Dual nature of the distal histidine residue in the autoxidation reaction of myoglobin and hemoglobin comparison of the H64 mutants

Synthetic heme protein models that function in aqueous solution

Supramolecular porphyrin–cyclodextrin complexes act as biomimetic heme protein models in aqueous solution.

Roles of N- and C-terminal domains in the ligand-binding properties of cytoglobin

Cytoglobin (Cygb) is a member of the hexacoordinated globin protein family and is expressed ubiquitously in rat and human tissues. Although Cygb is reportedly upregulated under hypoxic conditions both in vivo and in vitro, suggesting a physiological function to protect cells under hypoxic/ischemic conditions by scavenging reactive oxygen species or by signal transduction, the mechanisms associated with this function have not been fully elucidated. Recent studies comparing Cygbs among several species suggest that mammalian Cygbs show a distinctly longer C-terminal domain potentially involved in unique physiological functions. In this study, we prepared human Cygb mutants (ΔC, ΔN, and ΔNC) with either one or both terminal domains truncated and investigated the enzymatic functions and structural features by spectroscopic methods. Evaluation of the superoxide-scavenging activity between Cygb variants showed that the ΔC and ΔNC mutants exhibited slightly higher activity involving superoxide scavenging as compared with wild-type Cygb. Subsequent experiments involving ligand titration, flash photolysis, and resonance Raman spectroscopic studies suggested that the truncation of the C- and N-terminal domains resulted in less effective to dissociation constants and binding rates for carbon monoxide, respectively. Furthermore, structural stability was assessed by guanidine hydrochloride and revealed that the C-terminal domain might play a vital role in improving structure, whereas the N-terminal domain did not exert a similar effect. These findings indicated that long terminal domains could be important not only in regulating enzymatic activity but also for structural stability, and that the domains might be relevant to other hypothesized physiological functions for Cygb.

A method for extracting carboxy-myoglobin from beef

The present study was conducted to devise a method for the effective extraction of carboxy-myoglobin (COMb) from beef without carbon monoxide dissociation. The ratio of COMb to myoglobin was computed at absorptions of wavelengths 541 and 551 nm, which characterize COMb and the isosbestic point between COMb and deoxy-myoglobin, respectively. The COMb extraction rate was found to vary with temperature, pH and oxygen conditions. The decrease observed in this rate was inversely proportional to the rise in extraction temperature. The COMb extraction rate was also affected by pH, and the stability of COMb in the extract solution was the highest at pH 8.0-9.0. Moreover, the presence of oxygen was found to disturb COMb extraction. According to these results, nearly all COMb could be extracted from carbon-monoxide-treated beef under stirring conditions in pH 8.5 deoxidized buffer, at 1°C, and under N₂ flow with the improved extraction method in this study (98.1 ± 2.7%). The decrement of COMb in the extract was accelerated by light, and the COMb was stable for 20 min in the dark, at 1°C. The extraction conditions for COMb described above should allow the accurate evaluation of COMb in meat tissue.

Isolation and autoxidation profile of fish myoglobin from hoki (Macruronus magellanicus)

Myoglobin is known to be present exclusively in cardiac and red skeletal muscles, but not in white skeletal muscles. Thus, to date, only a few studies on myoglobin from fish species with white flesh have been reported. For comparative examination, we directly isolated myoglobin from cardiac muscle of hoki (Macruronus magellanicus), one of the most important commercial fish species with white muscle. The ferrous myoglobin was separated from its ferric met-form by anion exchange column chromatography. The absorption spectra of hoki myoglobin were similar to those of bigeye tuna skeletal myoglobin, in both oxy- and met-forms. However, hoki oxymyoglobin was found to be susceptible to autoxidation in 0.1 M buffer (pH 7.2) at 25°C, with its rate being more than 3 times higher than that of bigeye tuna oxymyoglobin.

