Autoxidation of oxymyoglobin: a meeting point of the stabilization and the activation of molecular oxygen

Potential role for Streptococcus gordonii-derived hydrogen peroxide in heme acquisition by Porphyromonas gingivalis

Streptococcus gordonii, an accessory pathogen and early colonizer of plaque, co-aggregates with many oral species including Porphyromonas gingivalis. It causes α-hemolysis on blood agar, a process mediated by H₂ O₂ and thought to involve concomitant oxidation of hemoglobin (Hb). Porphyromonas gingivalis has a growth requirement for heme, which is acquired mainly from Hb. The paradigm for Hb heme acquisition involves the initial oxidation of oxyhemoglobin (oxyHb) to methemoglobin (metHb), followed by heme release and extraction through the actions of K-gingipain protease and/or the HmuY hemophore-like protein. The ability of S. gordonii to mediate Hb oxidation may potentially aid heme capture during co-aggregation with P. gingivalis. Hemoglobin derived from zones of S. gordonii α-hemolysis was found to be metHb. Generation of metHb from oxyHb by S. gordonii cells was inhibited by catalase, and correlated with levels of cellular H₂ O₂ production. Generation of metHb by S. gordonii occurred through the higher Hb oxidation state of ferrylhemoglobin. Heme complexation by the P. gingivalis HmuY was employed as a measure of the ease of heme capture from metHb. HmuY was able to extract iron(III)protoporphyrin IX from metHb derived from zones of S. gordonii α-hemolysis and from metHb generated by the action of S. gordonii cells on isolated oxyHb. The rate of HmuY-Fe(III)heme complex formation from S. gordonii-mediated metHb was greater than from an equivalent concentration of auto-oxidized metHb. It is concluded that S. gordonii may potentially aid heme acquisition by P. gingivalis by facilitating metHb formation in the presence of oxyHb.

Heme acquisition mechanisms of Porphyromonas gingivalis - strategies used in a polymicrobial community in a heme-limited host environment

Porphyromonas gingivalis, a main etiologic agent and key pathogen responsible for initiation and progression of chronic periodontitis requires heme as a source of iron and protoporphyrin IX for its survival and the ability to establish an infection. Porphyromonas gingivalis is able to accumulate a defensive cell-surface heme-containing pigment in the form of μ-oxo bisheme. The main sources of heme for P. gingivalis in vivo are hemoproteins present in saliva, gingival crevicular fluid, and erythrocytes. To acquire heme, P. gingivalis uses several mechanisms. Among them, the best characterized are those employing hemagglutinins, hemolysins, and gingipains (Kgp, RgpA, RgpB), TonB-dependent outer-membrane receptors (HmuR, HusB, IhtA), and hemophore-like proteins (HmuY, HusA). Proteins involved in intracellular heme transport, storage, and processing are less well characterized (e.g. PgDps). Importantly, P. gingivalis may also use the heme acquisition systems of other bacteria to fulfill its own heme requirements. Porphyromonas gingivalis displays a novel paradigm for heme acquisition from hemoglobin, whereby the Fe(II)-containing oxyhemoglobin molecule must first be oxidized to methemoglobin to facilitate heme release. This process not only involves P. gingivalis arginine- and lysine-specific gingipains, but other proteases (e.g. interpain A from Prevotella intermedia) or pyocyanin produced by Pseudomonas aeruginosa. Porphyromonas gingivalis is then able to fully proteolyze the more susceptible methemoglobin substrate to release free heme or to wrest heme from it directly through the use of the HmuY hemophore.

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.

Role of the cysteine protease interpain A of Prevotella intermedia in breakdown and release of haem from haemoglobin

The gram-negative oral anaerobe Prevotella intermedia forms an iron(III) protoporphyrin IX pigment from haemoglobin. The bacterium expresses a 90 kDa cysteine protease, InpA (interpain A), a homologue of Streptococcus pyogenes streptopain (SpeB). The role of InpA in haemoglobin breakdown and haem release was investigated. At pH 7.5, InpA mediated oxidation of oxyhaemoglobin to hydroxymethaemoglobin [in which the haem iron is oxidized to the Fe(III) state and which carries OH- as the sixth co-ordinate ligand] by limited proteolysis of globin chains as indicated by SDS/PAGE and MALDI (matrix-assisted laser-desorption ionization)-TOF (time-of-flight) analysis. Prolonged incubation at pH 7.5 did not result in further haemoglobin protein breakdown, but in the formation of a haemoglobin haemichrome (where the haem Fe atom is co-ordinated by another amino acid ligand in addition to the proximal histidine residue) resistant to degradation by InpA. InpA-mediated haem release from hydroxymethaemoglobin-agarose was minimal compared with trypsin at pH 7.5. At pH 6.0, InpA increased oxidation at a rate greater than auto-oxidation, producing aquomethaemoglobin (with water as sixth co-ordinate ligand), and resulted in its complete breakdown and haem loss. Aquomethaemoglobin proteolysis and haem release was prevented by blocking haem dissociation by ligation with azide, whereas InpA proteolysis of haem-free globin was rapid, even at pH 7.5. Both oxidation of oxyhaemoglobin and breakdown of methaemoglobin by InpA were inhibited by the cysteine protease inhibitor E-64 [trans-epoxysuccinyl-L-leucylamido-(4-guanidino)butane]. In summary, we conclude that InpA may play a central role in haem acquisition by mediating oxyhaemoglobin oxidation, and by degrading aquomethaemoglobin in which haem-globin affinity is weakened under acidic conditions.

