Proton-linked subunit kinetic heterogeneity for carbon monoxide binding to hemoglobin from Chelidonichthys kumu

Mycobacterial and Human Ferrous Nitrobindins: Spectroscopic and Reactivity Properties

Structural and functional properties of ferrous Mycobacterium tuberculosis (Mt-Nb) and human (Hs-Nb) nitrobindins (Nbs) were investigated. At pH 7.0 and 25.0 °C, the unliganded Fe(II) species is penta-coordinated and unlike most other hemoproteins no pH-dependence of its coordination was detected over the pH range between 2.2 and 7.0. Further, despite a very open distal side of the heme pocket (as also indicated by the vanishingly small geminate recombination of CO for both Nbs), which exposes the heme pocket to the bulk solvent, their reactivity toward ligands, such as CO and NO, is significantly slower than in most hemoproteins, envisaging either a proximal barrier for ligand binding and/or crowding of H₂O molecules in the distal side of the heme pocket which impairs ligand binding to the heme Fe-atom. On the other hand, liganded species display already at pH 7.0 and 25 °C a severe weakening (in the case of CO) and a cleavage (in the case of NO) of the proximal Fe-His bond, suggesting that the ligand-linked movement of the Fe(II) atom onto the heme plane brings about a marked lengthening of the proximal Fe-imidazole bond, eventually leading to its rupture. This structural evidence is accompanied by a marked enhancement of both ligands dissociation rate constants. As a whole, these data clearly indicate that structural–functional relationships in Nbs strongly differ from what observed in mammalian and truncated hemoproteins, suggesting that Nbs play a functional role clearly distinct from other eukaryotic and prokaryotic hemoproteins.

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

Peroxynitrite-mediated oxidation of ferrous nitrosylated myoglobin (Mb(II)-NO) involves the transient ferric nitrosylated species (Mb(III)-NO), followed by ()NO dissociation and formation of ferric myoglobin (Mb(III)). In contrast, peroxynitrite-mediated oxidation of ferrous oxygenated myoglobin (Mb(II)-O2) involves the transient ferrous deoxygenated and ferryl derivatives (Mb(II) and Mb(IV)O, respectively), followed by Mb(III) formation. Here, kinetics of peroxynitrite-mediated oxidation of ferrous carbonylated horse heart myoglobin (Mb(II)-CO) is reported. Values of the first-order rate constant for peroxynitrite-mediated oxidation of Mb(II)-CO (i.e., for Mb(III) formation) and of the first-order rate constant for CO dissociation from Mb(II)-CO (i.e., for Mb(II) formation) are h=(1.2+/-0.2)x10(-2)s(-1) and k(off(CO))=(1.4+/-0.2)x10(-2)s(-1), respectively, at pH 7.2 and 20.0 degrees C. The coincidence of values of h and k(off(CO)) indicates that CO dissociation represents the rate limiting step of peroxynitrite-mediated oxidation of Mb(II)-CO.

The Oxygen Transport System in Three Species of the Boreal Fish Family Gadidae

The Arctic and Antarctic marine faunas differ by age and isolation. Fishes of the two polar regions have undergone different regional histories that have driven the physiological diversities. Antarctic fish are highly stenothermal, in keeping with stable water temperatures, whereas Arctic fish, being exposed to seasonal temperature variations, exhibit higher physiological plasticity. This study reports the characterization of the oxygen transport system of three Arctic species of the family Gadidae, namely the Arctic cod Arctogadus glacialis, the polar cod Boreogadus saida, and the Atlantic cod Gadus morhua. Unlike Antarctic notothenioids, the blood displays high multiplicity, i.e. it has three hemoglobins, similar to many other acanthomorph teleosts. In the most abundant hemoglobin, oxygen binding is modulated by heterotropic effectors, with marked Bohr and Root effects. Remarkably, in two species (A. glacialis and B. saida), the Hill coefficient is very close to one in the whole pH range, indicating the apparent absence of cooperativity. The amino acid sequences have been used to gain insight into the evolution history of globins of polar fish. The results indicate that Arctic and Antarctic globins have different phylogenies and lead us to suggest that the selective pressure of environment stability allows the phylogenetic signal to be maintained in the Antarctic sequences, whereas environmental variability would tend to disrupt this signal in the Gadidae sequences.