Unlocking the binding and reaction mechanism of hydroxyurea substrates as biological nitric oxide donors

Hydroxyurea is the only FDA approved treatment of sickle cell disease. It is believed that the primary mechanism of action is associated with the pharmacological elevation of nitric oxide in the blood; however, the exact details of this are still unclear. In the current work, we investigate the atomic level details of this process using a combination of flexible-ligand/flexible-receptor virtual screening coupled with energetic analysis that decomposes interaction energies. Utilizing these methods, we were able to elucidate the previously unknown substrate binding modes of a series of hydroxyurea analogs to hemoglobin and the concomitant structural changes of the enzyme. We identify a backbone carbonyl that forms a hydrogen bond with bound substrates. Our results are consistent with kinetic and electron paramagnetic resonance (EPR) measurements of hydroxyurea-hemoglobin reactions, and a full mechanism is proposed that offers new insights into possibly improving substrate binding and/or reactivity.

Ligand changes in ferric species of the giant extracellular hemoglobin ofGlossoscolex paulistusas function of pH: correlations between redox, spectroscopic and oligomeric properties and general implications with different hemoproteins

The present review is focused on the relationship between oligomeric and heme properties of HbGp, emphasizing the characteristics that can be generalized to other hemoproteins. This study represents the state-of-the-art with respect to the approaches for investigating giant extracellular hemoglobins as well as the correlation between oligomeric assembly alterations and their consequent changes in the first coordination sphere. A wide introduction focused on the properties of this hemoglobin is developed. Indeed, this hemoprotein is considered an interesting prototype of blood substitute and biosensor due to its peculiar properties, such as resistance to autoxidation and oligomeric stability. Previous studies by our group employing UV-vis, EPR and CD spectroscopies have been revised in a complete approach, in agreement with recent and relevant data from the literature. In fact, a consistent and inter-related spectroscopic study is described propitiating a wide assignment of "fingerprint" peaks found in the techniques evaluated in this paper. This review furnishes physicochemical information regarding the identification of ferric heme species of hemoproteins and metallic complexes through their spectroscopic bands. This effort at the attribution of UV-vis, EPR and CD peaks is not restricted to HbGp, and includes a comparative analysis of several hemoproteins involving relevant implications regarding several types of iron-porphyrin systems.

Autoxidation of giant extracellular hemoglobin of Glossoscolex paulistus: molecular mechanism and oligomeric implications

Giant extracellular hemoglobins present high redox stability due to their supramolecular architecture, high number of polypeptide chains and great compaction of protein subunits. The oligomeric assembly and the changes in the polypeptidic structure can influence the autoxidation rate of the heme proteins, being that different nucleophiles can act in this process due to pH alterations. In the present work, we have studied the autoxidation rate of whole Glossoscolex paulistus (HbGp) giant extracellular hemoglobin, as well as the autoxidation rate of the isolated d monomer of HbGp studied regarding pH variations. The kinetic decay behavior is dependent on pH, presenting mono-exponential or bi-exponential character, depending on the oligomeric state of the protein. Thus, the oligomeric dissociation in specific pH values demonstrated a bi-exponential kinetic decay. A mono-exponential kinetic behavior was verified in the pH range of 5.9-7.3, which is assigned to the native whole protein. In alkaline medium, the presence of hydroxide ions leads the autoxidation of whole hemoglobin to a complex behavior, which is described by the combination of two first-order kinetics. The slow process occurs due to the d monomer autoxidation. At pH 7.0, the kinetic is mono-exponential, indicating a highly conserved oligomeric structure. In acid medium, the proton-catalyzed autoxidation occurs both on the whole hemoglobin and in the d monomer. It has been found that proximal and distal histidines develop determinant roles regarding the autoxidation rate, being that the distal histidine controls the contact of ligands with the ferrous center through a very interesting "swinging door" mechanism. Despite the significant sensitivity of the distal histidine to the presence of protons, water molecules and anions, the influence of chemical changes around the heme, such as pH changes, is much more effective in hemoproteins without this amino acid as distal residue. This fact denotes the ability of HbGp to adapt to environmental disturbances caused by the presence of the distal histidine, which is responsible for the great redox and oligomeric stabilities encountered in HbGp.

