Haptoglobin heavy and light chains. Structural properties, reassembly, and formation of minicomplex with hemoglobin

Haptoglobin: From hemoglobin scavenging to human health

Haptoglobin (Hp) belongs to the family of acute-phase plasma proteins and represents the most important plasma detoxifier of hemoglobin (Hb). The basic Hp molecule is a tetrameric protein built by two α/β dimers. Each Hp α/β dimer is encoded by a single gene and is synthesized as a single polypeptide. Following post-translational protease-dependent cleavage of the Hp polypeptide, the α and β chains are linked by disulfide bridge(s) to generate the mature Hp protein. As human Hp gene is characterized by two common Hp1 and Hp2 alleles, three major genotypes can result (i.e., Hp1-1, Hp2-1, and Hp2-2). Hp regulates Hb clearance from circulation by the macrophage-specific receptor CD163, thus preventing Hb-mediated severe consequences for health. Indeed, the antioxidant and Hb binding properties of Hp as well as its ability to stimulate cells of the monocyte/macrophage lineage and to modulate the helper T-cell type 1 and type 2 balance significantly associate with a variety of pathogenic disorders (e.g., infectious diseases, diabetes, cardiovascular diseases, and cancer). Alternative functions of the variants Hp1 and Hp2 have been reported, particularly in the susceptibility and protection against infectious (e.g., pulmonary tuberculosis, HIV, and malaria) and non-infectious (e.g., diabetes, cardiovascular diseases and obesity) diseases. Both high and low levels of Hp are indicative of clinical conditions: Hp plasma levels increase during infections, inflammation, and various malignant diseases, and decrease during malnutrition, hemolysis, hepatic disease, allergic reactions, and seizure disorders. Of note, the Hp:Hb complexes display heme-based reactivity; in fact, they bind several ferrous and ferric ligands, including O₂, CO, and NO, and display (pseudo-)enzymatic properties (e.g., NO and peroxynitrite detoxification). Here, genetic, biochemical, biomedical, and biotechnological aspects of Hp are reviewed.

Haptoglobin preferentially binds β but not α subunits cross-linked hemoglobin tetramers with minimal effects on ligand and redox reactions

Human hemoglobin (Hb) and haptoglobin (Hp) exhibit an extremely high affinity for each other, and the dissociation of Hb tetramers into dimers is generally believed to be a prerequisite for complex formation. We have investigated Hp interactions with native Hb, αα, and ββ cross-linked Hb (ααXLHb and ββXLHb, respectively), and rapid kinetics of Hb ligand binding as well as the redox reactivity in the presence of and absence of Hp. The quaternary conformation of ββ subunit cross-linking results in a higher binding affinity than that of αα subunit cross-linked Hb. However, ββ cross-linked Hb exhibits a four fold slower association rate constant than the reaction rate of unmodified Hb with Hp. The Hp contact regions in the Hb dimer interfaces appear to be more readily exposed in ββXLHb than ααXLHb. In addition, apart from the functional changes caused by chemical modifications, Hp binding does not induce appreciable effects on the ligand binding and redox reactions of ββXLHb. Our findings may therefore be relevant to the design of safer Hb-based oxygen therapeutics by utilizing this preferential binding of ββXLHb to Hp. This may ultimately provide a safe oxidative inactivation and clearance pathway for chemically modified Hbs in circulation.

Coarse-grained modeling of protein second osmotic virial coefficients: sterics and short-ranged attractions

