Preparation of myoglobins

Biophysical and mass spectrometry based characterization of methylglyoxal-modified myoglobin: Role of advanced glycation end products in inducing protein structural alterations

Methylglyoxal (MG) is a highly reactive α-dicarbonyl compound which reacts with proteins to form advanced glycation end products (AGEs). MG-induced AGE (MAGE) formation is particularly significant in diabetic condition. In the current study, we have undertaken a time-dependant characterization of MG-modified myoglobin following incubation of the heme protein with the α-dicarbonyl compound for different time periods. Interestingly, mass spectrometric studies indicated modifications at two specific lysine residues, Lys-87 and Lys-133. The AGE adducts identified at Lys-87 were carboxymethyllysine and carboxyethyllysine, while those detected at Lys-133 included pyrraline-carboxymethyllysine and carboxyethyllysine, respectively. Far-UV CD studies revealed a decrease in the native α-helical content of the heme protein gradually with increasing time of MG incubation. In addition, MG modification was found to induce changes in tertiary structure as well as surface hydrophobicity of the heme protein. MG-derived AGE adducts thus appear to alter the structure of Mb considerably. Considering the increased level of MG in diabetic condition, the current study appears physiologically relevant in terms of understanding AGE-mediated protein modification and subsequent structural changes.

Common and unique strategies of myoglobin evolution for deep-sea adaptation of diving mammals

Myoglobin (Mb) is highly concentrated in the myocytes of diving mammals such as whales and seals, in comparison with land animals, and its molecular evolution has played a crucial role in their deep-sea adaptation. We previously resurrected ancestral whale Mbs and demonstrated the evolutional strategies for higher solubility under macromolecular crowding conditions. Pinnipeds, such as seals and sea lions, are also expert diving mammals with Mb-rich muscles. In the present study, we resurrected ancestral pinniped Mbs and investigated their biochemical and structural properties. Comparisons between pinniped and whale Mbs revealed the common and distinctive strategies for the deep-sea adaptation. The overall evolution processes, gaining precipitant tolerance and improving thermodynamic stability, were commonly observed. However, the strategies for improving the folding stability differed, and the pinniped Mbs exploited the shielding of hydrophobic surfaces more effectively than the whale Mbs.

Functional inhibition of redox regulated heme proteins: A novel mechanism towards oxidative stress induced by homocysteine

Homocysteine (Hcy) is a sulfur containing non-protein toxic amino acid synthesized from methionine. Elevated level of Hcy is associated with cardiovascular complications and neurodegeneration. Hcy is believed to induce organ damage and apoptosis via oxidative stress. The pro-oxidant nature of Hcy is considered to originate from the metal-induced oxidation of thiol group-containing molecules forming disulfides (Hcy-Hcy, Hcy-cysteine, Hcy-glutathione, etc) or with free cysteine residues of proteins (a process called protein S-homocysteinylation). Formation of such disulfides indeed results in the generation of reactive oxygen species (ROS) which eventually leads to loss of cellular integrity. In the present manuscript, we performed systematic investigation of the effect of Hcy on iron containing proteins. We discover a novel mechanism of Hcy toxicity wherein Hcy oxidation is linked with the functional loss of the protein with iron as cofactors. Our results indicate that redox regulated heme proteins might be primarily involved in the Hcy toxicity and associated oxidative stress.

Formation of Pentosidine Cross-Linking in Myoglobin by Glyoxal: Detection of Fluorescent Advanced Glycation End Product

Glyoxal, a reactive α-oxoaldehyde, increases in diabetic condition and reacts with proteins to form advanced glycation end products (AGEs) following Maillard-like reaction. Considering the significance of protein modification by glyoxal-derived AGEs, we investigated the in vitro effect of glyoxal (200 μM) on the monomeric heme protein myoglobin (Mb) (100 μM) after incubation for one week at 25 °C. Glyoxal-treated Mb exhibited increased absorbance around the Soret region, decreased α-helicity and thermal stability compared to control Mb. Intrinsic fluorescence spectrum of the treated Mb showed an additional signal in the 400-500 nm region on excitation at 280 nm that was absent in control Mb. When excited at 335 nm, the glyoxal-treated sample gave a strong fluorescence indicating AGE formation. Mass spectrometric studies revealed formation of glyoxal-derived fluorescent AGE adduct pentosidine between Lys-145 and Arg-139 residues of Mb. Other than pentosidine, additional AGE adducts, namely, carboxymethyllysine at Lys-133, hydroimidazolone at Arg-31 and pyrrolidone-carboxymethyllysine at Lys-145 were also detected. Lys-145 was thus found to contain two different types of AGE adducts, indicating the heterogeneous nature of in vitro glycation reaction. AGE-induced protein modifications might be associated with complications in disease conditions.

Methylglyoxal-induced modification causes aggregation of myoglobin

Post-translational modification of proteins by Maillard reaction, known as glycation, is thought to be the root cause of different complications, particularly in diabetes mellitus and age-related disorders. Methylglyoxal (MG), a reactive α-oxoaldehyde, increases in diabetic condition and reacts with proteins to form advanced glycation end products (AGEs) following Maillard-like reaction. We have investigated the in vitro effect of MG (200μM) on the monomeric heme protein myoglobin (Mb) (100μM) in a time-dependent manner (7 to 18days incubation at 25°C). MG induces significant structural alterations of the heme protein, including heme loss, changes in tryptophan fluorescence, decrease of α-helicity with increased β-sheet content etc. These changes occur gradually with increased period of incubation. Incubation of Mb with MG for 7days results in formation of the AGE adducts: carboxyethyllysine at Lys-16, carboxymethyllysine at Lys-87 and carboxyethyllysine or pyrraline-carboxymethyllysine at Lys-133. On increasing the period of incubation up to 14days, additional AGEs namely, carboxyethyllysine at Lys-42 and hydroimidazolone or argpyrimidine at Arg-31 and Arg-139 have been detected. MG also induces aggregation of Mb, which is clearly evident with longer period of incubation (18days), and appears to have amyloid nature. MG-derived AGEs may thus have an important role as the precursors of protein aggregation, which, in turn, may be associated with physiological complications.