The redox activity of hemoglobins: from physiologic functions to pathologic mechanisms

Pentacoordinate respiratory hemoproteins such as hemoglobin and myoglobin have evolved to supply cells with oxygen. However, these respiratory heme proteins are also known to function as redox enzymes, reacting with compounds such as nitric oxide and peroxides. The recent discoveries of hexacoordinate hemoglobins in vertebrates and nonsymbiotic plants suggest that the redox activity of globins is inherent to the molecule. The uncontrolled formation of radical species resulting from such redox chemistry on respiratory hemoproteins can lead to oxidative damage and cellular toxicity. In this review, we examine the functions of various globins and the mechanisms by which these globins act as redox enzymes under physiologic conditions. Evidence that redox reactions also occur under disease conditions, leading to pathologic complications, also is examined, focusing on recent discoveries showing that the ferryl oxidation state of these hemoproteins is present in these disease states in vivo. In addition, we review the latest advances in the understanding of globin redox mechanisms and how they might affect cellular signaling pathways and how they might be controlled therapeutically or, in the case of hemoglobin-based blood substitutes, through rational design.

Structure-Function Relationships in Unusual Nonvertebrate Globins

Based on the literature and our own results, this review summarizes the most recent state of nonvertebrate myoglobin (Mb) and hemoglobin (Hb) research, not as a general survey of the subject but as a case study. For this purpose, we have selected here four typical globins to discuss their unique structures and properties in detail. These include Aplysia myoglobin, which served as a prototype for the unusual globins lacking the distal histidine residue; midge larval hemoglobin showing a high degree of polymorphism; Tetrahymena hemoglobin evolved with a truncated structure; and yeast flavohemoglobin carrying an enigmatic two-domain structure. These proteins are not grouped by any common features other than the fact they have globin domains and heme groups. As a matter of course, various biochemical functions other than the conventional oxygen transport or storage have been proposed so far to these primitive or ancient hemoglobins or myoglobins, but the precise in vivo activity is still unclear. In this review, special emphasis is placed on the stability properties of the heme-bound O2. Whatever the possible roles of nonvertebrate myoglobins and hemoglobins may be (or might have been), the binding of molecular oxygen to iron(II) must be the primary event to manifest their physiological functions in vivo. However, the reversible and stable binding of O2 to iron(II) is not a simple process, since the oxygenated form of Mb or Hb is oxidized easily to its ferric met-form with the generation of superoxide anion. The metmyoglobin or methemoglobin thus produced cannot bind molecular oxygen and is therefore physiologically inactive. In this respect, protozoan ciliate myoglobin and yeast flavohemoglobin are of particular interest in their very unique structures. Indeed, both proteins have been found to have completely different strategies for overcoming many difficulties in the reversible and stable binding of molecular oxygen, as opposed to the irreversible oxidation of heme iron(II). Such comparative studies of the stability of MbO2 or HbO2 are of primary importance, not only for a full understanding of the globin evolution, but also for planning new molecular designs for synthetic oxygen carriers that may be able to function in aqueous solution and at physiological temperature.

DevS oxy complex stability identifies this heme protein as a gas sensor in Mycobacterium tuberculosis dormancy

DevS is one of the two sensing kinases responsible for DevR activation and the subsequent entry of Mycobacterium tuberculosis into dormancy. Full-length wild-type DevS forms a stable oxy-ferrous complex. The DevS autoxidation rates are extremely low (half-lives of >24 h) in the presence of cations such as K(+), Na(+), Mg(2+), and Ca(2+). At relatively high concentrations (100 mM), Cu(2+) accelerates autoxidation more than 1500-fold. Contrary to expectations, removal of the key hydrogen bond between the iron-coordinated oxygen and Tyr171 in the Y171F mutant provides a protein of comparable stability to autoxidation and similar oxygen dissociation rate. This correlates with our earlier finding that the Y171F mutant and wild-type kinase activities are similarly regulated by the binding of oxygen: namely, the ferrous five-coordinate complex is active, whereas the oxy-ferrous six-coordinate species is inactive. Our results indicate that DevS is a gas sensor in vivo rather than a redox sensor and that the stability of its ferrous-oxy complex is enhanced by interdomain interactions.

