For MP-11 in amine-rich and especially alcohol-rich environments, the enantioselectivity is vanishingly low, presumably because amino and hydroxyl groups cause a conformation change in the catalyst. In other environments, the MP-11 species are aggregated via intermolecular interactions in micellar (SDS) solution and on the surface of the silica gel, or via axial ligation in aqueous buffer at pH 6.0. Under these conditions, the enantioselectivity is enhanced; the enantiomeric excess (ee) becomes as high as 46%. An understanding of the effects of the aggregation state and consequent properties on the catalytic activity of MP-11 allowed us to control the yield and enantioselectivity of sulfoxidation reaction.

Covalently modified microperoxidases as heme-peptide models for peroxidases

Microperoxidase-8 (MP8) and microperoxidase-9 (MP9) have been covalently modified by attachment of proline-containing residues to the amino terminal peptide chain in order to obtain new peroxidase model systems. The catalytic activities of these derivatives in the oxidation of p-cresol by hydrogen peroxide have been compared to that of MP8. The presence of steric hindrance above the heme reduces the formation rate of the catalytically active species, while the reactivity is increased when the amino group of a proline residue is close to the iron. The modification of the catalyst affects the rate of degradation processes undergone by the heme group during catalysis. A bulky aromatic group on the distal side decreases the stability of the complex because it reduces the mobility of a phenoxy radical species formed during catalysis, while the presence of proline residues increases the number of turnovers of the heme catalysts before degradation. The complex Pro2-MP8 obtained by addition of two proline residues to MP8 exhibits the best catalytic performance in terms of activity and chemical stability.

The Alkaline Transition of Bis(N-acetylated) Heme Undecapeptide

Alkaline forms of the ferric bis(N-acetylated) heme undecapeptide of cytochrome c (N-ac-HUP) and some of its derivatives have been studied by electronic absorption and electron paramagnetic resonance spectroscopies. Surprisingly, even at pH >12, no evidence could be found for the formation of a hydroxyl ion adduct, in direct contrast to a previous report concerning ferric heme peptides encapsulated in detergent micelles (Mazumdar et al. Inorg. Chem. 1991, 30, 700-705). A spectroscopically determined pK(a) of approximately 9 is assigned to the deprotonation of the constituent histidine ligand of heme iron in N-ac-HUP. The present findings are not entirely in keeping with those of an earlier study concerning the properties of N-acetylated heme octapeptide (Wang et al. J. Biol. Chem. 1992, 35, 15310-15318), the differences observed being attributed to the buffering media employed in the two investigations. The implications of the current results in relation to a better understanding of the alkaline transitions observed in hemoglobins and myoglobins is considered.