In VitroNonenzymatic Glycation Enhances the Role of Myoglobin as a Source of Oxidative Stress

Metmyoglobin (Mb) was glycated by glucose in a non-enzymatic in vitro reaction. Amount of iron release from the heme pocket of myoglobin was found to be directly related with the extent of glycation. After in vitro glycation, the unchanged Mb and glycated myoglobin (GMb) were separated by ion exchange (BioRex 70) chromatography, which eliminated free iron from the protein fractions. Separated fractions of Mb and GMb were converted to their oxy forms -MbO2 and GMbO2, respectively. H2O2-induced iron release was significantly higher from GMbO2 than that from MbO2. This free iron, acting as a Fenton reagent, might produce free radicals and degrade different cell constituents. To verify this possibility, degradation of different cell constituents catalyzed by these fractions in the presence of H2O2 was studied. GMbO2 degraded arachidonic acid, deoxyribose and plasmid DNA more efficiently than MbO2. Arachidonic acid peroxidation and deoxyribose degradation were significantly inhibited by desferrioxamine (DFO), mannitol and catalase. However, besides free iron-mediated free radical reactions, role of iron of higher oxidation states, formed during interaction of H2O2 with myoglobin might also be involved in oxidative degradation processes. Formation of carbonyl content, an index of oxidative stress, was higher by GMbO2. Compared to MbO2, GMbO2 was rapidly autooxidized and co-oxidized with nitroblue tetrazolium, indicating increased rate of Mb and superoxide radical formation in GMbO2. GMb exhibited more peroxidase activity than Mb, which was positively correlated with ferrylmyoglobin formation in the presence of H2O2. These findings correlate glycation-induced modification of myoglobin and a mechanism of increased formation of free radicals. Although myoglobin glycation is not significant within muscle cells, free myoglobin in circulation, if becomes glycated, may pose a serious threat by eliciting oxidative stress, particularly in diabetic patients.

Evidences of xenon-induced structural changes in the active site of cyano-metmyoglobins: a 1H NMR study

Using xenon atoms as a biomolecular probe raises the concern of whether they may influence in some way the molecular and electronic structure of the system under study. In this paper, the relevance of guest-host interactions in xenon complexes with paramagnetic myoglobins (Mbs) is thoroughly analyzed, and the issue about the use of xenon to detect and characterize voids within flexible biomolecules is critically discussed. A detailed 1H NMR study useful for describing the hydrophobic cavities close to the active site of low-spin ferric myoglobins with respect to their interaction with the xenon atom is presented. The method is subsequently validated by the analysis of Xe-Mb with two different myoglobins, extracted from horse and pig. These myoglobins differ by 14 amino acids. One of these, Ile142 in horse Mb, is located in the proximal cavity, which is the main xenon binding site in horse Mb, and is replaced by Met142 in pig Mb. We demonstrated specific behaviors associated with the capacity of each of the two myoglobins to bind xenon and provided site-specific information on the host-guest interaction. Moreover, 1H NMR measurements produce a picture of xenon-related local distortions of the protein, associated with a functionally relevant residue located right at the active site, the proximal hystidine E7(His93). According to the 1H NMR data, xenon induces the tilt of the residue His93 relative to the heme plane and consequently causes an alteration of the magnetic axes. Similar conclusions are obtained both for pig cyano-myoglobin and for horse cyano-myoglobin, the structural deformation being in the former of minor entity.

Resonance Raman interrogation of the consequences of heme rotational disorder in myoglobin and its ligated derivatives

Resonance Raman spectroscopy is employed to characterize heme site structural changes arising from conformational heterogeneity in deoxyMb and ligated derivatives, i.e., the ferrous CO (MbCO) and ferric cyanide (MbCN) complexes. The spectra for the reversed forms of these derivatives have been extracted from the spectra of reconstituted samples. Dramatic changes in the low-frequency spectra are observed, where newly observed RR modes of the reversed forms are assigned using protohemes that are selectively deuterated at the four methyl groups or at the four methine carbons. Interestingly, while substantial changes in the disposition of the peripheral vinyl and propionate groups can be inferred from the dramatic spectral shifts, the bonds to the internal histidyl imidazole ligand and those of the Fe-CO and Fe-CN fragments are not significantly affected by the heme rotation, as judged by lack of significant shifts in the nu(Fe-N(His)), nu(Fe-C), and nu(C-O) modes. In fact, the apparent lack of an effect on these key vibrational parameters of the Fe-N(His), Fe-CO, and Fe-CN fragments is entirely consistent with previously reported equilibrium and kinetic studies that document virtually identical functional properties for the native and reversed forms.