The pH dependence of the activity of dehaloperoxidase from Amphitrite ornata

Dehaloperoxidase (DHP) from the terebellid polychaete, Amphitrite ornata, is the first hemoglobin that has peroxidase activity as part of its native function. The substrate 2,4,6-tribromophenol (TBP) is oxidatively debrominated by DHP to form 2,6-dibromoquinone (DBQ) in a two-electron process. There is a well-defined internal binding site for TBP above the heme, a feature not observed in other hemoglobins or peroxidases. A study of the pH dependence of the activity of DHP reveals a substantial difference in mechanism. From direct observation of the Soret band of the heme it is shown that the pKa for heme activation in protein DHP is 6.5. Below this pH the heme absorbance decreases in the presence of H2O2 with or without addition of substrate. The low pH data are consistent with significant heme degradation. Above pH 6.5 addition of H2O2 causes the heme to shift rapidly to a compound II spectrum and then slowly to an unidentified intermediate with an absorbance of 410 nm. However, the pKa of the substrate TBP is 6.8 and the greatest enzyme activity is observed above the pKa of TBP under conditions where the substrate is a phenolate anion (TPBO-). Although the mechanisms may differ, the data show that both neutral TBP and anionic TPBO- are converted to the quinone product. The mechanistic implications of the pH dependence are discussed by comparison other known peroxidases, which oxidize substrates at the heme edge.

Critical role of His369 in the reactivity of Pseudomonas aeruginosa cytochrome cd1nitrite reductase with oxygen

In the denitrification pathway, Pseudomonas aeruginosa cytochrome cd1 nitrite reductase catalyzes the reduction of nitrite to nitric oxide; in vitro, this enzyme is also competent in the reduction of O2 to 2H2O. In this article, we present a comparative kinetic study of the O2 reaction in the wild-type nitrite reductase and in three site-directed mutants (Tyr10-->Phe, His369-->Ala and His327-->Ala/His369-->Ala) of the amino acid residues close to the d1 heme on the distal side. The results clearly indicate that His369 is the key residue in the control of reactivity, as its substitution with Ala, previously shown to affect the reduction of nitrite, also impairs the reaction with O2, affecting both the properties and lifespan of the intermediate species. Our findings allow the presentation of an overall picture for the reactivity of cytochrome cd1 nitrite reductase and extend our previous conclusion that the conserved distal histidines are essential for the binding to reduced d1 heme of different anions, whether a substrate such as nitrite, a ligand such as cyanide, or an intermediate in the O2 reduction. Moreover, we propose that His369 also exerts a protective role against degradation of the d1 heme, by preventing the formation and adverse effects of the reactive O2 species (never present in significant amounts in wild-type cytochrome cd1 nitrite reductase), a finding with clear physiological implications.

Functional and structural characterization of the myoglobin from the polychaete Ophelia bicornis

The myoglobin of the polychaete annelid Ophelia bicornis was isolated, purified to homogeneity and characterized. The primary structure, obtained from cDNA and protein sequencing, consists of 139 amino acid residues. The alignment with other globin sequences showed that O. bicornis myoglobin misses the pre-A helix and the first six residues of the A helix. The presence of a PheB10-GlnE7 haem distal residue pair is in agreement with the measured oxygen affinity (P50=0.85 mmHg; 1 mmHg=0.133 kPa) and the only slightly higher autoxidation rate constant (0.28 h(-1)) with respect to that of the sperm whale myoglobin mutant E7 His-->Gln (0.21 h(-1)) and to elephant myoglobin (0.1 h(-1)). Oxygen-binding co-operativity was found to be absent under all the examined experimental conditions. The resistance of O. bicornis myoglobin towards autoxidation seems to confirm the important role of part of the A helix in the stability of the globin. The higher pKa of the acid-alkaline ferric transition of O. bicornis with respect to Asian elephant myoglobin, as well as the higher absorbance ratio of its ferric form to the oxy form measured in the Soret region (gammamet/gammaoxy) with respect to that of the African elephant myoglobin, suggested a stronger interaction between the distal glutamine and the water molecule at the sixth co-ordinate position.