A series of coarse-grained models, with different levels of structural resolution, were tested to calculate the steric contributions to protein osmotic second virial coefficients (B(22,S)) for proteins ranging from small single-domain molecules to large multidomain molecules, using the recently developed Mayer sampling method. B(22,S) was compared for different levels of coarse-graining: four-beads-per-amino-acid (4bAA), one-bead-per-amino-acid (1bAA), one-sphere-per-domain (1sD), and one-sphere-per-protein (1sP). Values for the 1bAA and 4bAA models were quantitatively indistinguishable for both spherical and nonspherical proteins, and the agreement with values from all-atom models improved with increasing protein size, making the CG approach attractive for large proteins of biotechnological interest. Interestingly, in the absence of detailed structural information, the hydrodynamic radius (R(h)) along with a simple 1sP approximation provided reasonably accurate values for B(22,S) for both globular and highly asymmetric protein structures, while other 1sP approximations gave poorer agreement; this helps to justify the currently empirical practice of estimating B(22,S) from R(h) for large proteins such as antibodies. The results also indicate that either 1bAA or 4bAA CG models may be good starting points for incorporating short-range attractions. Comparison of gD-crystallin B(22) values including both sterics and short-range attractions shows that 1bAA and 4bAA models give equivalent results when properly scaled to account for differences in the number of surface beads in the two CG descriptions. This provides a basis for future work that will also incorporate long-ranged electrostatic attractions and repulsions.

Glycoproteomic analysis and molecular modeling of haptoglobin multimers

Extra-thiol groups on the α-subunit allow haptoglobin (Hp) to form a variety of native multimers which influence the biophysical and biological properties of Hp. In this work, we demonstrated how differences of multimeric conformation alter the glycosylation of Hp. The isoform distributions of different multimers were examined by an alternative approach, i.e. 3-D-(Native/IEF/SDS)-PAGE, which revealed differences in N-glycosylation among individual multimers of the same Hp sample. Glycomic mapping of permethylated N-glycan indicated that the assembled monomer and multimeric conformation modulate the degree of glycosylation, especially the reduction in terminal sialic acid residues on the bi-antennary glycan. Loss of the terminal sialic acid in the higher order multimers increases the number of terminal galactose residues, which may contribute to conformation of Hp. A molecular model of the glycosylated Hp multimer was constructed, suggesting that the effect of steric hindrance on multimeric formation is critical for the enlargement of the glycan moieties on either side of the monomer. In addition, N241 of Hp was partially glycosylated, even though this site is unaffected by steric consideration. Thus, the present study provides evidence for the alteration of glycan structures on different multimeric conformations of Hp, improving our knowledge of conformation-dependent function of this glycoprotein.

Hemoglobin and heme scavenger receptors

Heme, the functional group of hemoglobin, myoglobin, and other hemoproteins, is a highly toxic substance when it appears in the extracellular milieu. To circumvent potential harmful effects of heme from hemoproteins released during physiological or pathological cell damage (such as hemolysis and rhabdomyolysis), specific high capacity scavenging systems have evolved in the mammalian organism. Two major systems, which essentially function in a similar way by means of a circulating latent plasma carrier protein that upon ligand binding is recognized by a receptor, are represented by a) the hemoglobin-binding haptoglobin and the receptor CD163, and b) the heme-binding hemopexin and the receptor low density lipoprotein receptor-related protein/CD91. Apart from the disclosure of the molecular basis for these important heme scavenging systems by identifying the functional link between the carrier proteins and the respective receptors, research over the last decade has shown how these systems, and the metabolic pathways they represent, closely relate to inflammation and other biological events.

Receptor targeting of hemoglobin mediated by the haptoglobins: roles beyond heme scavenging

Haptoglobin, the haptoglobin-hemoglobin receptor CD163, and the heme oxygenase-1 are proteins with a well-established function in the clearance and metabolism of "free" hemoglobin released during intravascular hemolysis. This scavenging system counteracts the potentially harmful oxidative and NO-scavenging effects associated with "free" hemoglobin, and, furthermore, elicits an anti-inflammatory response. In the late primate evolution, haptoglobin variants with distinct functions have arisen, including haptoglobin polymers and the haptoglobin-related protein. The latter associates with a subspecies of high-density lipoprotein (HDL) particles playing a crucial role in the innate immunity against certain trypanosome parasites. Recent studies have elucidated this fairly sophisticated immune defense mechanism that takes advantage of a trypanosomal haptoglobin-hemoglobin receptor evolved to supply the parasite with heme. Because of the high resemblance between haptoglobin and haptoglobin-related protein, the receptor also takes up the complex of hemoglobin and the HDL-bound haptoglobin-related protein. This tricks the parasite into internalizing another HDL-associated protein and toxin, apolipoprotein L-I, that kills the parasite. In conclusion, variant human homologous hemoglobin-binding proteins that collectively may be designated the haptoglobins have diverted from the haptoglobin gene. On hemoglobin and receptor interaction, these haptoglobins contribute to different biologic events that go beyond simple removal from plasma of the toxic hemoglobin.

