Cyanide binding to ferrous and ferric microperoxidase-11

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.

Hemeproteins as Targets for Sulfide Species

Significance: Sulfides are endogenous and ubiquitous signaling species that share the hemeproteins as biochemical targets with O₂, nitric oxide, and carbon monoxide. The description of the binding mechanisms is mandatory to anticipate the biochemical relevance of the interaction. Recent Advances: The binding of sulfide to ferric hemeproteins has been described in more than 40 systems, including native proteins, mutants, and model systems. Mechanisms of sulfide binding to ferric hemeproteins have been examined by a combination of kinetic and computational experiments. The distal control of the association process, dissected into the migration of the ligand to the active site and the binding event, reveals that neutral hydrogen sulfide (H₂S) reaches the active site and is the predominant binding ligand, while the HS^- is excluded by the protein matrix. Experiments with model compounds, devoid of a protein scaffold, reveal that both H₂S and HS^- can bind the ferric heme if accessing the site. A critical role of the proximal ligand in the prevention of the metal-centered reduction has been experimentally assessed. For metmyoglobin and methemoglobin, the coordination of sulfide leads to noncanonical functions: sulfide storage and its oxidative detoxification have been evidenced under physiological and excess sulfide concentrations, respectively. Critical Issues: The bound species is suggested to predominate in the monoprotonated form, although spectroscopic evidence is pending. Future Directions: A description of the role of hemeproteins as biochemical targets for inorganic sulfide requires understanding the reactivity of bound sulfide, for example: the metal-centered reduction, the reaction with excess sulfide, oxidants, or other gasotransmitters, among other biomolecules.

Reductive nitrosylation of ferric microperoxidase-11

Microperoxidase-11 (MP11) is an undecapeptide derived from horse heart cytochrome c, which is considered as a heme-protein model. Here, the reductive nitrosylation of ferric MP11 (MP11(III)) under anaerobic conditions has been investigated between pH 7.4 and 9.2, at T = 20.0 °C. At pH ≤ 7.7, NO binds reversibly to MP11(III) leading to the formation of the MP11(III)-NO complex. However, between pH 8.2 and 9.2, the addition of NO to MP11(III) leads to the formation of ferrous nitrosylated MP11(II) (MP11(II)-NO). In fact, the transient MP11{FeNO}⁶ species is converted to ferrous deoxygenated MP11 (MP11(II)) by OH^-- and H₂O-based catalysis, which represents the rate-limiting step of the whole reaction. Then, MP11(II) binds NO very rapidly leading to MP11(II)-NO formation. Over the whole pH range explored, the apparent values of k_on, k_off, and K (= k_off/k_on) for MP11(III)(-NO) (de)nitrosylation are essentially pH independent, ranging between 5.8 × 10⁵ M^-1 s^-1 and 1.6 × 10⁶ M^-1 s^-1, between 1.9 s^-1 and 3.7 s^-1, and between 1.4 × 10^-6 M and 4.6 × 10^-6 M, respectively. Values of the apparent pseudo-first-order rate constant for the MP11{FeNO}⁶ conversion to MP11(II) (i.e., h) increase linearly with pH; the apparent values [Formula: see text] and [Formula: see text] are 7.2 × 10² M^-1 s^-1 and 2.5 × 10^-4 s^-1, respectively. Present data confirm that MP11 is a useful molecular model to highlight the role of the protein matrix on the heme-based reactivity.

Distinguishing Active Site Characteristics of Chlorite Dismutases with Their Cyanide Complexes

O2-evolving chlorite dismutases (Clds) efficiently convert chlorite (ClO2-) to O2 and Cl-. Dechloromonas aromatica Cld ( DaCld) is a highly active chlorite-decomposing homopentameric enzyme, typical of Clds found in perchlorate- and chlorate-respiring bacteria. The Gram-negative, human pathogen Klebsiella pneumoniae contains a homodimeric Cld ( KpCld) that also decomposes ClO2-, albeit with an activity 10-fold lower and a turnover number lower than those of DaCld. The interactions between the distal pocket and heme ligand of the DaCld and KpCld active sites have been probed via kinetic, thermodynamic, and spectroscopic behaviors of their cyanide complexes for insight into active site characteristics that are deterministic for chlorite decomposition. At 4.7 × 10-9 M, the KD for the KpCld-CN- complex is 2 orders of magnitude smaller than that of DaCld-CN- and indicates an affinity for CN- that is greater than that of most heme proteins. The difference in CN- affinity between Kp- and DaClds is predominantly due to differences in koff. The kinetics of binding of cyanide to DaCld, DaCld(R183Q), and KpCld between pH 4 and 8.5 corroborate the importance of distal Arg183 and a p Ka of ∼7 in stabilizing complexes of anionic ligands, including the substrate. The Fe-C stretching and FeCN bending modes of the DaCld-CN- (νFe-C, 441 cm-1; δFeCN, 396 cm-1) and KpCld-CN- (νFe-C, 441 cm-1; δFeCN, 356 cm-1) complexes reveal differences in their FeCN angle, which suggest different distal pocket interactions with their bound cyanide. Conformational differences in their catalytic sites are also reported by the single ferrous KpCld carbonyl complex, which is in contrast to the two conformers observed for DaCld-CO.