Studies on the structure of hemoglobin. I. Physicochemical properties of human globin

Assessment of total and unbound cell-free heme in plasma of patients with sickle cell disease

Intravascular hemolysis results in the release of cell-free hemoglobin and heme in plasma. In sickle cell disease, the fragility of the sickle red blood cell leads to chronic hemolysis, which can contribute to oxidative damage and activation of inflammatory pathways. The scavenger proteins haptoglobin and hemopexin provide pathways to remove hemoglobin and heme, respectively, from the circulation. Heme also intercalates in membranes of blood cells and endothelial cells in the vasculature and associates with other plasma components such as albumin and lipoproteins. Hemopexin has a much higher affinity and can strip heme from the other pools and detoxify plasma from cell-free circulatory heme. However, due to chronic hemolysis, hemopexin is depleted in individuals with sickle cell disease. Thus, cell-free unbound heme is expected to accumulate in plasma. We developed a methodology for the accurate quantification of the fraction of heme, which is pathologically relevant in sickle cell disease, that does not appear to be sequestered to a plasma compartment. Our data show significant variation in the concentration of unbound heme, and rather unexpectedly, the size of the unbound fraction does not correlate to the degree of hemolysis, as measured by the concentration of bound heme. Very high heme concentrations (>150 µM) were obtained in some plasma with unbound concentrations that were several fold lower than in plasma with much lower hemolysis (<50 µM). These findings underscore the long-term effects of chronic hemolysis on the blood components and of the disruption of the essential equilibrium between release of hemoproteins/heme in the circulation and adaptative response of the scavenging/removal mechanisms. Understanding the clinical implications of this loss of response may provide insights into diagnostic and therapeutic targets in patients with sickle cell disease.

A Slam-dependent hemophore contributes to heme acquisition in the bacterial pathogen Acinetobacter baumannii

Nutrient acquisition systems are often crucial for pathogen growth and survival during infection, and represent attractive therapeutic targets. Here, we study the protein machinery required for heme uptake in the opportunistic pathogen Acinetobacter baumannii. We show that the hemO locus, which includes a gene encoding the heme-degrading enzyme, is required for high-affinity heme acquisition from hemoglobin and serum albumin. The hemO locus includes a gene coding for a heme scavenger (HphA), which is secreted by a Slam protein. Furthermore, heme uptake is dependent on a TonB-dependent receptor (HphR), which is important for survival and/or dissemination into the vasculature in a mouse model of pulmonary infection. Our results indicate that A. baumannii uses a two-component receptor system for the acquisition of heme from host heme reservoirs.

High-Yield Electrochemical Synthesis of Silver Nanoparticles by Enzyme-Modified Boron-Doped Diamond Electrodes

We report a novel electrochemical approach for synthesizing colloidal silver in an aqueous phase by a hemoglobin-modified boron-doped diamond electrode. The resulting Ag nanoparticles are within 10 nm in size and highly monodisperse with minimal electrode deposition. We also introduce a method for measuring the yield of synthesized nanoparticles using square-wave voltammetry as an alternative to UV-vis spectroscopy. More than 50% of the transferred electrons contributed directly to the formation of silver nanoparticles. This high yield indicates that such electrochemical synthesis is an efficient one-pot method for producing colloidal silver free of toxic reagents and offers a path toward green metal nanoparticle synthesis in solution. A comparative study using alternative electrodes, modifiers, and surfactants suggests a mechanism for the formation of silver nanoparticles mediated by hemoglobin-modified boron-doped diamond electrodes.

The Interplay between Molten Globules and Heme Disassociation Defines Human Hemoglobin Disassembly

Hemoglobin functions as a tetrameric oxygen transport protein, with each subunit containing a heme cofactor. Its denaturation, either in vivo or in vitro, involves autoxidation to methemoglobin, followed by cofactor loss and globin unfolding. We have proposed a global disassembly scheme for human methemoglobin, linking hemin (ferric protoporphyrin IX) disassociation and apoprotein unfolding pathways. The model is based on the evaluation of circular dichroism and visible absorbance measurements of guanidine-hydrochloride-induced disassembly of methemoglobin and previous measurements of apohemoglobin unfolding. The populations of holointermediates and equilibrium disassembly parameters were estimated quantitatively for adult and fetal hemoglobins. The key stages are characterized by hexacoordinated hemichrome intermediates, which are important for preventing hemin disassociation from partially unfolded, molten globular species during early disassembly and late-stage assembly events. Both unfolding experiments and independent small angle x-ray scattering measurements demonstrate that heme disassociation leads to the loss of tetrameric structural integrity. Our model predicts that after autoxidation, dimeric and monomeric hemichrome intermediates occur along the disassembly pathway inside red cells, where the hemoglobin concentration is very high. This prediction suggests why misassembled hemoglobins often get trapped as hemichromes that accumulate into insoluble Heinz bodies in the red cells of patients with unstable hemoglobinopathies. These Heinz bodies become deposited on the cell membranes and can lead to hemolysis. Alternatively, when acellular hemoglobin is diluted into blood plasma after red cell lysis, the disassembly pathway appears to be dominated by early hemin disassociation events, which leads to the generation of higher fractions of unfolded apo subunits and free hemin, which are known to damage the integrity of blood vessel walls. Thus, our model provides explanations of the pathophysiology of hemoglobinopathies and other disease states associated with unstable globins and red cell lysis and also insights into the factors governing hemoglobin assembly during erythropoiesis.