The ferrous-dioxygen intermediate in human cytochrome P450 3A4. Substrate dependence of formation and decay kinetics

The oxy-ferrous complex is the first of three branching intermediates in the catalytic cycle of cytochrome P450, in which the total efficiency of substrate turnover is curtailed by the side reaction of autoxidation. For human membrane-bound cytochromes P450, the oxy complex is believed to be the primary source of cytotoxic superoxide and peroxide, although information on the properties and stability of this intermediate is lacking. Here we document stopped-flow spectroscopic studies of the formation and decay of the oxy-ferrous complex in the most abundant human cytochrome P450 (CYP3A4) as a function of temperature in the substrate-free and substrate-bound form. CYP3A4 solubilized in purified monomeric form in nanoscale POPC bilayers is functionally and kinetically homogeneous. In substrate-free CYP3A4, the oxy complex is extremely unstable with a half-life of approximately 30 ms at 5 degrees C. Saturation with testosterone or bromocriptine stabilizes the oxy-ferrous intermediate. Comparison of the autoxidation rates with the available data on CYP3A4 turnover kinetics suggests that the oxy complex may be an important route for uncoupling.

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.

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 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.

Myoglobin-catalyzed tyrosine nitration: no need for peroxynitrite

The nitration of tyrosine residues in protein to yield 3-nitrotyrosine derivatives has been suggested to represent a specific footprint for peroxynitrite formation in vivo. However, recent studies suggest that certain hemoproteins such as peroxidases catalyze the H(2)O(2)-dependent nitration of tyrosine to yield 3-nitrotyrosine in a peroxynitrite-independent reaction. Because 3-nitrotyrosine has been shown to be present in the postischemic myocardium, we wished to assess the ability of myoglobin to catalyze the nitration of tyrosine in vitro. We found that myoglobin catalyzed the oxidation of nitrite and promoted the nitration of tyrosine. Both nitrite oxidation and tyrosine nitration were H(2)O(2)-dependent and required the formation of ferryl (Fe(+4)) myoglobin. In addition, nitrite oxidation and tyrosine nitration were pH-dependent with a pH optimum of approximately 6.0. Taken together, these data suggest that the acidic pH and low oxygen tension produced during myocardial ischemia will facilitate myoglobin-catalyzed, peroxyntrite-independent formation of 3-nitrotyrosine.

Comparative Analysis of Autoxidation of Haemoglobin

Autoxidation of oxyhaemoglobin (oxyHb) to methaemoglobin was measured at different temperatures in haemoglobin solutions from Atlantic hagfish, river lamprey, common carp, yellowfin tuna and pig. The aims were to evaluate the impact of the absent distal histidine in hagfish haemoglobin, the importance of oxyHb being either monomeric (hagfish and lamprey) or tetrameric (carp, tuna and pig) and to gain information on the temperature-sensitivity of autoxidation. The rate of autoxidation was lower in hagfish than in carp, yellowfin tuna and lamprey haemoglobins at any given temperature. Substitution of the distal histidine residue (His E7) with glutamine in hagfish haemoglobin was therefore not associated with an accelerated autoxidation, as might be expected on the basis of the normal protective role of His E7. Glutamine may have similar qualities to histidine and be involved in the low susceptibility to autoxidation. The low oxidation rate of hagfish haemoglobin, together with an oxidation rate of lamprey haemoglobin that did not differ from that of carp and yellowfin tuna haemoglobins, also revealed that autoxidation was not accelerated in the monomeric oxyhaemoglobins. Pig haemoglobin was oxidised more slowly than fish haemoglobins, demonstrating that fish haemoglobins are more sensitive to autoxidation than mammalian haemoglobins. The rate of autoxidation of hagfish haemoglobin was, however, only significantly greater than that of pig haemoglobin at high temperatures. Autoxidation was accelerated by rising temperature in all haemoglobins. Arrhenius plots of carp and yellowfin tuna haemoglobin revealed a break at 25°C, reflecting a lower temperature-sensitivity between 5 and 25°C than between 25 and 40°C.