Myoglobin-enhanced oxygen delivery to isolated cardiac mitochondria

The heart, red skeletal muscles and the nitrogen-fixing legume root nodule function in steady states of high oxygen influx, partial oxygenation of cytoplasmic myoglobin or leghemoglobin and correspondingly low oxygen partial pressure. Here, we ask: what conditions are required at the surface of actively respiring, state III, tightly coupled mitochondria to enhance oxygen flow to cytochrome oxidase? Pigeon heart mitochondria were isolated with minimal damage to the outer mitochondrial membrane and were incubated at low oxygen pressures, where respiration is oxygen limited, with solutions of each of six monomeric hemoglobins with widely divergent kinetics and equilibria in their reactions with oxygen: Busycon myoglobin, horse myoglobin, Lucina hemoglobins I and II, soybean leghemoglobin c and Gasterophilus hemoglobin. Each augments oxygen uptake. The declining fractional saturation of each hemoglobin with oxygen was monitored spectrophotometrically as mitochondrial respiration depleted the oxygen; the oxygen partial pressure at half-maximal rate of oxygen uptake was similar for each hemoglobin, supporting the conclusion that the hemoglobins did not interact with the mitochondrial surface in oxygen delivery. The oxygen pressure required to support state III mitochondrial oxygen uptake, 0.005 kPa (0.04 torr), is small compared with that obtained in the sarcoplasm and at the mitochondrial surface of the working heart, 0.32 kPa (2.4 torr). We conclude that, in normal steady states of contraction of the myoglobin-containing heart, oxygen utilization by mitochondrial cytochrome oxidase is not limited by oxygen availability.

Dual probe fluorescence monitoring of intracellular free calcium during ischemia in mouse heart by using continuous compensation for pH dependence of the dissociation constant of Fura-2, and the interference of myoglobin

Mitochondrial damage is the main source of cellular injury upon ischemia-reperfusion, and calcium loading has been implicated in this phenomenon. The use of optical probes for calcium monitoring of the intact heart is hampered by internal filter effects of intracellular hemoproteins, endogenous fluorescence, and their sensitivity to pH. We describe here a method for measurement of intracellular free calcium in isolated myoglobin-deficient perfused mouse hearts under conditions of large intracellular pH fluctuations by simultaneous fluorescence monitoring of the calcium-probe Fura-2 and the pH probe BCECF through dual wavelength excitation of both probes. In myoglobin-containing mouse heart endogenous chromophores interfere with Fura-2 fluorometry. It is shown that a paradoxical decrease in Fura-2 fluorescence occurs during ischemia in isolated mouse hearts. Simultaneous recording of BCECF fluorescence (calibrated against pH measurement with phosphorus NMR) and data reduction based on continual recalculation of the apparent dissociation constant of the calcium-probe complex revealed that a marked increase in intracellular free calcium occurs, and that the Fura-2 fluorescence decrease was caused by an increase in dissociation constant due to intracellular acidification. Intracellular free calcium rose almost linearly during a 20-min period of ischemia and returned to basal values rapidly upon the commencement of perfusion.

Molecular characterization of two myoglobins of Paragonimus westermani

Myoglobins (Mbs), globin proteins, are present in high concentrations in trematodes. In Paragonimus westermani, 2 cDNAs were found to encode Mbs. The first clone, Pwmyo1, codes a total of 149 amino acids with a calculated mass of 16.6 kDa. The second, Pwmyo2, encodes a 146-amino acid protein with a calculated mass of 16.2 kDa. The predicted secondary structures showed the presence of 8 helices, which is the basic characteristic of Mbs. Sequence alignment revealed a high homology with the other trematode Mbs. The 2 clones contained the characteristic tyrosyl residues at helical positions B10 and distal E7, which are substitutions that have been previously shown to contribute to the high oxygen affinity of Mbs. Polyclonal antibodies against the recombinant Mbs were raised with no cross-reactivity observed. Immunolocalization revealed the proteins to be distributed generally throughout the parenchymal tissues, but absent from the tegument and reproductive organs. The cell mass of the eggs of the worm stained positive to Pwmyo2 but not Pwmyo1, suggesting the stage-specific expression of these Mbs.

Effect of non-enzymatic glycation on esterase activities of hemoglobin and myoglobin

Heme proteins--hemoglobin and myoglobin possess esterase activities. Studies with purified hemoglobin from normal individuals and diabetic patients revealed that the esterase activity as measured from hydrolysis of p-nitrophenyl acetate (p-NPA) was higher in diabetic condition and increased progressively with extent of the disease. HbA(1c), the major glycated hemoglobin, which increases proportionately with blood glucose level in diabetes mellitus, exhibited more esterase activity than the non-glycated hemoglobin fraction, HbA(0), as demonstrated spectrophotometrically as well as by activity staining. Glycation influenced esterase activity of hemoglobin by increasing the affinity for the substrate and the rate of the reaction. Both HbA(0) and HbA(1c)-mediated catalysis of p-NPA hydrolysis was pH-dependent. Esterase activity of in vitro-glycated myoglobin (GMb) was also higher than that of its non-glycated analog (Mb). The amplified esterase activities of hemoglobin and myoglobin might be associated with glycation-induced structural modifications of the proteins.

Mutations of the βN102 Residue of HbA Not Only Inhibit the Ligand-Linked T to Re State Transition, but Also Profoundly Affect the Properties of the T State Itself

The properties of three HbA variants with different mutations at the beta102 position, betaN102Q, betaN102T, and betaN102A, have been examined. All three are inhibited in their ligand-linked transition from the low affinity T quaternary state to the high affinity Re quaternary state. In the presence of inositol hexaphosphate, IHP, none of them exhibits cooperativity in the binding of oxygen. This is consistent with the destabilization of the Re state as a result of the disruption of the hydrogen bond that normally forms between the beta102 asparagine residue and the alpha94 aspartate residue in the Re state. However, these three substitutions also alter the properties of the T state of the hemoglobin tetramer. In the presence of IHP, the first two substitutions result in large increases in the ligand affinities of the beta-subunits within the T state structure. The betaN102A variant, however, greatly reduces the pH dependencies of the affinities of the alpha and beta subunits, K1(alpha) and K1(beta), respectively, for the binding of the first oxygen molecule in the absence of IHP. In the presence of IHP, the T state of this variant is strikingly similar to that of HbA under the same conditions. For both hemoglobins, K1(alpha) and K1(beta) exhibit only small Bohr effects. In the absence of IHP, the affinities of the alpha and beta subunits of HbA for the first oxygen are increased, and both exhibit greatly increased Bohr effects. However, in contrast to the behavior of HbA, the ligand-binding properties of the T state tetramer of the betaN102A variant are little affected by the addition or removal of IHP. It appears that along with its effect on the stability of the liganded Re state, this mutation has an effect on the T state that mimics the effect of adding IHP to HbA. It inhibits the set of conformational changes, which are coupled to the K1 Bohr effects and normally accompany the binding of the first ligand to the HbA tetramer in the absence of organic phosphates.

