Peroxynitrite-mediated oxidation of ferrous carbonylated myoglobin is limited by carbon monoxide dissociation

Hydroxylamine-induced oxidation of ferrous nitrobindins

Hemoglobin and myoglobin are generally taken as molecular models of all-α-helical heme-proteins. On the other hand, nitrophorins and nitrobindins (Nb), which are arranged in 8 and 10 β-strands, respectively, represent the molecular models of all-β-barrel heme-proteins. Here, kinetics of the hydroxylamine- (HA-) mediated oxidation of ferrous Mycobacterium tuberculosis, Arabidopsis thaliana, and Homo sapiens nitrobindins (Mt-Nb(II), At-Nb(II), and Hs-Nb(II), respectively), at pH 7.0 and 20.0 °C, are reported. Of note, HA displays antibacterial properties and is a good candidate for the treatment and/or prevention of reactive nitrogen species- (RNS-) linked aging-related pathologies, such as macular degeneration. Under anaerobic conditions, mixing the Mt-Nb(II), At-Nb(II), and Hs-Nb(II) solutions with the HA solutions brings about absorbance spectral changes reflecting the formation of the ferric derivative (i.e., Mt-Nb(III), At-Nb(III), and Hs-Nb(III), respectively). Values of the second order rate constant for the HA-mediated oxidation of Mt-Nb(II), At-Nb(II), and Hs-Nb(II) are 1.1 × 10⁴ M^-1 s^-1, 6.5 × 10⁴ M^-1 s^-1, and 2.2 × 10⁴ M^-1 s^-1, respectively. Moreover, the HA:Nb(II) stoichiometry is 1:2 as reported for ferrous deoxygenated and carbonylated all-α-helical heme-proteins. A comparative look of the HA reduction kinetics by several ferrous heme-proteins suggests that an important role might be played by residues (such as His or Tyr) in the proximity of the heme-Fe atom either coordinating it or not. In this respect, Nbs seem to exploit somewhat different structural aspects, indicating that redox mechanisms for the heme-Fe(II)-to-heme-Fe(III) conversion might differ between all-α-helical and all-β-barrel heme-proteins.

Ferric nitrosylated myoglobin catalyzes peroxynitrite scavenging

Myoglobin (Mb), generally taken as the molecular model of monomeric globular heme-proteins, is devoted: (i) to act as an intracellular oxygen reservoir, (ii) to transport oxygen from the sarcolemma to the mitochondria of vertebrate heart and red muscle cells, and (iii) to act as a scavenger of nitrogen and oxygen reactive species protecting mitochondrial respiration. Here, the first evidence of ^·NO inhibition of ferric Mb- (Mb(III)) mediated detoxification of peroxynitrite is reported, at pH 7.2 and 20.0 °C. ^·NO binds to Mb(III) with a simple equilibrium; the value of the second-order rate constant for Mb(III) nitrosylation (i.e., ^·NOk_on) is (6.8 ± 0.7) × 10⁴ M^-1 s^-1 and the value of the first-order rate constant for Mb(III)-NO denitrosylation (i.e., ^·NOk_off) is 3.1 ± 0.3 s^-1. The calculated value of the dissociation equilibrium constant for Mb(III)-NO complex formation (i.e., ^·NOk_off/^·NOk_on = (4.6 ± 0.7) × 10^-5 M) is virtually the same as that directly measured (i.e., ^·NOK = (3.8 ± 0.5) × 10^-5 M). In the absence of ^·NO, Mb(III) catalyzes the conversion of peroxynitrite to NO₃^-, the value of the second-order rate constant (i.e., ^Pk_on) being (1.9 ± 0.2) × 10⁴ M^-1 s^-1. However, in the presence of ^·NO, Mb(III)-mediated detoxification of peroxynitrite is only partially inhibited, underlying the possibility that also Mb(III)-NO is able to catalyze the peroxynitrite isomerization, though with a reduced rate (^Pk_on* = (2.8 ± 0.3) × 10³ M^-1 s^-1). These data expand the multiple roles of ^·NO in modulating heme-protein actions, envisaging a delicate balancing between peroxynitrite and ^·NO, which is modulated through the relative amount of Mb(III) and Mb(III)-NO.