Photodynamic action of Hypericum perforatum hydrophilic extract against Staphylococcus aureus

Hypericin (Hyp) is one of the most effective, naturally occurring photodynamic agents, which proved effective against a wide array of microorganisms.

The human protein haptoglobin inhibits IsdH-mediated heme-sequestering by Staphylococcus aureus

Iron is an essential nutrient for all living organisms. To acquire iron, many pathogens have developed elaborate systems to steal it from their hosts. The iron acquisition system in the opportunistic pathogen Staphylococcus aureus comprises nine proteins, called iron-regulated surface determinants (Isds). The Isd components enable S. aureus to extract heme from hemoglobin (Hb), transport it into the bacterial cytoplasm, and ultimately release iron from the porphyrin ring. IsdB and IsdH act as hemoglobin receptors and are known to actively extract heme from extracellular Hb. To limit microbial pathogenicity during infection, host organisms attempt to restrict the availability of nutrient metals at the host-pathogen interface. The human acute phase protein haptoglobin (Hp) protects the host from oxidative damage by clearing hemoglobin that has leaked from red blood cells and also restricts the availability of extracellular Hb-bound iron to invading pathogens. To investigate whether Hp serves an additional role in nutritional immunity through a direct inhibition of IsdH-mediated iron acquisition, here we measured heme extraction from the Hp-Hb complex by UV-visible spectroscopy and determined the crystal structure of the Hp-Hb-IsdH complex at 2.9 Å resolution. We found that Hp strongly inhibits IsdH-mediated heme extraction and that Hp binding prevents local unfolding of the Hb heme pocket, leaving IsdH unable to wrest the heme from Hb. Furthermore, we noted that the Hp-Hb binding appears to trap IsdH in an initial state before heme transfer. Our findings provide insights into Hp-mediated IsdH inhibition and the dynamics of IsdH-mediated heme extraction.

Enhanced photosensitizing properties of protein bound curcumin

AIMS

The naturally occurring compound curcumin has been proposed for a number of pharmacological applications. In spite of the promising chemotherapeutic properties of the molecule, the use of curcumin has been largely limited by its chemical instability in water. In this work, we propose the use of water soluble proteins to overcome this issue in perspective applications to photodynamic therapy of tumors.

MATERIALS AND METHODS

Curcumin was bound to bovine serum albumin and its photophysical properties was studied as well as its effect on cell viability after light exposure through MTT assay and confocal imaging.

KEY FINDINGS

Bovine serum albumin binds curcumin with moderate affinity and solubilizes the hydrophobic compound preserving its photophysical properties for several hours. Cell viability assays demonstrate that when bound to serum albumin, curcumin is an effective photosensitizer for HeLa cells, with better performance than curcumin alone. Confocal fluorescence imaging reveals that when curcumin is delivered alone, it preferentially associates with mitochondria, whereas curcumin bound to bovine serum albumin is found in additional locations within the cell, a fact that may be related to the higher phototoxicity observed in this case.

SIGNIFICANCE

The higher bioavailability of the photosensitizing compound curcumin when bound to serum albumin may be exploited to increase the efficiency of the drug in photodynamic therapy of tumors.

Apomyoglobin is an efficient carrier for zinc phthalocyanine in photodynamic therapy of tumors

The spectral and the photophysical properties of phthalocyanines have made these dyes attractive for applications in photodynamic therapy of cancer. One important known issue of these compounds is their tendency to aggregate in aqueous media, which decreases their fluorescence, triplet, and singlet oxygen quantum yields. We report on the use of apomyoglobin as a carrier for zinc phthalocyanine (ZnPc) to overcome solubility limitations of the dye. We show that the protein is able to bind ZnPc in monomeric form, preserving its photophysics. Confocal fluorescence imaging of PC3 and HeLa cells, treated with the complex between ZnPc and apomyoglobin, demonstrates that the photosensitizer is uptaken quickly by cells. Illumination of treated cells strongly decreases viability, as demonstrated by live/dead fluorescence assay.

Hypericin-Apomyoglobin: An Enhanced Photosensitizer Complex for the Treatment of Tumor Cells

Bioavailability of photosensitizers for cancer photodynamic therapy is often hampered by their low solubility in water. Here, we overcome this issue by using the water-soluble protein apomyoglobin (apoMb) as a carrier for the photosensitizer hypericin (Hyp). The Hyp-apoMb complex is quickly uptaken by HeLa and PC3 cells at submicromolar concentrations. Fluorescence emission of Hyp-apoMb is exploited to localize the cellular distribution of the photosensitizer. The plasma membrane is rapidly and efficiently loaded, and fluorescence is observed in the cytoplasm only at later times and to a lesser extent. Comparison with cells loaded with Hyp alone demonstrates that the uptake of the photosensitizer without the protein carrier is a slower, less efficient process, that involves the whole cell structure without preferential accumulation at the plasma membrane. Cell viability assays demonstrate that the Hyp-apoMb exhibits superior performance over Hyp. Similar results were obtained using tumor spheroids as three-dimensional cell culture models.