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.

Intersubunit interactions associated with Tyr42 alpha stabilize the quaternary-T tetramer but are not major quaternary constraints in deoxyhemoglobin

Previous mutational studies on Tyr42alpha variants as well as the current studies on the mutant hemoglobin alphaY42A show that the intersubunit interactions associated with Tyr42alpha significantly stabilize the alpha1beta2 interface of the quaternary-T deoxyhemoglobin tetramer. However, crystallographic studies, UV and visible resonance Raman spectroscopy, CO combination kinetic measurements, and oxygen binding measurements on alphaY42A show that the intersubunit interactions formed by Tyr42alpha have only a modest influence on the structural properties and ligand affinity of the deoxyhemoglobin tetramer. Therefore, the alpha1beta2 interface interactions associated with Tyr42alpha do not contribute significantly to the quaternary constraints that are responsible for the low oxygen affinity of deoxyhemoglobin. The slight increase in the ligand affinity of deoxy alphaY42A correlates with small, mutation-induced structural changes that perturb the environment of Trp37beta, a critical region of the quaternary-T alpha1beta2 interface that has been shown to be the major source of quaternary constraint in deoxyhemoglobin.

Protoporphyrin IX-induced structural and functional changes in human red blood cells, haemoglobin and myoglobin

Protoporphyrin IX and its derivatives are used as photosensitizers in the photodynamic therapy of cancer. Protoporphyrin IX penetrates into human red blood cells and releases oxygen from them. This leads to a change in the morphology of the cells. Spectrophotometric studies reveal that protoporphyrin IX interacts with haemoglobin and myoglobin forming ground state complexes. For both proteins, the binding affinity constant decreases, while the possible number of binding sites increases, as the aggregation state of the porphyrin is increased. The interactions lead to conformational changes of both haemoglobin and myoglobin as observed in circular dichroism studies. Upon binding with the proteins, protoporphyrin IX releases the heme-bound oxygen from the oxyproteins, which is dependent on the stoichiometric ratios of the porphyrin : protein. The peroxidase activities of haemoglobin and myoglobin are potentiated by the protein-porphyrin complexation. Possible mechanisms underlying the relation between the porphyrin-induced structural modifications of the heme proteins and alterations in their functional properties have been discussed. The findings may have a role in establishing efficacy of therapeutic uses of porphyrins as well as in elucidating their mechanisms of action as therapeutic agents.

Effects of systematic peripheral group deuteration on the low-frequency resonance Raman spectra of myoglobin derivatives

Resonance Raman spectra are reported for a series of systematically deuterated analogues of myoglobin in its deoxy state as well as for its CO and O(2) adducts. Specifically, the myoglobin samples studied are those that have been reconstituted with deuterated protoheme analogues. These include the methine deuterated, protoheme-d4; analogue bearing C(2)H(3) groups at the 1, 3, 5, and 8 positions, protoheme-d12; the species bearing C(2)H(3) groups at the 1 and 3 positions only, 1,3-protoheme-d6; and the species bearing C(2)H(3) groups at the 5 and 8 positions only, 5,8-protoheme-d6. While the results are generally consistent with previously reported data for synthetic metalloporphyrin models and previous studies of labeled heme proteins, the high-quality low-frequency RR data reported here reveal several important aspects of these low-frequency modes. Of special interest is the fact that, using the two d6-protoheme analogues, it is shown that certain modes are apparently localized on particular pyrrole rings, while others are localized on different rings; i.e., several of these low-frequency modes are localized on one side of the heme.

A pH-dependent aquomet-to-hemichrome transition in crystalline horse methemoglobin

In 1947, Perutz and co-workers reported that crystalline horse methemoglobin undergoes a large lattice transition as the pH is decreased from 7.1 to 5.4. We have determined the pH 7.1 and 5.4 crystal structures of horse methemoglobin at 1.6 and 2.1 A resolution, respectively, and find that this lattice transition involves a 23 A translation of adjacent hemoglobin tetramers as well as changes in alpha heme ligation and the tertiary structure of the alpha subunits. Specifically, when the pH is lowered from 7.1 to 5.4, the Fe(3+) alpha heme groups (but not the beta heme groups) are converted from the aquomet form, in which the proximal histidine [His87(F8)alpha] and a water molecule are the axial heme ligands, to the hemichrome (bishistidine) form, in which the proximal histidine and the distal histidine [His58(E7)alpha] are the axial heme ligands. Hemichrome formation is coupled to a large tertiary structure transition in the eight-residue segment Pro44(CD2)alpha-Gly51(D7)alpha that converts from an extended loop structure at pH 7.1 to a pi-like helix at pH 5.4. The formation of the pi helix forces Phe46(CD4)alpha out of the alpha heme pocket and into the interface between adjacent hemoglobin tetramers where it participates in crystal lattice contacts unique to the pH 5.4 structure. In addition, the transition from aquomet alpha subunits to bishistidine alpha subunits is accompanied by an approximately 1.2 A movement of the alpha heme groups to a more solvent-exposed position as well as the creation of a solvent channel from the interior of the alpha heme pocket to the outside of the tetramer. These changes and the extensive rearrangement of the crystal lattice structure allow the alpha heme group of one tetramer to make direct contact with an alpha heme group on an adjacent tetramer. These results suggest possible functional roles for hemichrome formation in vivo.