Methemoglobin is a supplement for in vitro culture of human nasopharyngeal epithelial cells transformed by human papillomavirus type 16 DNA

NPC-N cells were normal human nasopharvngeal epithelial cells transformed by transfection with human papillomavirus type 16 deoxyribonucleic acid. Bovine pituitary extract (BPE) was one of the indispensable ingredients for in vitro culture of NPC-N cells in a serum-free medium. Chromatographic fractionation of BPE and subsequent immunoblotting analyses identified the hemoglobin growth-stimulating factor. Methemoglobin (metHb) was then synthesized, and also found to be growth stimulating. The growth-stimulating effect of metHb was abolished when NPC-N cells were cultured in a medium that also contained haptoglobin, a molecule that binds to hemoglobin. A defined medium consisting of insulin and metHb was then developed for optimal growth of NPC-N cells. MetHb kept under the conditions identical to those of cell culture released hemin which also enhanced the cell growth. Though all the degradation products of hemin are currently known to be physiologically significant. only ferric iron derived from metHb or hemin could stimulate the growth of NPC-N cells. Abnormal vasculature showing leaky walls and hemorrhage is a common feature of malignant tumors. Hemoglobin originating from extravasated red blood cells and subsequently oxidized to metHb because of the presence of activated inflammatory cells might contribute to the increased proliferation of cancerous cells.

Methaemoglobin enhances the proliferation of transformed human epithelial cells: a possible outcome of neovascularisation and haemorrhage in tumours?

The effect of human methaemoglobin (metHb), possibly derived from extravasated red blood cells in tumours showing neovascularisation and haemorrhage, on the growth of transformed human epithelial cells was investigated. MetHb stimulated the growth of immortalised epithelial cells or transformed cells at precrisis stage (cells have bypassed M1, but not M2, the two mortality checkpoints). The stimulatory effect was due to the release of haemin from metHb that was isolated by a Sephadex column and identified by its characteristic light absorption spectrum. Although all the degradation products of haemin are currently known to be physiologically significant, only ferric iron derived from metHb or haemin could stimulate cell growth. High concentrations of metHb or haemin inhibited cell growth possibly due to the generation of high concentrations of bilirubin. However, bilirubin formed in the cells of human body is known to be transported to the liver for further processing and excretion. Haemoglobin oxidised to where tumours show neovascularisation and haemorrhage likely contributes significantly to the increased proliferation of cancerous cells.

Comparison of haptoglobin and apolipoprotein A-I on biliary lipid particles involved in cholesterol crystallization

Several proteins are known to modulate cholesterol crystallization. We recently demonstrated that haptoglobin has cholesterol crystallization promoting activity. However, this effect is still not well understood mechanistically. The current study examined the distribution of haptoglobin compared to apolipoprotein A-I (apo A-I) to micelles, vesicles and crystals as an initial step in providing a focus for further studies of the mechanism of cholesterol crystallization activity. Specific protein purification was accomplished by immunoaffinity chromatography. The crystallization-promoting activity of biliary haptoglobin, albumin and commercial apo A-I was measured by a photometric crystal growth assay. The distribution of micelles, vesicles and proteins in model bile was determined by Sepharose CL-6B column chromatography. Detection of the presence of test proteins in cholesterol crystals was determined using specific 125I-radiolabelled proteins. Haptoglobin (20 micrograms/mL) showed a significant crystallization promoting-activity, whereas apo A-I (30 micrograms/mL) only tended to show a slight inhibitory activity. The cholesterol crystal-bound protein in each case was found to be less than 1% of the total concentration of that protein that had been added to the model bile system. The elution profile of commercial apo A-I from a Sepharose CL-6B column was strikingly altered when it was added to model bile prior to elution. In contrast, the column elution profiles for both haptoglobin and albumin were unchanged when model bile was similarly added to the sample. Haptoglobin increased the amount of cholesterol found in the vesicular fraction when compared to apo A-I. Haptoglobin does not bind tightly to either biliary lipid particles or to cholesterol crystals but does increase the amount of cholesterol in vesicles by inducing a shift from micellar cholesterol (P = 0.046). This shift appears to explain in part its promoting effect on cholesterol crystallization.

