Selective Oxidation of Methionine β(55)D6 at the α1β1 Interface in Hemoglobin Completely Destabilizes the T-state

Experimental basis for a new allosteric model for multisubunit proteins

Monod, Wyman, and Changeux (MWC) explained allostery in multisubunit proteins with a widely applied theoretical model in which binding of small molecules, so-called allosteric effectors, affects reactivity by altering the equilibrium between more reactive (R) and less reactive (T) quaternary structures. In their model, each quaternary structure has a single reactivity. Here, we use silica gels to trap protein conformations and a new kind of laser photolysis experiment to show that hemoglobin, the paradigm of allostery, exhibits two ligand binding phases with the same fast and slow rates in both R and T quaternary structures. Allosteric effectors change the fraction of each phase but not the rates. These surprising results are readily explained by the simplest possible extension of the MWC model to include a preequilibrium between two tertiary conformations that have the same functional properties within each quaternary structure. They also have important implications for the long-standing question of a structural explanation for the difference in hemoglobin oxygen affinity of the two quaternary structures.

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.

Symmetric Behavior of Hemoglobin α- and β- Subunits during Acid-Induced Denaturation Observed by Electrospray Mass Spectrometry

This work employs electrospray mass spectrometry (ESI-MS) and UV-vis spectroscopy for monitoring the mechanism of acid-induced hemoglobin (Hb) denaturation. The protein for these experiments has been freshly prepared from bovine blood. All three Hb derivatives studied (oxyHb, metHb, and cyanometHb) respond to gradual changes from pH 6.8 to 2.1 in a manner that can be described by a stepwise sequential unfolding mechanism: (alphahbetah)2 --> 2 alphahbetah --> 2 alphahfolded + 2 betahfolded --> 2 alphaaunfolded + 2 betaaunfolded + 4 heme (superscripts "h" and "a" refer to holo- and apo-forms, respectively). The results obtained on these freshly prepared samples are significantly different from those of similar experiments previously conducted on metHb obtained commercially as lyophilized powder. Those earlier experiments suggested a highly asymmetric behavior of the two globin chains, involving a heme-deficient dimer (alphahbetaa) as a mechanistically important intermediate on the (dis)assembly pathway. Importantly, heme-deficient dimers are virtually undetectable for the freshly prepared Hb derivatives studied herein at any pH. This apparent discrepancy is attributed to the occurrence of oxidative modifications in the commercial protein. Liquid chromatography and tandem mass spectrometry reveal significant levels of sulfoxide formation for all four methionine residues in commercially obtained metHb. The extent of these modifications for freshly prepared protein is lower by at least a factor of 10. It is concluded that the acid-induced denaturation of Hb follows a highly symmetric mechanism. The occurrence of other mechanisms (possibly involving asymmetric elements) under different solvent conditions cannot be ruled out.

First-generation blood substitutes: what have we learned? Biochemical and physiological perspectives

Chemically modified or recombinant hemoglobin (Hb)-based oxygen carriers (HBOCs) have been developed as oxygen therapeutics or 'blood substitutes' for use in a variety of clinical settings. Oxidative and nitrosative reactions of acellular Hb can limit the effectiveness and compromise the safety of HBOCs. The reactions between Hb and biologically relevant redox active molecules may also perturb redox sensitive signaling pathways. In recent years, systematic in vitro and in vivo structural and functional evaluation of several HBOCs has been carried out and, in some cases, delineated the 'structural' origin of their toxicity. This enables potential protective strategies against Hb-mediated side reactions to be rationally suggested. Here the authors provide an overview of their research experiences, novel insights into the molecular basis of toxicities of these products and some lessons learned.