Quantification of Active Apohemoglobin Heme-Binding Sites via Dicyanohemin Incorporation

Apohemoglobin (apoHb) is produced by removing heme from hemoglobin (Hb). However, preparations of apoHb may contain damaged globins, which render total protein assays inaccurate for active apoHb quantification. Fortunately, apoHb heme-binding sites react with heme via the proximal histidine-F8 (His-F8) residue, which can be monitored spectrophotometrically. The bond between the His-F8 residue of apoHb and heme is vital for maintenance of fully functional and cooperative Hb. Additionally, most apoHb drug delivery applications facilitate hydrophobic drug incorporation inside the apoHb hydrophobic heme-binding pocket in which the His-F8 residue resides. This makes the His-F8 residue a proper target for apoHb activity quantification. In this work, dicyanohemin (DCNh), a stable monomeric porphyrin species, was used as a probe molecule to quantify active apoHb through monocyanohemin-His-F8 bond formation. ApoHb activity was quantified via the analysis of the 420 nm equilibrium absorbance of DCNh and apoHb mixtures. His-F8 saturation was determined by the presence of an inflection point from a plot of the 420 nm absorbance of a fixed concentration of apoHb against an increasing DCNh concentration. Various concentrations of a stock apoHb solution were tested to demonstrate the precision of the assay. The accuracy of the assay was assessed via spectral deconvolution, confirming His-F8 saturation at the inflection point. The effect of the heme-binding protein bovine serum albumin and precipitated apoHb on assay sensitivity was not significant. An analysis of the biophysical properties of reconstituted Hb confirmed heme-binding pocket activity. Taken together, this assay provides a simple and reliable method for determination of apoHb activity.

Mechanism of Human Apohemoglobin Unfolding

Removal of heme from human hemoglobin (Hb) results in formation of an apoglobin heterodimer. Titration of this apodimer with guanidine hydrochloride (GdnHCl) leads to biphasic unfolding curves indicating two distinct steps. Initially, the heme pocket unfolds and generates a dimeric intermediate in which ∼50% of the original helicity is lost, but the α₁β₁ interface is still intact. At higher GdnHCl concentrations, this intermediate dissociates into unfolded monomers. This structural interpretation was verified by comparing GdnHCl titrations for adult human hemoglobin A (HbA), recombinant fetal human hemoglobin (HbF), recombinant Hb cross-linked with a single glycine linker between the α chains, and recombinant Hbs with apolar heme pocket mutations that markedly stabilize native conformations in both subunits. The first phase of apoHb unfolding is independent of protein concentration, little affected by genetic cross-linking, but significantly shifted toward higher GdnHCl concentrations by the stabilizing distal pocket mutations. The second phase depends on protein concentration and is shifted to higher GdnHCl concentrations by genetic cross-linking. This model for apoHb unfolding allowed us to quantitate subtle differences in stability between apoHbA and apoHbF, which suggest that the β and γ heme pockets have similar stabilities, whereas the α₁γ₁ interface is more resistant to dissociation than the α₁β₁ interface.

A New Haemoglobin Variant Haemoglobin J Birmingham a 120 (H3) Ala → Glu

An abnormal haemoglobin, which on paper electrophoresis has the mobility of Hb J, has been found in two brothers and, in the heterozygous state at least, is not associated with serious clinical abnormality. The structure of this hitherto unreported haemoglobin is α 120 (H3) Ala → Glu and it is named Haemoglobin J Birmingham.

Subdiffraction localization of a nanostructured photosensitizer in bacterial cells

Antibacterial treatments based on photosensitized production of reactive oxygen species is a promising approach to address local microbial infections. Given the small size of bacterial cells, identification of the sites of binding of the photosensitizing molecules is a difficult issue to address with conventional microscopy. We show that the excited state properties of the naturally occurring photosensitizer hypericin can be exploited to perform STED microscopy on bacteria incubated with the complex between hypericin and apomyoglobin, a self-assembled nanostructure that confers very good bioavailability to the photosensitizer. Hypericin fluorescence is mostly localized at the bacterial wall, and accumulates at the polar regions of the cell and at sites of cell wall growth. While these features are shared by Gram-negative and Gram-positive bacteria, only the latter are effectively photoinactivated by light exposure.