Cyanide Binding and Heme Cavity Conformational Transitions in Drosophila melanogaster Hexacoordinate Hemoglobin,

The reason for the presence of hemoglobin-like molecules in insects, such as Drosophila melanogaster, that live in fully aerobic environments has yet to be determined. Heme endogenous hexacoordination (where HisE7 and HisF8 axial ligands to the heme Fe atom are both provided by the protein) is a recently discovered mechanism proposed to modulate O(2) affinity in hemoglobins from different species. Previous results have shown that D. melanogaster hemoglobin 1 (product of the glob1 gene) displays heme endogenous hexacoordination in both the ferrous and ferric states. Here we present kinetic data characterizing the exogenous cyanide ligand binding process, and the three-dimensional structure (at 1.4 A resolution) of the ensuing cyano-met D. melanogaster hemoglobin. Comparison with the crystal structure of the endogenously hexacoordinated D. melanogaster hemoglobin shows that the transition to the cyano-met form is supported by conformational readjustment in the CD-D-E region of the protein, which removes HisE7 from the heme. The structural and functional features of D. melanogaster hemoglobin are examined in light of previous results achieved for human and mouse neuroglobins and for human cytoglobin, which display heme endogenous hexacoordination. The study shows that, despite the rather constant value for cyanide association rate constants for the ferric hemoproteins, different distal site conformational readjustments and/or heme sliding mechanisms are displayed by the known hexacoordinate hemoglobins as a result of exogenous ligand binding.

Proton-linked subunit heterogeneity in ferrous nitrosylated human adult hemoglobin: an EPR study

The effect of pH on the X-band electron paramagnetic resonance (EPR) spectrum of ferrous nitrosylated human adult tetrameric hemoglobin (HbNO) as well as of ferrous nitrosylated monomeric alpha- and beta-chains has been investigated, at -163 degrees C. At pH 7.3, the X-band EPR spectrum of tetrameric HbNO and ferrous nitrosylated monomeric alpha- and beta-chains displays a rhombic shape. Lowering the pH from 7.3 to 3.0, tetrameric HbNO and ferrous nitrosylated monomeric alpha- and beta-chains undergo a transition towards a species characterized by a X-band EPR spectrum with a three-line splitting centered at 334mT. These pH-dependent spectroscopic changes may be taken as indicative of the cleavage, or the severe weakening, of the proximal HisF8-Fe bond. In tetrameric HbNO, the pH-dependent spectroscopic changes depend on the acid-base equilibrium of two apparent ionizing groups with pK(a) values of 5.8 and 3.8. By contrast, the pH-dependent spectroscopic changes occurring in ferrous nitrosylated monomeric alpha- and beta-chains depend on the acid-base equilibrium of one apparent ionizing group with pK(a) values of 4.8 and 4.7, respectively. The different pK(a) values for the proton-linked spectroscopic transition(s) of tetrameric HbNO and ferrous nitrosylated monomeric alpha- and beta-chains suggest that the quaternary assembly drastically affects the strength of the proximal HisF8-Fe bond in both subunits. This probably reflects a 'quaternary effect', i.e., structural changes in both subunits upon tetrameric assembly, which is associated to a relevant variation of functional properties (i.e., proton affinity).