Folding and assembly of hemoglobin monitored by electrospray mass spectrometry using an on-line dialysis system

The native structure of hemoglobin (Hb) comprises two alpha- and two beta-subunits, each of which carries a heme group. There appear to be no previous studies that report the in vitro folding and assembly of Hb from highly unfolded alpha- and beta-globin in a "one-pot" reaction. One difficulty that has to be overcome for studies of this kind is the tendency of Hb to aggregate during refolding. This work demonstrates that denaturation of Hb in 40% acetonitrile at pH 10.0 is reversible. A dialysis-mediated solvent change to a purely aqueous environment of pH 8.0 results in Hb refolding without any apparent aggregation. Fluorescence, Soret absorption, circular dichroism, and ESI mass spectra of the protein recorded before unfolding and after refolding are almost identical. By employing an externally pressurized dialysis cell that is coupled on-line to an ESI mass spectrometer, changes in heme binding behavior, protein conformation, and quaternary structure can be monitored as a function of time. The process occurs in a stepwise sequential manner, leading from monomeric alpha- and beta-globin to heterodimeric species, which then assemble into tetramers. Overall, this mechanism is consistent with previous studies employing the mixing of folded alpha- and beta-globin. However, some unexpected features are observed, e.g., a heme-deficient beta-globin dimer that represents an off-pathway intermediate. Monomeric beta-globin is capable of binding heme before forming a complex with an alpha-subunit. This observation suggests that holo-alpha-apo-beta globin does not represent an obligatory intermediate during Hb assembly, as had been proposed previously. The on-line dialysis/ESI-MS approach developed for this work represents a widely applicable tool for studying the folding and self-assembly of noncovalent biological complexes.

Substrates of the methionine sulfoxide reductase system and their physiological relevance

Posttranslational modifications can change a protein's structure, function, and solubility. One specific modification caused by reactive oxygen species is the oxidation of the sulfur atom in the methionine (Met) side chain. This modified amino acid is denoted as methionine sulfoxide (MetO). MetOs in proteins are of considerable interest as they are involved in early posttranslational modification events. Thus, various organisms produce specific enzymes that can reverse these modifications. MetO reductases, known collectively as the methionine sulfoxide reductase (Msr) system, are the only known enzymes that can reduce MetOs. The current research field of Met redox cycles is consumed with elucidating its role in regulation, redox homeostasis, prevention of irreversible modifications, pathogenesis, and the aging process. Substrates of the Msr system can be loosely classified by the overall effect of the MetO on the protein. Regulated substrates utilize Met as a molecular switch to modulate activation; scavenging substrates use Mets to detoxify oxidants and protect important regions of the protein; and modified substrates are altered by Met oxidation resulting in various changes in their properties, including function, activity, structure, and degradation resistance.

Structural basis of peroxide-mediated changes in human hemoglobin: a novel oxidative pathway

Hydrogen peroxide (H(2)O(2)) triggers a redox cycle between ferric and ferryl hemoglobin (Hb) leading to the formation of a transient protein radical and a covalent hemeprotein cross-link. Addition of H(2)O(2) to highly purified human hemoglobin (HbA(0)) induced structural changes that primarily resided within beta subunits followed by the internalization of the heme moiety within alpha subunits. These modifications were observed when an equal molar concentration of H(2)O(2) was added to HbA(0) yet became more abundant with greater concentrations of H(2)O(2). Mass spectrometric and amino acid analysis revealed for the first time that betaCys-93 and betaCys-112 were oxidized extensively and irreversibly to cysteic acid when HbA(0) was treated with H(2)O(2). Oxidation of further amino acids in HbA(0) exclusive to the beta-globin chain included modification of betaTrp-15 to oxyindolyl and kynureninyl products as well as betaMet-55 to methionine sulfoxide. These findings may therefore explain the premature collapse of the beta subunits as a result of the H(2)O(2) attack. Analysis of a tryptic digest of the main reversed phase-high pressure liquid chromatography fraction revealed two alpha-peptide fragments (alpha128-alpha139) and a heme moiety with the loss of iron, cross-linked between alphaSer-138 and the porphyrin ring. The novel oxidative pathway of HbA(0) modification detailed here may explain the diverse oxidative, toxic, and potentially immunogenic effects associated with the release of hemoglobin from red blood cells during hemolytic diseases and/or when cell-free Hb is used as a blood substitute.