Kinetic Analysis of a Globin-Coupled Histidine Kinase, AfGcHK: Effects of the Heme Iron Complex, Response Regulator, and Metal Cations on Autophosphorylation Activity

The globin-coupled histidine kinase, AfGcHK, is a part of the two-component signal transduction system from the soil bacterium Anaeromyxobacter sp. Fw109-5. Activation of its sensor domain significantly increases its autophosphorylation activity, which targets the His183 residue of its functional domain. The phosphate group of phosphorylated AfGcHK is then transferred to the cognate response regulator. We investigated the effects of selected variables on the autophosphorylation reaction's kinetics. The kcat values of the heme Fe(III)-OH(-), Fe(III)-cyanide, Fe(III)-imidazole, and Fe(II)-O2 bound active AfGcHK forms were 1.1-1.2 min(-1), and their Km(ATP) values were 18.9-35.4 μM. However, the active form bearing a CO-bound Fe(II) heme had a kcat of 1.0 min(-1) but a very high Km(ATP) value of 357 μM, suggesting that its active site structure differs strongly from the other active forms. The Fe(II) heme-bound inactive form had kcat and Km(ATP) values of 0.4 min(-1) and 78 μM, respectively, suggesting that its low activity reflects a low affinity for ATP relative to that of the Fe(III) form. The heme-free form exhibited low activity, with kcat and Km(ATP) values of 0.3 min(-1) and 33.6 μM, respectively, suggesting that the heme iron complex is essential for high catalytic activity. Overall, our results indicate that the coordination and oxidation state of the sensor domain heme iron profoundly affect the enzyme's catalytic activity because they modulate its ATP binding affinity and thus change its kcat/Km(ATP) value. The effects of the response regulator and different divalent metal cations on the autophosphorylation reaction are also discussed.

Replacing heme with paclitaxel to prepare drug-loaded globin nanoassembles for CD163 targeting

Protein-based nanoparticles hold great promises in both preclinical and clinical practices, such as oncology diagnosis and treatment, because of their high biocompatibility and biodegradability. However, the complicated preparation and lack of targeting specific cells or tissues may limit their further uses. To overcome these limitations, we developed a novel replacing method for preparing dual-functional protein nanocarrier, such that one function is capable of encapsulating small molecule into protein, whereas the other function is cable of recognizing CD163 receptor [hemoglobin (Hb) scavenger receptor]. In this study, Hb was chosen as the targeting drug carrier. First, the heme group in the Hb was removed and replaced by paclitaxel (PTX) to form nanoparticles (Gb-NPs-PTX). The resulted Gb-NPs-PTX showed spherical shape and their diameter could be controlled in the range of 120-160 nm by altering the ratio of PTX to Hb. The binding activity of Gb-NPs-PTX to CD163 was confirmed by cell uptake in CD163(+) Chinese hamster ovary cells. Results in vivo also showed a CD163-dependent tissue accumulation of Gb-NPs-PTX in mice. In summary, by using the novel replacing method, PTX could be easily encapsulated into Hb nanoparticles and the targeting effects of Hb could also be kept.

Introduction of water into the heme distal side by Leu65 mutations of an oxygen sensor, YddV, generates verdoheme and carbon monoxide, exerting the heme oxygenase reaction

The globin-coupled oxygen sensor, YddV, is a heme-based oxygen sensor diguanylate cyclase. Oxygen binding to the heme Fe(II) complex in the N-terminal sensor domain of this enzyme substantially enhances its diguanylate cyclase activity which is conducted in the C-terminal functional domain. Leu65 is located on the heme distal side and is important for keeping the stability of the heme Fe(II)-O2 complex by preventing the entry of the water molecule to the heme complex. In the present study, it was found that (i) Escherichia coli-overexpressed and purified L65N mutant of the isolated heme-bound domain of YddV (YddV-heme) contained the verdoheme iron complex and other modified heme complexes as determined by optical absorption spectroscopy and mass spectrometry; (ii) CO was generated in the reconstituted system composed of heme-bound L65N and NADPH:cytochrome P450 reductase as confirmed by gas chromatography; (iii) CO generation of heme-bound L65N in the reconstituted system was inhibited by superoxide dismutase and catalase. In a concordance with the result, the reactive oxygen species increased the CO generation; (iv) the E. coli cells overexpressing the L65N protein of YddV-heme also formed significant amounts of CO compared to the cells overexpressing the wild type protein; (v) generation of verdoheme and CO was also observed for other mutants at Leu65 as well, but to a lesser extent. Since Leu65 mutations are assumed to introduce the water molecule into the heme distal side of YddV-heme, it is suggested that the water molecule would significantly contribute to facilitating heme oxygenase reactions for the Leu65 mutants.

Spectroscopic characterization of (57)Fe-enriched cytochrome c

Investigation of the heme iron dynamics in cytochrome c with Mössbauer spectroscopy and especially nuclear resonance vibrational spectroscopy requires the replacement of the natural abundant heme iron with the (57)Fe isotope. For demetallization, we use a safer and milder ferrous sulfate-hydrochloric acid method in addition to the harsher commonly used hydrofluoric acid-based procedure. The structural integrity of the (57)Fe-reconstituted protein in both oxidation states is confirmed from absorption spectra and a detailed analysis of the rich resonance Raman spectra. These results reinforce the application of metal-substituted heme c proteins as reliable models for the native proteins.