Identification of the bile acid binding proteins in human serum by photoaffinity labeling

The binding of conjugated and unconjugated bile acids to human serum lipoproteins was investigated by density gradient centrifugation and photoaffinity labeling studies. The binding of bile acids to high-density lipoprotein increased by substitution of the 3 alpha-hydroxy group in cholate and taurocholate by a photolabile 3-azido or 3-azi-function. The affinity of bile acid derivatives to HDL showed the following ranking: 3 beta-azido-7 alpha,12 alpha-dihydroxy-,3,3-azo-7 alpha,12 alpha-dihydroxy- > 3 alpha,7 alpha,12 alpha-trihydroxy-,11 xi-azido-3 alpha,7 alpha,12 xi-trihydroxy- > 11 xi-azido-12-oxo-3 alpha,7 alpha-dihydroxy- > 7,7-azo-3 alpha,12 alpha-dihydroxy-,3 alpha,7 alpha-dihydroxy-,3 alpha,12 alpha-dihydroxy- > 3 alpha-hydroxy-cholan-24-oic acid. Based on the actual serum concentrations of albumin and HDL, a preference of hydrophilic bile acids to HDL is evident, the 3-azido- and 3-azi-derivatives showing a 5-23-fold higher binding to HDL compared to soluble serum proteins. For the identification of the bile acid binding proteins in human blood, photoaffinity labeling with a variety of photolabile conjugated and unconjugated bile acid derivatives was performed with subsequent analysis of radiolabeled serum proteins by one- and two-dimensional gel electrophoresis. In addition to albumin and the apolipoproteins A-I and A-II of high-density lipoproteins (Kramer et al. (1979) Eur. J. Biochem. 102, 1-9), three further proteins in the lipoprotein free serum fraction of M(r) 41,000, 50,000 and 83,000 were specifically labeled. By two-dimensional electrophoresis and by immunoprecipitation these proteins were identified as alpha 1-acid glycoprotein (M(r) 41,000), alpha 1-antitrypsin (M(r) 50,000) and transferrin (M(r) 83,000). No binding of bile acids to haptoglobin, alpha 2-HS-glycoprotein, hemopexin or alpha 1-fetoprotein occurred. In conclusion, these studies show that bile acid derivatives bind to several serum proteins in addition to albumin and furthermore that the substituent in position 3 of the steroid nucleus greatly influences the affinity of bile acids to high density lipoproteins.

Hemoglobin binding with haptoglobin: delineation of the haptoglobin binding site on the alpha-chain of human hemoglobin

Previous studies from this laboratory employing a comprehensive synthetic overlapping peptide strategy showed that the alpha-chain of human hemoglobin (Hb) contains a single haptoglobin (HP) binding region residing within residues alpha 121-135. The present study describes a precise delineation of this Hp-binding site on the alpha-chain. Two overlapping peptides (alpha 111-125 and alpha 121-135) spanning this region and a panel of five peptides decreasing at the C-terminal from residue 135 by decrements of two residues (alpha 119-135, alpha 119-133, alpha 119-131, alpha 119-129, and alpha 119-127) were synthesized, purified, and characterized. Quantitative radiometric titration of 125I-labeled human HP (type 2-1) with adsorbents of each of these synthetic peptides showed that the peptide alpha 119-127 retained a Hp-binding activity equivalent to that of peptide alpha 121-135. This finding indicated that Lys-127 marked the C-terminal boundary of the binding site. Another panel of eight peptides was then synthesized, which had their C-terminus fixed at Lys-127 and increased at the N-terminus by one-residue increments from residue 122 up to residue 115 (alpha 122-127, alpha 121-127, alpha 120-127, alpha 119-127, alpha 118-127, alpha 117-127, alpha 116-127, and alpha 115-127). The binding of 125I-Hp to adsorbents of these peptides demonstrated that the N-terminal boundary of the site did not extend beyond Valine 121. It is, therefore, concluded that the Hp-binding site on the alpha-chain of human Hb comprises residues alpha 121-127.