Nature of the FeO2 bonding in myoglobin and hemoglobin: A new molecular paradigm

The iron(II)-dioxygen bond in myoglobin and hemoglobin is a subject of wide interest. Studies range from examinations of physical-chemical properties dependent on its electronic structure, to investigations of the stability as a function of oxygen supply. Among these, stability properties are of particular importance in vivo. Like all known dioxygen carriers synthesized so far with transition metals, the oxygenated forms of myoglobin and hemoglobin are known to be oxidized easily to their ferric met-forms, which cannot bind molecular oxygen and are therefore physiologically inactive. The mechanistic details of this autoxidation reaction, which are of clinical, as well as of physical-chemical, interest, have long been investigated by a number of authors, but a full understanding of the heme oxidation has not been reached so far. Recent kinetic and thermodynamic studies of the stability of oxymyoglobin (MbO2) and oxyhemoglobin (HbO2) have revealed new features in the FeO2 bonding. In vivo, the iron center is always subject to a nucleophilic attack of the water molecule or hydroxyl ion, which can enter the heme pocket from the surrounding solvent and thereby irreversibly displace the bound dioxygen from MbO2 or HbO2 in the form of O2- so that the iron is converted to the ferric met-form. Since the autoxidation reaction of MbO2 or HbO2 proceeds through a nucleophilic displacement following one-electron transfer from iron(II) to the bound O2, this reaction may be viewed as a meeting point of the stabilization and the activation of molecular oxygen performed by hemoproteins. Along with these lines of evidence, we finally discuss the stability property of human HbO2 and provide with the most recent state of hemoglobin research. The HbA molecule contains two types of alphabeta contacts and seems to differentiate them quite properly for its functional properties. The alpha1beta2 or alpha2beta1 contact is associated with the cooperative oxygen binding, whereas the alpha1beta1 or alpha2beta2 contact is used for controlling the stability of the bound O2. We can thus form a unified picture for hemoglobin function by closely integrating the cooperative and the stable binding of molecular oxygen with iron(II) in aqueous solvent. These new views on the nature of FeO2 bonding and the possible role of globin moiety in stabilizing MbO2 and HbO2 are of primary importance, not only for a full understanding of various hemoprotein reactions with O2, but also for planning new molecular designs for synthetic oxygen carriers which may be able to function in aqueous solvent and at physiological temperature.

A Biophysical Investigation of Recombinant Hemoglobins with Aromatic B10 Mutations in the Distal Heme Pockets,

This study examines the structural and functional effects of amino acid substitutions in the distal side of both the alpha- and beta-chain heme pockets of human normal adult hemoglobin (Hb A). Using our Escherichia coli expression system, we have constructed four recombinant hemoglobins: rHb(alphaL29F), rHb(alphaL29W), rHb(betaL28F), and rHb(betaL28W). The alpha29 and beta28 residues are located in the B10 helix of the alpha- and beta-chains of Hb A, respectively. The B10 helix is significant because of its proximity to the ligand-binding site. Previous work showed the ability of the L29F mutation to inhibit oxidation. rHb(alphaL29W), rHb(betaL28F), and rHb(betaL28W) exhibit very low oxygen affinity and reduced cooperativity compared to those of Hb A, while the previously studied rHb(alphaL29F) exhibits high oxygen affinity. Proton nuclear magnetic resonance spectroscopy indicates that these mutations in the B10 helix do not significantly perturb the alpha(1)beta(1) and alpha(1)beta(2) subunit interfaces, while as expected, the tertiary structures near the heme pockets are affected. Experiments in which visible spectrophotometry was utilized reveal that rHb(alphaL29F) has equivalent or slower rates of autoxidation and azide-induced oxidation than does Hb A, while rHb(alphaL29W), rHb(betaL28F), and rHb(betaL28W) have increased rates. Bimolecular rate constants for NO-induced oxidation have been determined using a stopped-flow apparatus. These findings indicate that amino acid residues in the B10 helix of the alpha- and beta-chains can play different roles in regulating the functional properties and stability of the hemoglobin molecule. These results may provide new insights for designing a new generation of hemoglobin-based oxygen carriers.

Autoxidation studies of extracellular hemoglobin of Glossoscolex paulistus at pH 9: cyanide and hydroxyl effect

