Ferric nitrosylated myoglobin catalyzes peroxynitrite scavenging

Ligand-Based Regulation of Dynamics and Reactivity of Hemoproteins

Hemoproteins include several heme-binding proteins with distinct structure and function. The presence of the heme group confers specific reactivity and spectroscopic properties to hemoproteins. In this review, we provide an overview of five families of hemoproteins in terms of dynamics and reactivity. First, we describe how ligands modulate cooperativity and reactivity in globins, such as myoglobin and hemoglobin. Second, we move on to another family of hemoproteins devoted to electron transport, such as cytochromes. Later, we consider heme-based reactivity in hemopexin, the main heme-scavenging protein. Then, we focus on heme-albumin, a chronosteric hemoprotein with peculiar spectroscopic and enzymatic properties. Eventually, we analyze the reactivity and dynamics of the most recently discovered family of hemoproteins, i.e., nitrobindins.

Nitrosylation of ferric zebrafish nitrobindin: A spectroscopic, kinetic, and thermodynamic study

Nitrobindins (Nbs) are all-β-barrel heme-proteins present in all the living kingdoms. Nbs inactivate reactive nitrogen species by sequestering NO, converting NO to HNO₂, and isomerizing peroxynitrite to NO₃^- and NO₂^-. Here, the spectroscopic characterization of ferric Danio rerio Nb (Dr-Nb(III)) and NO scavenging through the reductive nitrosylation of the metal center are reported, both processes being relevant for the regulation of blood flow in fishes through poorly oxygenated tissues, such as retina. Both UV-Vis and resonance Raman spectroscopies indicate that Dr-Nb(III) is a mixture of a six-coordinated aquo- and a five-coordinated species, whose relative abundancies depend on pH. At pH ≤ 7.0, Dr-Nb(III) binds reversibly NO, whereas at pH ≥ 7.8 NO induces the conversion of Dr-Nb(III) to Dr-Nb(II)-NO. The conversion of Dr-Nb(III) to Dr-Nb(II)-NO is a monophasic process, suggesting that the formation of the transient Dr-Nb(III)-NO species is lost in the mixing time of the rapid-mixing stopped-flow apparatus (∼ 1.5 ms). The pseudo-first-order rate constant for the reductive nitrosylation of Dr-Nb(III) is not linearly dependent on the NO concentration but tends to level off. Values of the rate-limiting constant (i.e., k_lim) increase linearly with the OH^- concentration, indicating that the conversion of Dr-Nb(III) to Dr-Nb(II)-NO is limited by the OH^--based catalysis. From the dependence of k_lim on [OH^-], the value of the second-order rate constant k_OH- was obtained (5.2 × 10³ M^-1 s^-1). Reductive nitrosylation of Dr-Nb(III) leads to the inactivation of two NO molecules: one being converted to HNO_2, and the other being tightly bound to the heme-Fe(II) atom.

Oxygen-mediated oxidation of ferrous nitrosylated nitrobindins

The O₂-mediated oxidation of all-β-barrel ferrous nitrosylated nitrobindin from Arabidopsis thaliana (At-Nb(II)-NO), Mycobacterium tuberculosis (Mt-Nb(II)-NO), and Homo sapiens (Hs-Nb(II)-NO) to ferric derivative (At-Nb(III), Mt-Nb(III), and Hs-Nb(III), respectively) has been investigated at pH 7.0 and 20.0 °C. Unlike ferrous nitrosylated horse myoglobin, human serum heme-albumin and human hemoglobin, the process in Nb(II)-NO is mono-exponential and linearly dependent on the O₂ concentration, displaying a bimolecular behavior, characterized by k_on = (6.3 ± 0.8) × 10³ M^-1 s^-1, (1.4 ± 0.2) × 10³ M^-1 s^-1, and (3.9 ± 0.5) × 10³ M^-1 s^-1 for At-Nb(II)-NO, Mt-Nb(II)-NO, and Hs-Nb(II)-NO, respectively. No intermediate is detected, indicating that the O₂ reaction with Nb(II)-NO is the rate-limiting step and that the subsequent conversion of the heme-Fe(III)-N(O)OO^- species (i.e., N-bound peroxynitrite to heme-Fe(III)) to heme-Fe(III) and NO₃^- is much faster. A similar mechanism can be invoked for ferrous nitrosylated human neuroglobin and rabbit hemopexin, in which the heme-Fe(III)-N(O)OO^- species is formed as well, although the rate-limiting step seems represented by the reshaping of the six-coordinated heme-Fe(III) complex. Although At-Nb(II)-NO and Mt-Nb(II)-NO are partially (while Hs-Nb(II)-NO is almost completely) penta-coordinated, density functional theory (DFT) calculations rule out that the cleavage of the proximal heme-Fe-His bond in Nb(II)-NO is responsible for the more stable heme-Fe(III)-N(O)OO^- species. Moreover, the oxidation of the penta-coordinated heme-Fe(II)-NO adduct does not depend on O₂ binding at the proximal side of the metal center. These features may instead reflect the peculiarity of Nb folding and of the heme environment, with a reduced steric constraint for the formation of the heme-Fe(III)-N(O)OO^- complex.