Methods to quantitate human haptoglobin by complexation with hemoglobin

Native human haptoglobin isolated from normal human plasma by affinity chromatography on chicken hemoglobin -Sepharose was used as standard antigen. A direct sandwich ELISA for haptoglobin was developed, with human hemoglobin as a capturing agent. The peroxidase activity of the complex was measured as a means of detecting functional haptoglobin. The reactivities of monoclonal antibody vs. polyclonal antisera on the haptoglobin-hemoglobin complex were compared. Adopting a monoclonal antibody, clone 21.7, which is directed to the alpha chain of haptoglobin, a specific method to quantitate the native haptoglobin which can complex with hemoglobin has been developed.

Hemoglobin binding to deglycosylated haptoglobin

The carbohydrate portion of polymeric haptoglobin was gradually removed by exoglycosidases in order to investigate its role in complex formation between haptoglobin and hemoglobin. Total removal of sialic acid diminished the haptoglobin-hemoglobin complex formation 15%. Removal of about 25% of the galactose residues from asialohaptoglobin, i.e., about 40% of the total weight of the carbohydrate moiety, totally inhibited the ability of haptoglobin to form complex with hemoglobin and react with haptoglobin-specific antibodies. Liberation of further galactose residues resulted in slow precipitation of the protein. Removal of a similar part of the carbohydrate moiety from haptoglobin-hemoglobin complex did not liberate hemoglobin from it, and the complex reacted with haptoglobin antibodies. The combined data indicate that the carbohydrate portion is essential for the functionally active form of polymeric haptoglobin to complex with hemoglobin, but it hardly has any direct role in the binding event, and other factors are responsible for the stability of the complex.

A general method for the isolation of haptoglobin 1-1, 2-1, and 2-2 from human plasma

A general method for the isolation of haptoglobin 1-1, 2-1, and 2-2 human plasma is described. Plasma is fractionated by affinity chromatography on chicken hemoglobin-Sepharose using the full capacity of the column; then after washing the column thoroughly, haptoglobin is eluted with 8 M urea and the eluate is collected in fractions to separate active and denatured haptoglobin. The urea-free, active fractions of haptoglobin are fractionated by affinity chromatography on Affi-Gel Con A to remove nonglycoproteins, principally apolipoprotein A-I, and the haptoglobin is eluted with 0.5 M glucose. Then the haptoglobin-containing fractions are fractionated by negative immunoadsorption chromatography on anti-chicken hemoglobin-protein A-Sepharose to remove chicken hemoglobin-human haptoglobin complexes. Haptoglobin prepared by this three-step procedure is biologically active and nearly homogeneous. The recovery is approximately 70%, irrespective of phenotype. The procedure can be completed in 3 days. A partial purification of apolipoprotein A-I is obtained simultaneously by this method.

Structure of haptoglobin and the haptoglobin-hemoglobin complex by electron microscopy

The human serum protein, haptoglobin, forms a stable, irreversible complex with hemoglobin. Haptoglobin is composed of two H chains, which are connected via two smaller L chains to give a protein of 85,000 Mr. In the complex, each H chain binds an alpha beta dimer of hemoglobin for a total molecular weight of 150,000. The scanning transmission electron microscope has been used to derive new information about the shape and structure of haptoglobin and hemoglobin, and about their relative orientation in the complex. The micrographs of negatively stained images show that haptoglobin has the shape of a barbell with two spherical head groups, which are the H chains. These are connected by a thin filament with a central knob, which corresponds to the L chains. The overall length of the molecule is about 124(+/- 8) A and the interhead distance is 87 (+/- 7) A. In the haptoglobin-hemoglobin complex, the head groups are ellipsoidal and under optimal staining conditions bilobal . Thus, the alpha beta dimers are binding to the H chains, but off the long axis of the barbell by 127 degrees in a trans configuration. This angle considerably restricts the region on the surface of the H chain structure that can contain the hemoglobin binding site. The interhead group distance for complex is 116.5(+/- 6.3) A or 30 A greater than for haptoglobin. The N terminus of the beta chain was located on the trans off-axis configured barbell structure of complex by using a hemoglobin that was crosslinked between the alpha beta dimers in the region of the beta N terminus. The distances and angles that are measured on the micrographs for the native and crosslinked complex molecules permit the directions of two of the alpha beta dimer ellipsoid axes to be assigned. Taken together, these data provide an approximate relative orientation for the binding of the alpha beta dimer to the H chain of haptoglobin.