Hemoglobin promotes Staphylococcus aureus nasal colonization

Staphylococcus aureus nasal colonization is an important risk factor for community and nosocomial infection. Despite the importance of S. aureus to human health, molecular mechanisms and host factors influencing nasal colonization are not well understood. To identify host factors contributing to nasal colonization, we collected human nasal secretions and analyzed their ability to promote S. aureus surface colonization. Some individuals produced secretions possessing the ability to significantly promote S. aureus surface colonization. Nasal secretions pretreated with protease no longer promoted S. aureus surface colonization, suggesting the involvement of protein factors. The major protein components of secretions were identified and subsequent analysis revealed that hemoglobin possessed the ability to promote S. aureus surface colonization. Immunoprecipitation of hemoglobin from nasal secretions resulted in reduced S. aureus surface colonization. Furthermore, exogenously added hemoglobin significantly decreased the inoculum necessary for nasal colonization in a rodent model. Finally, we found that hemoglobin prevented expression of the agr quorum sensing system and that aberrant constitutive expression of the agr effector molecule, RNAIII, resulted in reduced nasal colonization of S. aureus. Collectively our results suggest that the presence of hemoglobin in nasal secretions contributes to S. aureus nasal colonization.

Staphylococcus aureus is a medically important human pathogen that is found in the nasal passages of approximately 1/3 of the population. The nose serves as a reservoir for spread of this pathogen and predisposes the host to potential infection. Factors contributing to S. aureus nasal colonization are only beginning to be elucidated. The collection and analysis of human nasal secretions provided evidence that the presence of hemoglobin in nasal secretions can promote S. aureus nasal colonization. Hemoglobin reduced expression of the S. aureus agr quorum sensing regulatory system known to be involved in surface colonization, and it was found that induction of the agr system reduced nasal colonization. These findings suggest that individuals experiencing frequent nosebleeds would be prone to S. aureus colonization and epidemiological data supports these findings. By understanding host factors and bacterial molecular mechanisms involved in nasal colonization we may one day be able to design novel decolonization strategies.

ERYTHROPOIESIS DURING AMPHIBIAN METAMORPHOSIS : II. Immunochemical Study of Larval and Adult Hemoglobins of Rana catesbeiana

Rabbit antibodies were prepared against the major hemoglobin components of the larval and adult stages of R. catesbeiana. The properties of the antisera were studied by double immunodiffusion, precipitation, and complement fixation. The antisera to tadpole and frog hemoglobins did not cross-react with either hemoglobin or apohemoglobin. The anti-serum against frog hemoglobin was used for the detection of frog hemoglobin in tadpoles undergoing either natural or thyroxine-induced metamorphosis. It was shown that frog hemoglobin is detectable first in the liver, indicating that the liver is the site of erythrocyte maturation during metamorphosis.

ERYTHROPOIESIS DURING AMPHIBIAN METAMORPHOSIS : III. Immunochemical Detection of Tadpole and Frog Hemoglobins (Rana catesbeiana) in Single Erythrocytes

Rabbit antibodies specific for the major tadpole and frog hemoglobin components of R. catesbeiana were used for the detection of the two hemoglobins inside single cells. The antisera, after fractionation by ammonium sulfate precipitation and diethylaminoethyl (DEAE)-cellulose chromatography, were conjugated with fluorescein isothiocyanate for the antifrog hemoglobin serum and tetramethylrhodamine isothiocyanate for the antitadpole hemoglobin serum. The conjugated fractions, refractionated by stepwise elution from a DEAE-cellulose column, were used for the fluorescent staining of blood smears, liver tissue imprints, and smears of liver cell suspensions. Both simultaneous and sequential staining with the two fluorescent preparations indicated that larval and adult hemoglobins were not present within the same erythrocyte during metamorphosis. In other experiments, erythroid cells from animals in metamorphosis were spread on agar containing specific antiserum. Precipitates were formed around the cells which contain the particular hemoglobin. The percentages of cells containing either tadpole or frog hemoglobin were estimated within the experimental error of the method. The data showed that the two hemoglobins are in different cells. It is concluded that the hemoglobin change observed during the metamorphosis of R. catesbeiana is due to the appearance of a new population of erythroid cells containing exclusively frog hemoglobin.

Quantitative determination and localization of cathepsin D and its inhibitors

A literature survey was performed of the methods of quantitative assessment of the activity and concentration of cathepsin D and its inhibitors. Usefulness of non-modified and modified proteins and synthetic peptides as measurement substrates was evaluated. The survey includes also chemical and immunochemical methods used to determine the distribution of cathepsin D and its inhibitors in cells and tissues.