Highly asymmetric interactions between globin chains during hemoglobin assembly revealed by electrospray ionization mass spectrometry

Dynamics of bovine hemoglobin assembly was investigated by monitoring monomers/oligomers equilibria in solution with electrospray ionization mass spectrometry and circular dichroism spectroscopy. Intensities of ionic signals corresponding to various protein species (tetramers, dimers, heme-deficient dimers, as well as apo- and holo-monomers) were used to estimate relative fractions of these species in solution as a function of pH. The fraction of folded protein for each observed species was estimated based on charge-state distributions of corresponding ionic species in the mass spectra. The cumulative numbers (averaged across the entire protein population) were in good agreement with circular dichroism data at the Soret band and in the far-UV region, respectively. The mass spectral data confirm that hemoglobin dissociation involves a step where heme is first lost from the beta-chain of the alpha beta-dimer to form a heme-deficient dimeric species. This dimer dissociates further to produce a holo-alpha-chain and an apo-beta-chain. The former is tightly folded into a comparatively compact structure at neutral pH, while the latter always exhibits significant backbone disorder. Acidification of the protein solution to pH 4 leads to partial heme dissociation and significant increase of the backbone flexibility in the alpha-chains as well. Complete dissociation of the heme from the alpha-chains at a pH below 4 coincides with the total disappearance of the dimeric and tetrameric hemoglobin species from the mass spectra. The experimental data provide strong evidence that binding of a partially unstructured apo-beta-chain to a tightly folded holo-alpha-chain to form a heme-deficient dimer is the initial step of hemoglobin assembly. Such binding locks the beta-chain in a highly ordered conformation, which allows for an efficient heme acquisition, followed by docking of two hemoglobin dimers to form a tetrameric form of the protein. The asymmetry of the roles of the two chains in the assembly process is surprising, given a rather high sequence homology (ca. 43%) and highlights functional importance of intrinsic protein disorder. The study also demonstrates a tremendous potential of mass spectrometry as an analytical tool capable of elucidating protein interaction mechanisms in highly heterogeneous systems.

Position-dependent interactions between cysteine residues and the helix dipole

A protein model was developed for studying the interaction between cysteine residues and the helix dipole. Site-directed mutagenesis was used to introduce cysteine residues at the N-terminus of helix H in recombinant sperm whale myoglobin. Based on the difference in thiol pK(a) between folded proteins and an unfolded peptide, the energy of interaction between the thiolate and the helix dipole was determined. Thiolates at the N1 and N2 positions of the helix were stabilized by 0.3 kcal/mole and 0.7 kcal/mole, respectively. A thiolate at the Ncap position was stabilized by 2.8 kcal/mole, and may involve a hydrogen bond. In context with other studies, an experimentally observed helix dipole effect may be defined in terms of two distinct components. A charge-dipole component involves electrostatic interactions with peptide bond dipoles in the first two turns of the helix and affects residues at all positions of the terminus; a hydrogen bond component involves one or more backbone amide groups and is only possible at the capping position due to conformational restraints elsewhere. The nature and magnitude of the helix dipole effect is, therefore, position-dependent. Results from this model system were used to interpret cysteine reactivity in rodent hemoglobins and the thioredoxin family.

Myoglobin deficiency in the hearts of phylogenetically diverse temperate-zone fish species

Previous studies relying upon spectrophotometric methods reported low levels of myoglobin, an intracellular oxygen-binding protein, in oxidative muscles of some sluggish benthic fishes distributed throughout the North Atlantic Ocean. Using immunochemical techniques we show that myoglobin is not expressed in the heart ventricles of Cyclop terus lumpus (Cyclopteridae), Anarhichas lupus (Anarhichadidae), Macrozoarces americanus (Zoarcidae), and Lophius americanus (Lophiidae). Hemitripterus americanus (Hemitripteridae) expresses myoglobin at 2.3 ± 0.2 mg·g wet mass–1(mean ± SD). Myoglobin was not detected in oxidative skeletal muscle (pectoral adductor profundus) in either the white-hearted fishes examined or red-hearted H. americanus. Supporting these results, myoglobin messenger RNA was not detected in cardiac muscles of white-hearted fishes by means of either direct Northern blot analysis or by the reverse transcriptase – polymerase chain reaction followed by amplification of cDNA product. The partial cDNA sequence of H. americanus myoglobin was determined and shows 86.9% identity with a known teleost myoglobin cDNA from Chionodraco rastrospinosus. The 3' untranslated region of H. americanus is 255 nucleotides longer than the 3' untranslated region of C. rastrospinosus. Comparisons of the deduced amino acid sequence of H. americanus with those of other teleosts show 66.2% sequence identity with Cyprinus carpio, 74.6% with Scomber japonicus, and 80.3% with Thunnus albacares and C. rastrospinosus.