Hydroxylamine-induced oxidation of ferrous CO-bound carboxymethylated-cytochrome c

The hexa-coordinated metal center of horse heart cyt[Formula: see text] (cyt[Formula: see text] is at the root of its low reactivity. In contrast, carboxymethylated cyt[Formula: see text] (CM-cyt[Formula: see text] displays myoglobin-like properties. Herein, kinetics of CO binding to ferrous CM-cyt[Formula: see text] (CM-cyt[Formula: see text](II)) and of the irreversible oxidation of ferrous carbonylated CM-cyt[Formula: see text] (CM-cyt[Formula: see text](II)-CO) by hydroxylamine (HA), at pH 5.8 and 20.0 [Formula: see text]C, are reported. HA irreversibly oxidizes CM-cyt[Formula: see text](II)-CO with the 1:2 stoichiometry leading to the formation of the ferric species (CM-cyt[Formula: see text](III)) without the observation of intermediates. Present data indicate that: (i) the rate of CO dissociation from CM-cyt[Formula: see text](II)-CO represents the rate-limiting step of HA-mediated oxidation of the carbonylated metal center, (ii) the fast oxidation of CM-cyt[Formula: see text](II)-CO from HA reflects the penta-coordination of the transient CM-cyt[Formula: see text](II) species, (iii) the HA-catalyzed conversion of CM-cyt[Formula: see text](II)-CO to CM-cyt[Formula: see text](III) could proceed via the geminate mechanism, (iv) values of the second-order rate constants for the carbonylation and the HA-mediated oxidation of ferrous heme-proteins are linearly correlated reflecting the penta- or hexa-coordination of the metal center, the free energy for the in-plane positioning of the heme-Fe atom in the unliganded species, and the arrangement of the distal portion of the heme pocket that affects ligand and/or electron transfer.

Hydroxylamine-induced oxidation of ferrous carbonylated truncated hemoglobins from Mycobacterium tuberculosis and Campylobacter jejuni is limited by carbon monoxide dissociation

Hydroxylamine (HA) is an oxidant of ferrous globins and its action has been reported to be inhibited by CO, even though this mechanism has not been clarified. Here, kinetics of the HA-mediated oxidation of ferrous carbonylated Mycobacterium tuberculosis truncated hemoglobin N and O (Mt-trHbN(II)-CO and Mt-trHbO(II)-CO, respectively) and Campylobacter jejuni truncated hemoglobin P (Cj-trHbP(II)-CO), at pH 7.2 and 20.0 °C, are reported. Mixing Mt-trHbN(II)-CO, Mt-trHbO(II)-CO, and Cj-trHbP(II)-CO solution with the HA solution brings about absorption spectral changes reflecting the disappearance of the ferrous carbonylated derivatives with the concomitant formation of the ferric species. HA oxidizes irreversibly Mt-trHbN(II)-CO, Mt-trHbO(II)-CO, and Cj-trHbP(II)-CO with the 1:2 stoichiometry. The dissociation of CO turns out to be the rate-limiting step for the oxidation of Mt-trHbN(II)-CO, Mt-trHbO(II)-CO, and Cj-trHbP(II)-CO by HA. Values of the second-order rate constant for HA-mediated oxidation of Mt-trHbN(II)-CO, Mt-trHbO(II)-CO, and Cj-trHbP(II)-CO range between 8.8 × 10⁴ and 8.6 × 10⁷ M^-1 s^-1, reflecting different structural features of the heme distal pocket. This study (1) demonstrates that the inhibitory effect of CO is linked to the dissociation of this ligand, giving a functional basis to previous studies, (2) represents the first comparative investigation of the oxidation of ferrous carbonylated bacterial 2/2 globins belonging to the N, O, and P groups by HA, (3) casts light on the correlation between kinetics of HA-mediated oxidation and carbonylation of globins, and (4) focuses on structural determinants modulating the HA-induced oxidation process.

Peroxynitrite-An ugly biofactor?

Cellular damage occurring under oxidative conditions has been ascribed mainly to the formation of peroxynitrite (ONOOH/ONOO(-)) that originates from the reaction of NO(*) with O(2) (*-). The detrimental effects of peroxynitrite are exacerbated by the reaction with CO(2) that leads to ONOOC(O)O(-), which further decays to the strong oxidant radicals NO(2) (*) and CO(3) (*-). The reaction with CO(2), however, may redirect peroxynitrite specificity. An excessive formation of peroxynitrite represents an important mechanism contributing to the DNA damage, the inactivation of metabolic enzymes, ionic pumps, and structural proteins, and the disruption of cell membranes. Because of its ability to oxidize biomolecules, peroxynitrite is implicated in an increasing list of diseases, including neurodegenerative and cardiovascular disorders, inflammation, pain, autoimmunity, cancer, and aging. However, peroxynitrite displays also protective activities: (i) at high concentrations, it shows anti-viral, anti-microbial, and anti-parasitic actions; and (ii) at low concentrations, it stimulates protective mechanisms in the cardiovascular, nervous, and respiratory systems. The detrimental effects of peroxynitrite and related reactive species are impaired by (pseudo-) enzymatic systems, mainly represented by heme-proteins (e.g., hemoglobin and myoglobin). Here, we report biochemical aspects of peroxynitrite actions being at the root of its biomedical effects.