The complex oligomeric assembly of the hemoglobin subunits may influence the autoxidation rate. To understand this relation, the rate of autoxidation was studied at pH 9.0, where the Glossoscolex paulistus Hemoglobin (GpHb) dissociates. At alkaline pH, this hemoglobin is dissociated into monomers, trimers and tetramers, allowing the study of the integral protein and monomer subunit autoxidation on independent experiments. The autoxidation rate was evaluated in the presence and absence of cyanide (CN(-)), a strong field ligand to the ferric ion. The oxidation kinetic was monitored using the UV-vis absorption at 415 nm, and resulted in: i) bi-exponential kinetics for the whole hemoglobin (indicating a fast and a slow oxidative process) and ii) mono-exponential for the monomer (indicating a single process). To understand the specific characteristics of each autoxidation process, Arrhenius plots allowed the determination of the activation energy. The experimental results indicate for the whole hemoglobin in the absence of CN(-) an activation energy of 150 +/- 10 kJ mol(-1) for the fast and the slow processes. Under the same conditions the monomer displayed an activation energy of 160 +/- 10 kJ mol(-1), very close to the value obtained for the integral protein. The pseudo-second order rate constant for the whole protein autoxidation by CN(-) showed two different behaviors characterized by a rate constant k(CN1)' = 0.11 +/- 0.02 s(-1) mol(-1) L for CN(-) concentrations lower than 0.012 mol L(-1); and k(CN1)" = 0.76 +/- 0.04 s(-1) mol(-1) L at higher concentrations for the fast process, while the slow process remain constant with k(CN2) = 0.033 +/- 0.002 s(-1) mol(-1) L. The monomer has a characteristic rate constant of 0.041 +/- 0.002 s(-1) mol(-1) L for all cyanide concentrations. Comparing the results for the slow process of the whole hemoglobin and the oxidation of the monomer, it is possible to infer that the slow process has a strong contribution of the monomer in the whole hemoglobin kinetic. Moreover, as disulfide linkers sustain the trimer assembly, cooperativity may explain the higher kinetic constant for this subunit.

Human myoglobin recognition of oxygen: dynamics of the energy landscape

Femtosecond to nanosecond dynamics of O(2) rebinding to human WT myoglobin and its mutants, V68F and I107F, have been studied by using transient absorption. The results are compared with NO rebinding. Even though the immediate environment around the heme binding site is changed by the mutations, the picosecond geminate rebinding of oxygen is at most minimally affected. On the other hand, the V68F (E11) mutation causes drastic differences in rebinding on the nanosecond time scale, whereas the effect of the I107F (G8) mutation remains relatively small within our 10-ns time window. Unlike traditional homogeneous kinetics and molecular dynamics collisional simulations, we propose a "bifurcation model" for populations of directed and undirected dynamics on the ultrafast time scale, reflecting the distribution of initial protein conformations. The major mutation effect occurs on the time scale on which global protein conformational change is possible, consistent with transitions between the conformations of directed and undirected population playing a role in the O(2) binding. We discuss the relevance of these findings to the bimolecular function of the protein.

Human haemoglobin: a new paradigm for oxygen binding involving two types of alphabeta contacts

This review summarizes the most recent state of haemoglobin (Hb) research based on the literature and our own results. In particular, an attempt is made to form a unified picture for haemoglobin function by reconciling the cooperative oxygen binding with the stabilization of the bound dioxygen in aqueous solvent. The HbA molecule contains two types of alphabeta contacts. One type is the alpha1beta2 or alpha2beta1 contacts, called sliding contacts, and these are strongly associated with the cooperative binding of O2 to the alpha2beta2 tetramer. The other type is the alpha1beta1 or alpha2beta2 contacts, called packing contacts, but whose role in Hb function was not clear until quite recently. However, detailed pH-dependence studies of the autoxidation rate of HbO2 have revealed that the alpha1beta1 and alpha2beta2 interfaces are used for controlling the stability of the bound O2. When the alpha1beta1 or alpha2beta2 contact is formed, the beta chain is subjected to a conformational constraint which causes the distal (E7) histidine to be tilted slightly away from the bound dioxygen, preventing the proton-catalysed nucleophilic displacement of O2- from the FeO2 by an entering water molecule. This is one of the most characteristic features of HbO2 stability. Finally we discuss the role of the alpha1beta1 or alpha2beta2 contacts by providing some examples of unstable haemoglobin mutants. These pathological mutations are found mostly on the beta chain, especially in the alpha1beta1 contact regions. In this way, HbA seems to differentiate two types of alphabeta contacts for its functional properties.