Nitrosylation of rabbit ferrous heme-hemopexin

Hemopexin (HPX) serves as a trap for toxic plasma heme, ensuring its complete clearance by transportation to the liver. Moreover, HPX-heme has been postulated to play a key role in the homeostasis of nitric oxide (NO). Here, the thermodynamics for NO binding to rabbit ferrous HPX-heme as well as the EPR and optical absorption spectroscopic properties of rabbit ferrous nitrosylated HPX-heme (HPX-heme-NO) are reported. The value of the dissociation equilibrium constant for NO binding to rabbit ferrous HPX-heme (i.e., H) is (1.4+/-0.2)x10(-7) M, at pH 7.0 and 10.0 degrees C; the value of H is unaffected by sodium chloride. At pH 7.0, rabbit ferrous HPX-heme-NO is a six-coordinate heme-iron species, characterized by an X-band EPR spectrum with an axial geometry and by epsilon=146 mM(-1) cm(-1) at 419 nm. At pH 4.0, rabbit ferrous HPX-heme-NO is a five-coordinate heme-iron species, characterized by an X-band EPR spectrum with three-line splitting centered at 334 mT and by epsilon=74 mM(-1) cm(-1) at 387 nm. The p K(a) value of the reversible pH-induced six- to five-coordinate spectroscopic transition is 4.8+/-0.1 in the absence of sodium chloride and 4.3+/-0.1 in the presence of 1.5x10(-1) M sodium chloride. This result is in agreement with the effect of sodium chloride on rabbit HPX-heme stability. The present data have been analyzed in parallel with those of a related heme model compound and heme-protein systems.

The functionally distinct hemoglobins of the Arctic spotted wolffish Anarhichas minor

The Arctic fish Anarhichas minor, a benthic sedentary species, displays high hemoglobin multiplicity. The three major hemoglobins (Hb 1, Hb 2, and Hb 3) show important functional differences in pH and organophosphate regulation, subunit cooperativity, and response of oxygen binding to temperature. Hb 1 and Hb 2 display a low, effector-enhanced Bohr effect and no Root effect. In contrast, Hb 3 displays pronounced Bohr and Root effects, accompanied by strong organophosphate regulation. Hb 1 has the beta (beta(1)) chain in common with Hb 2; Hb 3 and Hb 2 share the alpha (alpha(2)) chain. The amino acid sequences have been established. Several substitutions in crucial positions were observed, such as Cys in place of C-terminal His in the beta(1) chain of Hb 1 and Hb 2. In Hb 3, Val E11 of the beta(2) chain is replaced by Ile. Homology modeling revealed an unusual structure of the Hb 3 binding site of inositol hexakisphoshate. Phylogenetic analysis indicated that only Hb 2 displays higher overall similarity with the major Antarctic hemoglobins. The oxygen transport system of A. minor differs remarkably from those of Antarctic Notothenioidei, indicating distinct evolutionary pathways in the regulatory mechanisms of the fish respiratory system in the two polar environments.

Anticooperative ligand binding properties of recombinant ferric Vitreoscilla homodimeric hemoglobin: a thermodynamic, kinetic and X-ray crystallographic study

Thermodynamics and kinetics for cyanide, azide, thiocyanate and imidazole binding to recombinant ferric Vitreoscilla sp. homodimeric hemoglobin (Vitreoscilla Hb) have been determined at pH 6.4 and 7.0, and 20.0 degrees C, in solution and in the crystalline state. Moreover, the three-dimensional structures of the diligated thiocyanate and imidazole derivatives of recombinant ferric Vitreoscilla Hb have been determined by X-ray crystallography at 1.8 A (Rfactor=19.9%) and 2.1 A (Rfactor=23.8%) resolution, respectively. Ferric Vitreoscilla Hb displays an anticooperative ligand binding behaviour in solution. This very unusual feature can only be accounted for by assuming ligand-linked conformational changes in the monoligated species, which lead to the observed 300-fold decrease in the affinity of cyanide, azide, thiocyanate and imidazole for the monoligated ferric Vitreoscilla Hb with respect to that of the fully unligated homodimer. In the crystalline state, thermodynamics for azide and imidazole binding to ferric Vitreoscilla Hb may be described as a simple process with an overall ligand affinity for the homodimer corresponding to that for diligation in solution. These data suggest that the ligand-free homodimer, observed in the crystalline state, is constrained in a low affinity conformation whose ligand binding properties closely resemble those of the monoligated species in solution. From the kinetic viewpoint, anticooperativity is reflected by the 300-fold decrease of the second-order rate constant for cyanide and imidazole binding to the monoligated ferric Vitreoscilla Hb with respect to that for ligand association to the ligand-free homodimer in solution. On the other hand, values of the first-order rate constant for cyanide and imidazole dissociation from the diligated and monoligated derivatives of ferric Vitreoscilla Hb in solution are closely similar. As a whole, ligand binding and structural properties of ferric Vitreoscilla Hb appear to be unique among all Hbs investigated to date.