Probing the role of hydration in the unfolding transitions of carbonmonoxy myoglobin and apomyoglobin

We show that the equilibrium unfolding transition of horse carbonmonoxy myoglobin monitored by the stretching vibration of the CO ligand, a local environmental probe, is very sharp and, thus, quite different from those measured by global conformational reporters. In addition, the denatured protein exhibits an A(0)-like CO band. We hypothesize that this sharp transition reports penetration of water into the heme pocket of the protein. Parallel experiments on horse apomyoglobin, wherein an environment-sensitive fluorescent probe, nile red, was used, also reveals a similar putative hydration event. Given the importance of dehydration in protein folding and also the recent debate over the interpretation of probe-dependent unfolding transitions, these results have strong implications on the mechanism of protein folding.

Primary structure of myoglobins from 31 species of birds

Primary structure of myoglobins (Mbs) from 31 avian species of 15 orders were reported, although portions of the structures in the 2 species could not be determined. At least 68 of the total 153 amino acid sites were invariant all through the avian, reptilian and human Mbs, and 20 of these sites were "internal", forming the internal hydrophobic cavities in which the heme group remains wrapped. Furthermore, at 27 sites, if replaced, the replacements were mostly conservative, and 13 of the conservative sites were "internal". Thus the all 33 "internal" sites, important for structural and functional stability of the protein, have been well preserved, either invariant or conserved, during evolution from reptiles to birds and mammals. The residue 71 (E14) in 4 penguin species was not deleted as previously reported in emperor penguin Mb but occupied by Gln. The residue 121 (GH3) was deleted in all 3 species studied of Falconiformes. Out of 9 anseriforms, 5 species of different genera showed the identical structure. Secondary structures as viewed by hydropathy profiles were highly similar throughout the reptilian, avian and mammalian Mbs.

Recombinant expression and partial characterization of an active soluble histo-aspartic protease from Plasmodium falciparum

Malaria aspartic proteases are attractive drug targets for the treatment of malaria, however, recombinant expression of active histo-aspartic proteinase (HAP) to facilitate its characterization has proven elusive. The present study reports on the first recombinant expression of soluble, active histo-aspartic proteinase from Plasmodium falciparum as a thioredoxin fusion protein. A truncated form of HAP (77p-451) was fused to thioredoxin in the pET32b(+) vector and the fusion protein (Trx-tHAP) was expressed in Escherichia coli Rosetta-gami B (DE3)pLysS. The fusion protein was partially purified from the culture medium using a combination of anion exchange and Ni(2+) affinity chromatography. Soluble tHAP was subsequently purified by enterokinase treatment and removal, followed by gel filtration chromatography. Although truncated HAP was incapable of autocatalytic activation, enterokinase digestion of partially purified fusion protein released the truncated prosegment yielding a mature form of tHAP (mtHAP). N-terminal sequencing of mtHAP indicated that enterokinase cleavage took place at Lys119-Ser120, four residues upstream of the native cleavage site (Gly123-Ser124). Initial activity tests showed that mtHAP was capable of hydrolyzing acid-denatured globin as well as cleavage of the synthetic substrate EDANS-CO-CH(2)-CH(2)-CO-ALERMFLSFP-Dap(DABCYL)-OH. Inhibition studies showed that the activity of mtHAP was completely inhibited by pepstatin A and to a lesser degree, PMSF. Using the synthetic substrate, mtHAP showed a pH optimum of 5.2, and Km=3.4 microM and kcat=1.6 x 10(-3)s(-1). The successful expression of active recombinant HAP from E. coli will accelerate the investigation of the structure-function relationships of HAP and facilitate the development of specific inhibitors with antimalarial activities.

Acid-induced unfolding of myoglobin triggered by a laser pH jump method

Using 1-(2-nitrophenyl)ethyl sulfate (caged sulfate) as a photoactivatable caged proton, we could induce complete acid unfolding of myoglobin with a single nanosecond laser pulse. This was possible because of the high ( approximately mM) concentration of protons released by the photolabile compound. The ability of the compound to produce a large pH jump arises because the other photoproducts (2-nitrosoacetophenone and sulfate ion) do not buffer the released protons. The complete time course of the unfolding kinetics, spanning a range from milliseconds to several seconds, could be accurately reproduced by monitoring absorbance changes in the visible spectrum at 633 nm.

Cooperative Sub-Millisecond Folding Kinetics of Apomyoglobin pH 4 Intermediate

For small single-domain proteins, formation of the native conformation (N) from a fully unfolded form (U) or from a partially folded intermediate (I) occurs typically in a highly cooperative process that can be described by a two-state model. However, it is not clear whether cooperativity arises early along the folding reaction and whether folding intermediates are also formed in highly cooperative processes. Here, we show that each previously identified step leading apomyoglobin from its unfolded form to its native form, namely, the U <= => Ia, the Ia <= => Ib, and the Ib <= => N reactions, exhibits typical features of a two-state reaction. First, refolding and unfolding kinetics of the earliest U <= => Ia reaction are measurable at pH 4.2 within the urea-induced unfolding transition [Jamin, M., and Baldwin, R. L. (1996) Nat. Struct. Biol. 3, 613-618; Jamin, M., and Baldwin, R. L. (1998) J. Mol. Biol. 276, 491-504], and we report here that sub-millisecond kinetics measured by far-UV circular dichroism (CD), a probe of secondary structure, are similar to those measured by Trp fluorescence, a probe of hydrophobic core formation and chain collapse. These results confirm that folding of the earliest intermediate, Ia, occurs in a highly cooperative process, in which hydrophobic collapse and secondary structure formation occur concomitantly in the A(B)GH core. Second, when the refolding of N is measured at high pH, starting from the acid-unfolded ensemble, the formation of Ia occurs in the mixing time of the sub-millisecond stopped-flow, but the subsequent steps, the Ia <= => Ib and Ib <= => N reactions, exhibit similar kinetics by far-UV CD and Trp fluorescence, indicating that these two late stages of the apoMb folding process also occur in highly cooperative, two-state reactions.