Structure and assembly of haptoglobin polymers by electron microscopy

Haptoglobin (Hp) consists of light (L) and heavy (H) chains, the latter of which combine with hemoglobin alpha beta dimers to form a highly stable complex. Human haptoglobin assembles as HL units that occur in two allelic forms; HL1 , which is monovalent, and HL2 , which is divalent. As a result, three phenotypic forms exist in the human population: Hp1-1, the homozygous form in which the monovalent HL1 unit occurs as a dimer; Hp2-2, the homozygous form of the divalent HL2 unit, which gives a series of polymers; and the heterozygous Hp2-1 form, which gives a different series of polymers. We have investigated the structures and assembly properties of these two haptoglobin polymeric series in their complexes with hemoglobin using high-resolution scanning transmission electron microscopy. Polymers of complex are composed of ellipsoidal or bilobal head groups, which are the H alpha beta subunits connected by thin filament-like structures, which are the L chains. Polymers of size up to pentamers can be identified easily by counting the number of head groups in the molecule. Complex 2-1 and complex 2-2 trimers were studied extensively. The differences in detailed morphology show that while the 2-1 trimer is a linear polymer, the 2-2 trimer is a closed circular molecule. The micrograph images suggest that complex 2-2 tetramers and pentamers, and perhaps higher forms may also be cyclic. The structure of the L2 subunit of haptoglobin is shown to be composed of two domains, which may be similar in structure to the single domain of the monovalent L1 chain. The two L2 domains are connected by a hinge that has quite limited flexibility. Using these structural models, assembly characteristics and structural properties of the trimers and tetramers of complex 2-1 and complex 2-2 are described.

Undecagold labeling of a glycoprotein: STEM visualization of an undecagoldphosphine cluster labeling the carbohydrate sites of human haptoglobin-hemoglobin complex

A new type of label for electron microscopy has been introduced recently which consists of 11 gold atoms in a compact stable cluster with an organic shell composed of primary amine-substituted phosphine ligands. The radius of the cluster is about 10 A. The (phosphine ligand) amines can be derivatized or allowed to react directly forming covalent bonds to specific sites of other molecules. This report describes the specific labeling of carbohydrate moietis on the glycoprotein human haptoglobin (Hp) in the haptoglobin-hemoglobin complex (Hp X Hb). The Hp X Hb complex is easily recognized in the EM as a barbell-shaped molecule. Only the Hp portion contains carbohydrate (eight carbohydrate chains per Hp X Hb). The carbohydrate moieties of the Hp X Hb complex were oxidized by sodium periodate to produce aldehydes. The primary amines on the undecagold cluster were allowed to react with the aldehyde residues to produce Schiff's base linkages which were subsequently reduced with sodium borohydride. Micrographs obtained on the Brookhaven National Laboratory high-resolution scanning transmission electron microscope (STEM) showed the undecagold label to be localized in a region known to be occupied by the heavy chains of haptoglobin. The amount of labeling was found to be two to four gold clusters per molecule when excess label was reacted. The variation in position of the label is discussed and may be due to flexibility of the carbohydrate chains. Control experiments ruled out nonspecific binding of the gold cluster to the Hp X Hb. The high chemical specificity of the reaction and the high resolution of the gold cluster should make this new label of widespread value in studies of other glycoproteins or carbohydrate-bearing molecules.