The swinging movement of the distal histidine residue and the autoxidation reaction for midge larval hemoglobins

Some insects have a globin exclusively in their fast-growing larval stage. This is the case in the 4th-instar larva of Tokunagayusurika akamusi, a common midge found in Japan. In the polymorphic hemoglobin comprised of 11 separable components, hemoglobin VII (Ta-VII Hb) was of particular interest. When its ferric met-form was exposed to pH 5.0 from 7.2, the distal histidine was found to swing away from the E7 position. As a result, the iron(III) was converted from a hexacoordinate to a pentacoordinate form by a concomitant loss of the axial water ligand. The corresponding spectral changes in the Soret band were therefore followed by stopped-flow and rapid-scan techniques, and the observed first-order rate constants of k(out) = 25 s(-1) and kin = 128 s(-1) were obtained for the outward and inward movements, respectively, of the distal histidine residue in 0.1 m buffer at 25 degrees C. For O2 affinity, Ta-VII Hb showed a value of P50 = 1.7 Torr at pH 7.4, accompanied with a remarkable Bohr effect (deltaH+ = -0.58) almost equal to that of mammalian hemoglobins. We have also investigated the stability property of Ta-VII HbO2 in terms of the autoxidation rate over a wide range of pH from 4 to 11. The resulting pH-dependence curve was compared with those of another component Ta-V HbO2 and sperm whale MbO2, and described based on a nucleophilic displacement mechanism. In light of the O2 binding affinity, Bohr effect and considerable stability of the bound O2 against acidic autoxidation, we conclude that T. akamusi Hb VII can play an important role in O2 transport and storage as the major component in the larval hemolymph.

Yeast flavohemoglobin from Candida norvegensis. Its structural, spectral, and stability properties

Flavohemoglobin was isolated directly from the yeast Candida norvegensis and studied on its structural, spectral, and stability properties. In Candida flavohemoglobin, the 155 N-terminal residues make a heme-containing domain, while the remaining 234 C-terminal residues serve as a FAD-containing reductase domain. A pair of His-95 and Gln-63 was assigned to the proximal and distal residues, respectively. In purification procedure FAD was partially dissociated on a Butyl-Toyopearl column, so that FAD-lacking flavohemoglobin was also obtainable. In this ferric species, the Soret and charge-transfer bands were all characteristic of a penta-coordinate form. Compared with the recombinant heme domain expressed in Escherichia coli, we have measured the autoxidation rate over a wide pH range. The resulting pH dependence curves were then analyzed in terms of a nucleophilic displacement mechanism. As a result, the heme domain was found to be extremely susceptible to autoxidation, its rate being more than 100 times higher than that of sperm whale MbO2. However, this inherently high oxidation rate was dramatically suppressed in Candida flavohemoglobin to an extent almost comparable to the stability of mammalian myoglobins. These new findings lead us to conclude that Candida flavohemoglobin, differently from bacterial flavohemoglobins, can serve as an oxygen storage protein in aerobic conditions.

The alpha 1 beta 1 contact of human hemoglobin plays a key role in stabilizing the bound dioxygen

When the alpha and beta chains were separated from human oxyhemoglobin (HbO(2)), each individual chain was oxidized easily to the ferric form, their rates being almost the same with a very strong acid-catalysis. In the HbO(2) tetramer, on the other hand, both chains become considerably resistant to autoxidation over a wide range of pH values (pH 5-11). Moreover, HbA showed a biphasic autoxidation curve containing the two rate constants, i.e. k(f) for the fast oxidation due to the alpha chains, and k(s) for the slow oxidation to the beta chains. The k(f)/k(s) ratio increased from 3.2 at pH 7.5-7.3 at pH 5.8, but became 1 : 1 at pH values higher than 8.5. In the present work, we used the valency hybrid tetramers such as (alpha(3+))2(beta O(2))(2) and (alpha O(2)(2)(beta(3+))(2), and demonstrated that the autoxidation rate of either the alpha or beta chains (when O2- ligated) is independent of the valency state of the corresponding counterpart chains. From these results, we have concluded that the formation of the alpha 1 beta 1 or alpha 2 beta 2 contact suppresses remarkably the autoxidation rate of the beta chain and thus plays a key role in stabilizing the HbO(2) tetramer. Its mechanistic details were also given in terms of a nucleophilic displacement of O(2)(-) from the FeO(2) center, and the emphasis was placed on the proton-catalyzed process performed by the distal histidine residue.