Mutational effects at the subunit interfaces of human hemoglobin: evidence for a unique sensitivity of the T quaternary state to changes in the hinge region of the alpha 1 beta 2 interface

A set of variant human hemoglobins, each with an Ala or Gly substitution at a single residue, has been prepared, and the kinetics of their reactions with carbon monoxide have been measured. This reaction is rate-limited by the binding of the first CO to the deoxygenated T state of the protein. The magnitudes of the effects of the mutations on CO combination vary widely, and, with the exception of beta Y145, the residues with the most significant effects on these kinetics are found in the hinge region of the alpha 1 beta 2 interface. Mixed-metal hybrids, with zinc protoporphyrin IX in place of heme on both alpha or both beta subunits, were prepared for beta W37E, beta W37A, alpha Y140G, and alpha Y140A, hinge region variants causing large kinetic changes, and for beta Y145G. Such hybrids permit measurements of the kinetics of CO binding to only the heme-containing alpha or beta subunits within the unliganded hemoglobin tetramer. Mutations at beta 37 and alpha 140 have global effects on the T state, increasing the rates of CO binding to both types of subunits. Mutation of beta Y145 has a large effect on the beta subunits in the deoxygenated T state, but very little effect on the alpha subunits. Oxygen equilibria measurements on the crystalline T state of beta W37E also indicate large affinity increases in both subunits of this variant. The overall oxygen equilibria of the variant hemoglobins in solution are sensitive to numerous variables besides the properties of the deoxygenated T state. In contrast to CO combination kinetics, the residues whose alterations cause the largest changes in overall oxygen equilibria in solution are scattered seemingly randomly within the alpha 1 beta 2 interface.

Interface sliding as illustrated by the multiple quaternary structures of liganded hemoglobin

Initial crystallographic studies suggested that fully liganded mammalian hemoglobin can adopt only a single quaternary structure, the quaternary R structure. However, more recent crystallographic studies revealed the existence of a second quaternary structure for liganded hemoglobin, the quaternary R2 structure. Since these quaternary structures can be crystallized, both must be energetically accessible structures that coexist in solution. Unanswered questions include (i) the relative abundance of the R and R2 structures under various solution conditions and (ii) whether other quaternary structures are energetically accessible for the liganded alpha(2)beta(2) hemoglobin tetramer. Although crystallographic methods cannot directly answer the first question, they represent the most direct and most accurate approach to answering the second question. We now have determined and refined three different crystal structures of bovine carbonmonoxyhemoglobin. These structures provide clear evidence that the dimer-dimer interface of liganded hemoglobin has a wide range of energetically accessible structures that are related to each other by a simple sliding motion. The dimer-dimer interface acts as a "molecular slide bearing" that allows the two alpha beta dimers to slide back and forth without greatly altering the number or the nature of the intersubunit contacts. Since the general stereochemical features of this interface are not unusual, it is likely that interface sliding of the kind displayed by fully liganded hemoglobin plays important structural and functional roles in many other protein assemblies.

Structure-Function Analysis of the Emulsifying and Interfacial Properties of Apomyoglobin and Derived Peptides

Apomyoglobin was chosen as a model to study the emulsifying properties of proteins. It was cleaved into three peptides using cyanogen bromide and these peptides were purified and tested for emulsifying and interfacial properties. Two of the peptides had improved emulsifying activity compared to the whole protein. The peptide (residues 1-55) with the highest emulsifying activity and whole apomyoglobin were studied further. The amount of protein or peptide adsorbed at the oil-water interface of an emulsion was measured and the surface area occupied per molecule was calculated. For apomyoglobin, at maximal surface coverage each molecule occupied a surface area of approximately 8 nm2. This is consistent with a packed monolayer, based on the approximate dimensions of apomyoglobin. For peptide (1-55), at maximal surface coverage each molecule occupied a surface area of approximately 3 nm2. This is consistent with the area that the two amphipathic alpha-helices which are potentially present in this molecule would cover, if they were aligned along the oil-water interface. The different structural characteristics of these molecules responsible for their emulsifying properties are discussed. Copyright 1999 Academic Press.

Inhibition of xanthine oxidase and xanthine dehydrogenase by nitric oxide. Nitric oxide converts reduced xanthine-oxidizing enzymes into the desulfo-type inactive form

Xanthine oxidase (XO) and xanthine dehydrogenase (XDH) were inactivated by incubation with nitric oxide under anaerobic conditions in the presence of xanthine or allopurinol. The inactivation was not pronounced in the absence of an electron donor, indicating that only the reduced enzyme form was inactivated by nitric oxide. The second-order rate constant of the reaction between reduced XO and nitric oxide was determined to be 14.8 +/- 1.4 M-1 s-1 at 25 degrees C. The inactivated enzymes lacked xanthine-dichlorophenolindophenol activity, and the oxypurinol-bound form of XO was partly protected from the inactivation. The absorption spectrum of the inactivated enzyme was not markedly different from that of the normal enzyme. The flavin and iron-sulfur centers of inactivated XO were reduced by dithionite and reoxidized readily with oxygen, and inactivated XDH retained electron transfer activities from NADH to electron acceptors, consistent with the conclusion that the flavin and iron-sulfur centers of the inactivated enzyme both remained intact. Inactivated XO reduced with 6-methylpurine showed no "very rapid" spectra, indicating that the molybdopterin moiety was damaged. Furthermore, inactivated XO reduced by dithionite showed the same slow Mo(V) spectrum as that derived from the desulfo-type enzyme. On the other hand, inactivated XO reduced by dithionite exhibited the same signals for iron-sulfur centers as the normal enzyme. Inactivated XO recovered its activity in the presence of a sulfide-generating system. It is concluded that nitric oxide reacts with an essential sulfur of the reduced molybdenum center of XO and XDH to produce desulfo-type inactive enzymes.

Hemoglobin and myoglobin content in muscles of broiler chickens

The two main heme proteins, hemoglobin and myoglobin, are important factors determining meat quality aspects such as color and hemorrhage. The extent of hemorrhage in muscle tissue can probably be determined by measuring the hemoglobin content. The objective of this study was twofold: 1) to develop a specific and reproducible method to quantify the hemoglobin and myoglobin content in muscle tissue of broiler chickens, and 2) to study the effect of hemorrhage on the hemoglobin content in muscle tissue. We tested several methods to determine the total heme, hemoglobin, and myoglobin content in broiler chicken muscles on their specificity, sensitivity, and reproducibility. Methods based on immunological techniques appeared to be very specific and sensitive. The results obtained applying these methods on muscle tissue extracts were, however, not reproducible due to concentration effects. A combination of spectrophotometric analysis of the total heme protein concentration and measurement of the myoglobin concentration, applying size exclusion chromatography, proved to be a reliable and reproducible method to determine the hemoglobin and myoglobin content in chicken muscles. The total heme, hemoglobin, and myoglobin contents were related to muscle type. Extensive hemorrhage increased the hemoglobin content.