Vascular damage by unstable hemoglobins: The role of heme-depleted globin

The study compared the damage inflicted to endothelial cells (ECs) by intact hemoglobin (Hb) and isolated chains. To compare optional in vivo contact of acellular Hb with the endothelium, oxy-forms of Hb and its isolated alpha- and beta-chains existing in the thalassemias were added to primary confluent cultures of bovine aorta EC. Cell damage was followed by morphological changes or leakage of lactic dehydrogenase and pre-inserted 51Cr from the cells, followed for 27 h. Under these experimental conditions, Hb did not affect the cells but its chains inflicted damage, beta- more than alpha-chains. Based on the literature and our data, we hypothesized that hemin and/or globin should be responsible for the increased endothelial damage by beta-chains. While hemin hardly affected ECs, globin, unlike the plasma protein hemopexin, was harmful. Since hemin release leaves globin with a large hydrophobic surface, the globin-damage appears to result from adsorptive pinocytosis to endothelial membrane.

Molecular mechanism of high altitude respiration: primary structure of a minor hemoglobin component from Tufted duck (Aythya fuligula, Anseriformes)

Avian hemoglobins have attracted much attention in view of the unique oxygen transport characteristics. The present study describes the primary structure of minor hemoglobin component HbD from Tufted duck (Aythya fuligula), a migratory bird seen in Pakistan during the winter season. Separation of the polypeptide subunits was achieved by ion exchange chromatography in the presence of 8M urea. Molecular masses of the intact protein as well as peptides obtained from chemical and enzymatic cleavages were determined by electrospray ionization mass spectrometry. The sequence was studied by automatic Edman degradation of the native chains and their tryptic/hydrolytic fragments in a gas-phase sequencer. Comparison of the hemoglobin sequence with the corresponding sequences of Anseriform representatives and other avian species shows residues like alpha(D)23 Asp, alpha(D)120 Asp as being specific to Tufted duck. The three-dimensional structure analyzed with the protein structure modeling package, WHAT IF, using the crystal structure coordinates of chicken hemoglobin (PDB code=1hbr) shows alpha(D)34 Val, alpha(D)38 Gln, and alpha(D)94 Asp as possible mediators offering alternate pathway for oxygen uptake and release thereby leading to distinct hypoxia tolerance in the Tufted ducks. Results are discussed with reference to function and evolution in the Anseriform representatives.

Protein self-association in crowded protein solutions: a time-resolved fluorescence polarization study

The self-association equilibrium of a tracer protein, apomyoglobin (apoMb), in highly concentrated crowded solutions of ribonuclease A (RNase A) and human serum albumin (HSA), has been studied as a model system of protein interactions that occur in crowded macromolecular environments. The rotational diffusion of the tracer protein labeled with two different fluorescent dyes, 8-anilinonaphthalene-1-sulfonate and fluorescein isothiocyanate, was successfully recorded as a function of the two crowder concentrations in the 50-200 mg/mL range, using picosecond-resolved fluorescence anisotropy methods. It was found that apoMb molecules self-associate at high RNase A concentration to yield a flexible dimer. The apparent dimerization constant, which increases with RNase A concentration, could also be estimated from the fractional contribution of monomeric and dimeric species to the total fluorescence anisotropy of the samples. In contrast, an equivalent mass concentration of HSA does not result in tracer dimerization. This different effect of RNase A and HSA is much larger than that predicted from simple models based only on the free volume available to apoMb, indicating that additional, nonspecific interactions between tracer and crowder should come into play. The time-resolved fluorescence polarization methods described here are expected to be of general applicability to the detection and quantification of crowding effects in a variety of macromolecules of biological relevance.