Structural analysis of modified forms of recombinant IFN-beta produced under stress-simulating conditions

The present study focused on the investigation of the chemical stability of recombinant human interferon-beta (rhIFN-beta) tested in vitro by chemical treatments that simulate stress-induced conditions that may occur during handling, storage or ageing of protein samples. Mild oxidation and/or alkylation of the recombinant protein showed that the four methionines occurring in the interferon displayed different chemical susceptibility in that Met36 and Met117 were fully modified, whereas Met1 showed only little modification and Met62 was completely resistant. Moreover, incubation of rhIFN-beta under alkaline conditions resulted in the formation of a covalent dimeric species stabilised by an intermolecular disulphide bridge involving the free SH group of Cys17 from each polypeptide chain. Analysis of biological activity of the different IFN-beta derivatives showed that rhIFN-beta fully retains its specific activity following mild oxidation treatments whereas reaction with a high concentration of alkylating agents or incubation under alkaline conditions strongly reduce its specific antiviral activity.

Fast-reacting thiols in rat hemoglobins can intercept damaging species in erythrocytes more efficiently than glutathione

The S-conjugation rates of the free-reacting thiols present on each component of rat hemoglobin with 5,5-dithio-bis(2,2-nitrobenzoic acid) (DTNB) have been studied under a variety of conditions. On the basis of their reactivity with DTNB (0.5 mM), three classes of thiols have been defined as follows: fast reacting (fHbSH), with t1/2 <100 ms; slow reacting (sHbSH), with t1/2 30-50 s; and very slow reacting (vsHbSH), with t1/2 180-270 s. Under paraphysiological conditions, fHbSH (identified with Cys-125beta(H3)) conjugates with DTNB 100 times faster than glutathione and approximately 4000 times more rapidly than (v)sHbSH (Cys-13alpha(A11) and Cys-93beta(F9)). Such characteristics of fHbSH reactivity that are independent of the quaternary state of hemoglobin are mainly due to the following: (i) its low pK (approximately 6.9, the cysteinyl anion being stabilized by a hydrogen bond with Ser-123beta(H1)) and (ii) the large exposure to the solvent (as measured by analysis of a model of the molecular surface) and make these thiols the kinetically preferred groups in rat erythrocytes for S-conjugation. In addition, because of the high cellular concentration (8 mM, i.e. four times that of glutathione), fHbSHs are expected to intercept damaging species in erythrocytes more efficiently than glutathione, thus adding a new physiopathological role (direct involvement in cellular strategies of antioxidant defense) to cysteinyl residues in proteins.

Oxidation of a critical methionine modulates DNA binding of the Drosophila melanogaster high mobility group protein, HMG-D

HMG-D is a major high mobility group chromosomal protein present during early embryogenesis in Drosophila melanogaster. During overexpression and purification of HMG-D from E. coli, a key DNA binding residue, methionine 13, undergoes oxidation to methionine sulfoxide. Oxidation of this critical residue decreases the affinity of HMG-D for DNA by three-fold, altering the structure of the HMG-D-DNA complex without affecting the structure of the free protein. This work shows that minor modification of DNA intercalating residues may be used to fine tune the DNA binding affinity of HMG domain proteins.

In vitro methionine oxidation of Escherichia coli-derived human stem cell factor: effects on the molecular structure, biological activity, and dimerization

The effect of oxidation of the methionine residues of Escherichia coli-derived recombinant human stem cell factor (huSCF) to methionine sulfoxide on the structure and activity of SCF was examined. Oxidation was performed using hydrogen peroxide under acidic conditions (pH 5.0). The kinetics of oxidation of the individual methionine residues was determined by quantitation of oxidized and unoxidized methionine-containing peptides, using RP-HPLC of Asp-N endoproteinase digests. The initial oxidation rates for Met159, Met-1, Met27, Met36, and Met48 were 0.11 min-1, 0.098 min-1, 0.033 min-1, 0.0063 min-1, and 0.00035 min-1, respectively, when SCF was incubated in 0.5% H2O2 at room temperature. Although oxidation of these methionines does not affect the secondary structure of SCF, the oxidation of Met36 and Met48 affects the local structure as indicated by CD and fluorescence spectroscopy. The 295-nm Trp peak in the near-UV CD is decreased upon oxidation of Met36, and lost completely following the oxidation of Met48, indicating that the Trp44 environment is becoming significantly less rigid than it is in native SCF. Consistent with this result, the fluorescence spectra revealed that Trp44 becomes more solvent exposed as the methionines are oxidized, with the hydrophobicity of the Trp44 environment decreasing significantly. The oxidations of Met36 and Met48 decrease biological activity by 40% and 60%, respectively, while increasing the dissociation rate constant of SCF dimer by two- and threefold. These results imply that the oxidation of Met36 and Met48 affects SCF dimerization and tertiary structure, and decreases biological activity.