The myoglobin of Emperor penguin (Aptenodytes forsteri): amino acid sequence and functional adaptation to extreme conditions

In the framework of a study on molecular adaptations of the oxygen-transport and storage systems to extreme conditions in Antarctic marine organisms, we have investigated the structure/function relationship in Emperor penguin (Aptenodytes forsteri) myoglobin, in search of correlation with the bird life style. In contrast with previous reports, the revised amino acid sequence contains one additional residue and 15 differences. The oxygen-binding parameters seem well adapted to the diving behaviour of the penguin and to the environmental conditions of the Antarctic habitat. Addition of lactate has no major effect on myoglobin oxygenation over a large temperature range. Therefore, metabolic acidosis does not impair myoglobin function under conditions of prolonged physical effort, such as diving.

Preparation and kinetic characterization of a series of betaW37 variants of human hemoglobin A: evidence for high-affinity T quaternary structures

Four variants of human beta globin in which the Trp at position 37 has been replaced with a Tyr, Ala, Gly, or Glu have been expressed in Escherichia coli. These globins have been combined with normal human alpha chains and heme to form tetrameric hemoglobin molecules. A technique for the preparation of alpha chain dimers, which are cross-linked between their alpha99 lysine residues, has been developed, and these alpha dimers were combined with two of the beta globins, betaW37G and betaW37E, to form the corresponding cross-linked variants. The kinetics of CO binding to the deoxygenated derivatives following rapid mixing and of CO rebinding following flash photolysis have been examined as functions of pH in the presence and absence of the organic phosphate inositol hexaphosphate, IHP. The kinetic measurements indicate that replacement of the tryptophan with other residues destabilizes the hemoglobin tetramer, resulting in considerable dissociation of even the deoxygenated hemoglobins into alphabeta dimers at micromolar protein concentrations. Substitutions at beta37 also alter the properties of the deoxygenated hemoglobin tetramer. The alteration of the functional properties of the T states of these variants as well as the tendency of the deoxygenated derivatives to dissociate into alphabeta dimers increases in the order HbA < betaW37Y < betaW37A < betaW37G < betaW37E. Stabilizing the betaW37G or betaW37E tetramers by addition of IHP or by cross-linking does not restore the normal functional properties of the T state. Measurements of the geminate rebinding of CO establish a kinetic difference between the normal R state tetramer and the alphabeta dimer consistent with quaternary enhancement, the greater affinity of oxygen for the R state tetramer than for the alphabeta dimer. Kinetics of geminate rebinding also suggest that quaternary enhancement may be altered by substitutions at the beta37 position.

High-resolution crystal structures of human hemoglobin with mutations at tryptophan 37beta: structural basis for a high-affinity T-state,

The high-resolution X-ray structures of the deoxy forms of four recombinant hemoglobins in which Trp37(C3)beta is replaced with Tyr (betaW37Y), Ala (betaW37A), Glu (betaW37E), or Gly (betaW37G) have been refined and analyzed with superposition methods that partition mutation-induced perturbations into quaternary structure changes and tertiary structure changes. In addition, a new cross-validation statistic that is sensitive to local changes in structure (a "local Rfree" parameter) was used as an objective measure of the significance of the tertiary structure changes. No significant mutation-induced changes in tertiary structure are detected at the mutation site itself for any of the four mutants studied. Instead, disruption of the intersubunit contacts associated with Trp37(C3)beta results in (1) a change in quaternary structure at the alpha1beta2 interface, (2) alpha subunit tertiary structure changes that are centered at Asp94(G1)alpha-Pro95(G2)alpha, (3) beta subunit tertiary structure changes that are located between residues Asp99(G1)beta and Asn102(G4)beta, (4) increased mobility of the alpha subunit COOH-terminal dipeptide, and (5) shortening of the Fe-Nepsilon2His(F8) bond in the alpha and beta subunits of the betaW37G and betaW37E mutants. In each case, the magnitude of the change in a particular structural parameter increases in the order betaW37Y < betaW37A < betaW37E approximately betaW37G, which corresponds closely to the degree of functional disruption documented in the preceding papers.

Non-oxidative modification of low density lipoprotein by ruptured myocytes

In this study, the interaction of ruptured cardiac myocytes with low density lipoprotein (LDL) has been investigated and the consequent extent of uptake by macrophages. The results show that lysate released from ruptured myocytes is capable of inducing LDL oxidation and that the resulting modified form is recognised and degraded by macrophages. Peroxyl radical scavengers inhibit the LDL oxidation but not the macrophage uptake suggesting that LDL can be modified by mechanisms that are independent of oxidative processes by intracellular constituents of cardiac myocytes.

Kinetic characterization of myoglobins from vertebrates with vastly different body temperatures

Fish myoglobins are structurally distinct from the previously characterized mammalian myoglobins. Teleost fishes express generally lower levels of myoglobin than those found in mammals. Although the oxygen binding affinity is essentially the same as mammalian myoglobins, oxygen dissociation rates and carbon monoxide combination rates of the teleost myoglobins studied are significantly faster. Thus, the kinetic parameters of myoglobin from two Antarctic teleost species, measured close to their body temperature of -1 degree C, are comparable to those of mammalian myoglobins with higher body temperatures. These data suggest myoglobins from Antarctic teleosts may function at extreme environmental temperatures.