Identification of 4-hydroxy-2-nonenal-modified peptides within unfractionated digests using matrix-assisted laser desorption/ionization time-of-flight mass spectrometry

The lipid peroxidation product 4-hydroxy-2-nonenal (HNE) is generated as a consequence of oxidative stress and can readily react with nucleophilic sites of proteins (e.g., histidine residues), mainly via a Michael addition. The formation of such lipid-protein conjugates can alter protein properties and biological functions, thus leading to highly deleterious effects. The present work describes a rapid (very limited sample preparation) and sensitive (low-femtomole range) procedure to identify HNE-modified peptides (Michael adducts) within unfractionated tryptic digests. The protocol involves the formation of dinitrophenylhydrazones of the Michael adducts, when using 2,4-dinitrophenylhydrazine as reactive matrix, followed by analysis using matrix-assisted laser desorption/ionization time-of-flight mass spectrometry (MALDI-TOFMS). The hydrazone derivatives present high desorption/ionization yield and can thus be preferentially detected compared to unmodified peptides. The MALDI mass spectrum obtained is therefore drastically different from the one obtained with the classical 4-hydroxy-alpha-cyanocinnamic acid matrix. Moreover, the presence of HNE, or more generally speaking carbonylated peptides, could be highlighted by 180 mass units differences (corresponding to the dinitrophenylhydrazone moiety) between these two MALDI mass spectra. Further information (e.g., localization/identification of the modified residues, peptide sequences) could be obtained by performing MALDI postsource decay (or electrospray) MS/MS experiments on the ions of interest.

Origin of the anomalous circular dichroism spectra of many apomyoglobin mutants

Several authors have reported that many sperm whale apomyoglobin mutants show anomalous circular dichroism spectra. These mutants have a low molar ellipticity compared to the wild-type protein but in several cases have the same stability of unfolding. A model in which native apomyoglobin is not folded in the same manner as that in other proteins and in which mutants show progressive reductions in their degree of folding has been suggested to explain this phenomenon. However, nuclear magnetic resonance of the native apomyoglobin conformation has shown that this state is folded and compact, raising the possibility that the anomalous circular dichroism spectra could have another explanation. We studied several mutants with anomalous circular dichroism spectra and found that these proteins were all contaminated with nucleic acid that contributed to the ultraviolet absorption and caused uncertainty in the determination of protein concentration. The resulting overestimation of the concentration of apomyoglobin explains the phenomenon of anomalous circular dichroism spectra. We describe a procedure to remove the contaminant nucleic acid which yields accurate protein concentration measurements and provides the normal circular dichroism spectra. Our findings support a well-structured native conformation for apomyoglobin and may also be of the interest to scientists working with the purification of recombinant proteins.

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.

Characterization of non-covalent oligomers of proteins treated with hypochlorous acid

Hypochlorous acid (HOCl) is a potent oxidant produced by myeloperoxidase that causes aggregation of many proteins. Treatment of apohaemoglobin and apomyoglobin with HOCl produced a regular series of oligomer bands when the proteins were separated by SDS/PAGE under reducing conditions. Aggregation was detectable at a HOCl/protein molar ratio of 0.5:1 and was maximal at ratios of 10:1-20:1. Dimers formed within 1 min of adding HOCl, and further aggregation occurred over the next 30 min. No convincing evidence for covalent cross-linking was obtained by amino acid analysis, peptide analysis or electrospray ionization-MS of HOCl-modified apomyoglobin. The latter showed an increase in mass consistent with conversion of the two methionine residues into sulphoxides. A 5-fold excess of HOCl generated approximately three chloramines on the apomyoglobin. These underwent slow decay. Protein carbonyls were formed and were almost entirely located only on the polymer bands. Conversion of positively into negatively charged groups on the protein by succinylation caused preformed aggregates to dissociate. Treatment of apomyoglobin with taurine chloramine generated methionine sulphoxides but few protein carbonyls, and did not result in aggregation. We conclude that aggregation was due to strong, non-covalent interactions between protein chains. We propose that formation of protein carbonyls and possibly chloramines, along with methionine oxidation, alters protein folding to expose hydrophobic areas on neighbouring molecules that associate to form dimers and higher-molecular-mass aggregates. This process could lead to the formation of aggregated proteins at sites of myeloperoxidase activity and contribute to inflammatory tissue injury.

Functional and spectroscopic characterization of half-liganded iron-zinc hybrid hemoglobin: evidence for conformational plasticity within the T state

Functionally distinct conformations of HbA (human adult hemoglobin) were probed using deoxy and diliganded derivatives of symmetric Fe-Zn hybrids of HbA. To expand the range of accessible structures, different environments were utilized including solution, sol-gel encapsulation, and crystals. Further structural and functional modulation was achieved by the addition of allosteric effectors. Functional characterization included oxygen affinity measurements, CO combination rates, and geminate and bimolecular CO recombination, after photodissociation. The conformational properties were studied using visible resonance Raman spectroscopy as a probe of local tertiary structure at the iron-containing hemes and UV resonance Raman spectroscopy as a probe of elements of the globin known to be sensitive to quaternary structure. The combined results show a pattern in which there is a progression of conformational and functional properties that are consistent with a picture in which the T quaternary structure can accommodate a range of tertiary conformations (plasticity). At one end of the distribution is the equilibrium deoxy T state conformation that has the lowest ligand reactivity. At the other end of the distribution are T state conformations with higher ligand reactivity that exhibit "loosened" T state constraints within the globin including the alpha(1)beta(2) interface and reduced proximal strain at the heme.