Probing the alpha 1 beta 2 interface of human hemoglobin by mutagenesis. Role of the FG-C contact regions

The allosteric transition of hemoglobin involves an extensive reorganization of the alpha 1 beta 2 interface, in which two contact regions have been identified. This paper concerns at the effect of two mutations located in the "switch" (alpha C3 Thr --> Trp) and the "flexible joint" (beta C3 Trp --> Thr). We have expressed and characterized one double and two single mutants: Hb alpha T38W/beta W37T, Hb beta W37T, and Hb alpha T38W, whose structure has been determined by crystallography. We present data on: (i) the interface structure in the contact regions, (ii) oxygen and CO binding kinetics and cooperativity, (iii) dissociation rates of deoxy tetramers and association rates of deoxy dimers, and (iv) the effect of NaI on deoxy tetramer dissociation rate constant. All the mutants are tetrameric and T-state in the deoxygenated derivative. Reassociation of deoxygenated dimers is not modified by interface mutations. DeoxyHb alpha T38W/beta W37T dissociate much faster. We propose a binding site for I- at the switch region. The single mutants binds O2 cooperatively; the double one is almost non-cooperative, a feature confirmed by CO binding. The functional data, analyzed with the two-state model, indicate that these mutations reduce the value of the allosteric constant LO.

Iron site structure of two irreversible hemichromes from human hemoglobin, untreated and oxidized to sulfoxide at MetD6(55)beta

The Fe K-edge X-ray absorption near-edge structure (XANES) spectra of two irreversible human hemichromes, spontaneously formed from HbA and HbMetSO (a hemoglobin derivative, where MetD6(55)beta has been previously oxidized to sulfoxide by chloramine T) were determined. The results show that the hemichrome from HbMetSO is characterized by the distal histidyl imidazole moved within the bonding distance of the heme iron. Such structure is different from that of the hemichrome spontaneously produced from native human hemoglobin, which probably has a hydroxide group as sixth heme ligand.

Selective oxidation of Met-192 in bovine alpha-chymotrypsin. Effect on catalytic and inhibitor binding properties

Catalytic and inhibitor binding properties of bovine alpha-chymotrypsin, in which the Met-192 residue has been converted by treatment with chloramine T to the sulfoxide derivative (Met(O)192 alpha-chymotrypsin), have been examined relative to the native enzyme (alpha-chymotrypsin), between pH 4.5 and 8.0 (mu = 0.1), and/or 5.0 degrees C and 40.0 degrees C. Values of kcat, k+2 and/or k+3 for the hydrolysis of all the substrates examined (i.e., tMetAcONp, ZAlaONp, ZLeuONp, ZLysONp and ZTyrONp) catalyzed by native and Met(O)192 alpha-chymotrypsin are similar, as well as values of Km for the hydrolysis of ZLeuONp, ZLysONp and ZTyrONp. On the other hand, Ks and Km values for the hydrolysis of ZAlaONp and tMetAcONp are decreased by about 5-fold. Met-192 oxidation does not affect the kinetic and thermodynamic parameters for the (de)stabilization of the complex formed between the proteinase and the bovine basic pancreatic trypsin inhibitor. On the other hand, the recognition process between between alpha-chymotrypsin and the recombinant proteinase inhibitor eglin c from the leech Hirudo medicinalis is influenced by the oxidation event. Considering known molecular models, the observed catalytic and inhibitor binding properties of native and Met(O)192 alpha-chymotrypsin were related to the inferred stereochemistry of the proteinase-substrate and proteinase-inhibitor contact region(s).