Trematode myoglobins, functional molecules with a distal tyrosine

The myoglobins of two trematodes, Paramphistomum epiclitum and Isoparorchis hypselobagri, were isolated to homogeneity. The native molecules are monomeric with Mr 16,000-17,000 and pI 6.5-7.5. In each species, at least four different globin isoforms occur. Primary structure was determined at the protein level. The globin chains contain 147 amino acid residues. Although major determinants of the globin fold are conserved, characteristic substitutions are present. A Tyr residue occurs at the helical positions B10 and E7 (distal position). This is confirmed by NMR measurements (Zhang, W., Rashid, K. A., Haque, M., Siddiqi, A. H., Vinogradov, S. N., Moens, L. & La Mar, G. N. (1997) J. Biol. Chem. 272, 3000-3006). A distal Tyr normally provokes oxidation of the iron atom and the inability to bind oxygen, whereas a Tyr-B10 is indicative for a high oxygen affinity. In contrast, trematode myoglobins are functional molecules with a high oxygen affinity. Molecular modeling predicts two possible positions for the aromatic ring of Tyr-E7: one being outside the heme pocket making it freely accessible to the ligand and one within the heme pocket potentially able to form a second hydrogen bond with the iron-bound oxygen. A hydrogen bond between Tyr-B10 and the bound oxygen as in the Ascaris hemoglobin is predicted as well. The predicted structure may explain the high oxygen affinity of the trematode myoglobins.

Crystal structure of Asian elephant (Elephas maximus) cyano-metmyoglobin at 1.78-A resolution. Phe29(B10) accounts for its unusual ligand binding properties

The crystal structure of Asian elephant cyano-metmyoglobin which has a glutamine instead of the usual distal site histidine has been determined to high resolution. In addition to this replacement, the substitution of a conserved leucine residue in position 29(B10) at the distal side by a phenylalanine was unambiguously identified based on the available electron density. The suspicion, that there were errors in the original sequence which has caused some confusion, is thus confirmed. Comparison with other myoglobin structures in various ligated forms reveals an essentially unchanged tertiary structure in elephant myoglobin despite the two amino acid substitutions in the heme pocket. Our current structural model shows that the N epsilon 2 atom of Gln64(E7) has moved with respect to the corresponding nitrogen position of His64(E7) in the CO complex of sperm whale myoglobin. The newly assigned residue Phe29(B10) penetrates into the distal side of the heme pocket approaching the ligand within van der Waals distance and causing a much more crowded heme pocket compared to other myoglobins. Kinetic properties of Asian elephant myoglobin, wild type, and recombinant sperm whale myoglobins are discussed in relation to the structural consequences of the two amino acid substitutions H64Q and L29F.

Structure-dynamics-function relationships in Asian elephant (Elephas maximus) myoglobin. An optical spectroscopy and flash photolysis study on functionally important motions

In this work we report the thermal behavior (10-300 K) of the Soret band lineshape of deoxy and carbonmonoxy derivatives of Asian elephant (Elephas maximus) and horse myoglobins together with their carbon monoxide recombination kinetics after flash photolysis; the results are compared to analogous data relative to sperm whale myoglobin. The Soret band profile is modeled as a Voigt function that accounts for the coupling with high and low frequency vibrational modes, while inhomogeneous broadening is taken into account with suitable distributions of purely electronic transition frequencies. This analysis makes it possible to isolate the various contributions to the overall lineshape that; in turn, give information on structural and dynamic properties of the systems studied. The optical spectroscopy data point out sizable differences between elephant myoglobin on one hand and horse and sperm whale myoglobins on the other. These differences, more pronounced in deoxy derivatives, involve both the structure and dynamics of the heme pocket; in particular, elephant myoglobin appears to be characterized by larger anharmonic contributions to soft modes than the other two proteins. Flash photolysis data are analyzed as sums of kinetic processes with temperature-dependent fractional amplitudes, characterized by discrete pre-exponentials and either discrete or distributed activation enthalpies. In the whole temperature range investigated the behavior of elephant myoglobin appears to be more complex than that of horse and sperm whale myoglobins, which is in agreement with the increased anharmonic contributions to soft modes found in the former protein. Thus, to satisfactorily fit the time courses for CO recombination to elephant myoglobin five distinct processes are needed, only one of which is populated over the whole temperature range investigated. The remarkable convergence and complementarity between optical spectroscopy and flash photolysis data confirms the utility of combining these two experimental techniques in order to gain new and deeper insights into the functional relevance of protein fluctuations.

Oxidant-induced haemoprotein degradation in rat tissue slices: effect of bromotrichloromethane, antioxidants and chelators

Haemoprotein degradation and lipid peroxidation were evaluated in rat liver, kidney and heart slices incubated for 2 h in the presence and absence of bromotrichloromethane, antioxidants and chelators to obtain information about the relationship between oxidants and damage to haemoproteins. Haemoproteins were modified by bromotrichloromethane, and this modification, measured as loss of ferrohaemoproteins, generally was concurrent with lipid peroxidation measured as thiobarbituric acid-reactive substances. These two processes occurred simultaneously as a function of incubation time and oxidant concentration. Inhibition of the two processes by nordihydroguaiaretic acid, butylated hydroxyanisole and Trolox C, and lack of inhibition by mannitol, catalase and superoxide dismutase also were coincident. However, Methylene blue, EDTA, sodium fluoride, 2,4-dinitrophenol, N-ethylmaleimide and o-phenanthroline affected the two processes differently. The results suggested that haemoproteins may compete with other molecules for oxidant radicals, thus serving as protectors of cells against oxidant radicals. Products of haemoprotein degradation such as protein polymers, free amino acids and bilirubin may be indicators of in vivo oxidative stress.