The Scavenging Effect of Myoglobin from Meat Extracts toward Peroxynitrite Studied with a Flow Injection System Based on Electrochemical Reduction over a Screen-Printed Carbon Electrode Modified with Cobalt Phthalocyanine: Quantification and Kinetics

The scavenging activity of myoglobin toward peroxynitrite (PON) was studied in meat extracts, using a new developed electrochemical method (based on cobalt phthalocyanine-modified screen-printed carbon electrode, SPCE/CoPc) and calculating kinetic parameters of PON decay (such as half-time and apparent rate constants). As reactive oxygen/nitrogen species (ROS/RNS) affect the food quality, the consumers can be negatively influenced. The discoloration, rancidity, and flavor of meat are altered in the presence of these species, such as PON. Our new highly thermically stable, cost-effective, rapid, and simple electrocatalytical method was combined with a flow injection analysis system to achieve high sensitivity (10.843 nA µM⁻¹) at a nanomolar level LoD (400 nM), within a linear range of 3–180 µM. The proposed biosensor was fully characterized using SEM, FTIR, Raman spectroscopy, Cyclic Voltammetry (CV), Differential Pulse Voltammetry (DPV), and Linear Sweep Voltammetry (LSV). These achievements were obtained due to the CoPc-mediated reduction of PON at very low potentials (around 0.1 V vs. Ag/AgCl pseudoreference). We also proposed a redox mechanism involving two electrons in the reduction of peroxynitrite to nitrite and studied some important interfering species (nitrite, nitrate, hydrogen peroxide, dopamine, ascorbic acid), which showed that our method is highly selective. These features make our work relevant, as it could be further applied to study the kinetics of important oxidative processes in vivo or in vitro, as PON is usually present in the nanomolar or micromolar range in physiological conditions, and our method is sensitive enough to be applied.

Abiological catalysis by myoglobin mutant with a genetically incorporated unnatural amino acid

To inculcate biocatalytic activity in the oxygen-storage protein myoglobin (Mb), a genetically engineered myoglobin mutant H64DOPA (DOPA = L-3,4-dihydroxyphenylalanine) has been created. Incorporation of unnatural amino acids has already demonstrated their ability to accomplish many non-natural functions in proteins efficiently. Herein, the presence of redox-active DOPA residue in the active site of mutant Mb presumably stabilizes the compound I in the catalytic oxidation process by participating in an additional hydrogen bonding (H-bonding) as compared to the WT Mb. Specifically, a general acid-base catalytic pathway was achieved due to the availability of the hydroxyl moieties of DOPA. The reduction potential values of WT (E° = -260 mV) and mutant Mb (E° = -300 mV), w.r.t. Ag/AgCl reference electrode, in the presence of hydrogen peroxide, indicated an additional H-bonding in the mutant protein, which is responsible for the peroxidase activity of the mutant Mb. We observed that in the presence of 5 mM H2O2, H64DOPA Mb oxidizes thioanisole and benzaldehyde with a 10 and 54 folds higher rate, respectively, as opposed to WT Mb. Based on spectroscopic, kinetic, and electrochemical studies, we deduce that DOPA residue, when present within the distal pocket of mutant Mb, alone serves the role of His/Arg-pair of peroxidases.

Mycobacterial and Human Nitrobindins: Structure and Function

Aims: Nitrobindins (Nbs) are evolutionary conserved all-β-barrel heme-proteins displaying a highly solvent-exposed heme-Fe(III) atom. The physiological role(s) of Nbs is almost unknown. Here, the structural and functional properties of ferric Mycobacterium tuberculosis Nb (Mt-Nb(III)) and ferric Homo sapiens Nb (Hs-Nb(III)) have been investigated and compared with those of ferric Arabidopsis thaliana Nb (At-Nb(III), Rhodnius prolixus nitrophorins (Rp-NP(III)s), and mammalian myoglobins. Results: Data here reported demonstrate that Mt-Nb(III), At-Nb(III), and Hs-Nb(III) share with Rp-NP(III)s the capability to bind selectively nitric oxide, but display a very low reactivity, if any, toward histamine. Data obtained overexpressing Hs-Nb in human embryonic kidney 293 cells indicate that Hs-Nb localizes mainly in the cytoplasm and partially in the nucleus, thanks to a nuclear localization sequence encompassing residues Glu124-Leu154. Human Hs-Nb corresponds to the C-terminal domain of the human nuclear protein THAP4 suggesting that Nb may act as a sensor possibly modulating the THAP4 transcriptional activity residing in the N-terminal region. Finally, we provide strong evidence that both Mt-Nb(III) and Hs-Nb(III) are able to scavenge peroxynitrite and to protect free l-tyrosine against peroxynitrite-mediated nitration. Innovation: Data here reported suggest an evolutionarily conserved function of Nbs related to their role as nitric oxide sensors and components of antioxidant systems. Conclusion: Human THAP4 may act as a sensing protein that couples the heme-based Nb(III) reactivity with gene transcription. Mt-Nb(III) seems to be part of the pool of proteins required to scavenge reactive nitrogen and oxygen species produced by the host during the immunity response.