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

The oxygenated form of myoglobin or hemoglobin is oxidized easily to the ferric met-form with generation of the superoxide anion. To make clear the possible role(s) of the distal histidine (H64) residue in the reaction, we have carried out detailed pH-dependence studies of the autoxidation rate, using some typical H64 mutants of sperm whale myoglobin, over the wide range of pH 5-12 in 0.1 M buffer at 25 degrees C. Each mutation caused a dramatic increase in the autoxidation rate with the trend H64V >/= H64G >/= H64L >> H64Q > H64 (wild-type) at pH 7.0, whereas each mutant protein showed a characteristic pH-profile which is essentially different from that of the wild-type or native sperm whale MbO2. In particular, all the mutants have lost the acid-catalyzed process that can play a dominant role in the autoxidation reaction of most mammalian myoglobins or hemoglobins. Kinetic analyses of various types of pH-profiles lead us to conclude that the distal histidine residue can play a dual role in the nucleophilic displacement of O2- from MbO2 or HbO2 in protic, aqueous solution. One is in a proton-relay mechanism via its imidazole ring, and the other is in the maximum protection of the FeO2 center against a water molecule or an hydroxyl ion that can enter the heme pocket from the surrounding solvent.

Role of globin moiety in the autoxidation reaction of oxymyoglobin: effect of 8 M urea

It is in the ferrous form that myoglobin or hemoglobin can bind molecular oxygen reversibly and carry out its function. To understand the possible role of the globin moiety in stabilizing the FeO2 bond in these proteins, we examined the autoxidation rate of bovine heart oxymyoglobin (MbO2) to its ferric met-form (metMb) in the presence of 8 M urea at 25 degrees C and found that the rate was markedly enhanced above the normal autoxidation in buffer alone over the whole range of pH 5-13. Taking into account the concomitant process of unfolding of the protein in 8 M urea, we then formulated a kinetic procedure to estimate the autoxidation rate of the unfolded form of MbO2 that might appear transiently in the possible pathway of denaturation. As a result, the fully denatured MbO2 was disclosed to be extremely susceptible to autoxidation with an almost constant rate over a wide range of pH 5-11. At pH 8.5, for instance, its rate was nearly 1000 times higher than the corresponding value of native MbO2. These findings lead us to conclude that the unfolding of the globin moiety allows much easier attack of the solvent water molecule or hydroxyl ion on the FeO2 center and causes a very rapid formation of the ferric met-species by the nucleophilic displacement mechanism. In the molecular evolution from simple ferrous complexes to myoglobin and hemoglobin molecules, therefore, the protein matrix can be depicted as a breakwater of the FeO2 bonding against protic, aqueous solvents.

Decomposition of hydrogen peroxide by metmyoglobin: a cyclic formation of the ferryl intermediate

1. Metmyoglobin reacted with H2O2 to form ferryl-myoglobin, which reverted back spontaneously to the met-form. 2. Through this cyclic reaction of myoglobin between metMb(III) and ferryl-Mb(IV), we proposed that H2O2, one of the potent oxidants in vivo, can be decomposed continuously in cardiac and skeletal muscle tissues in the absence of catalase and peroxidase.

Hydrogen peroxide plays a key role in the oxidation reaction of myoglobin by molecular oxygen. A computer simulation

The stability properties of the iron(II)-dioxygen bond in myoglobin and hemoglobin are of particular importance, because both proteins are oxidized easily to the ferric met-form, which cannot be oxygenated and is therefore physiologically inactive. In this paper, we have formulated all the possible pathways leading to the oxidation of myoglobin to metmyoglobin with each required rate constant in 0.1 M buffer (pH 7.0) at 25 degrees C, and have set up six rate equations for the elementary processes going on in a simultaneous way. By using the Runge-Kutta method to solve these differential equations, the concentration progress curves were then displayed for all the reactive species involved. In this complex reaction, the primary event was the autoxidation of MbO2 to metMb with generation of the superoxide anion, this anion being converted immediately and almost completely into H2O2 by the spontaneous dismutation. Under air-saturated conditions (PO2 = 150 Torr), the H2O2 produced was decomposed mostly by the metMb resulting from the autoxidation of MbO2. At lower pressures of O2, however, H2O2 can act as the most potent oxidant of the deoxyMb, which increases with decreasing O2 pressures, so that there appeared a well defined maximum rate in the formation of metMb at approximately 5 Torr of oxygen. Such examinations with the aid of a computer provide us, for the first time, with a full picture of the oxidation reaction of myoglobin as a function of oxygen pressures. These results also seem to be of primary importance from a point of view of clinical biochemistry of the oxygen supply, as well as of pathophysiology of ischemia, in red muscles such as cardiac and skeletal muscle tissues.