Structure of the haptoglobin-haemoglobin complex

Department of Biomedicine, Aarhus University, 8000 Aarhus C, Denmark.
Nature (Impact Factor: 41.46). 08/2012; 489(7416):456-9. DOI: 10.1038/nature11369
Source: PubMed


Red cell haemoglobin is the fundamental oxygen-transporting molecule in blood, but also a potentially tissue-damaging compound owing to its highly reactive haem groups. During intravascular haemolysis, such as in malaria and haemoglobinopathies, haemoglobin is released into the plasma, where it is captured by the protective acute-phase protein haptoglobin. This leads to formation of the haptoglobin-haemoglobin complex, which represents a virtually irreversible non-covalent protein-protein interaction. Here we present the crystal structure of the dimeric porcine haptoglobin-haemoglobin complex determined at 2.9 Å resolution. This structure reveals that haptoglobin molecules dimerize through an unexpected β-strand swap between two complement control protein (CCP) domains, defining a new fusion CCP domain structure. The haptoglobin serine protease domain forms extensive interactions with both the α- and β-subunits of haemoglobin, explaining the tight binding between haptoglobin and haemoglobin. The haemoglobin-interacting region in the αβ dimer is highly overlapping with the interface between the two αβ dimers that constitute the native haemoglobin tetramer. Several haemoglobin residues prone to oxidative modification after exposure to haem-induced reactive oxygen species are buried in the haptoglobin-haemoglobin interface, thus showing a direct protective role of haptoglobin. The haptoglobin loop previously shown to be essential for binding of haptoglobin-haemoglobin to the macrophage scavenger receptor CD163 (ref. 3) protrudes from the surface of the distal end of the complex, adjacent to the associated haemoglobin α-subunit. Small-angle X-ray scattering measurements of human haptoglobin-haemoglobin bound to the ligand-binding fragment of CD163 confirm receptor binding in this area, and show that the rigid dimeric complex can bind two receptors. Such receptor cross-linkage may facilitate scavenging and explain the increased functional affinity of multimeric haptoglobin-haemoglobin for CD163 (ref. 4).

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    • "Haptoglobin-haemoglobin consists of a dimer of haptoglobin chains, each interacting with an αβ dimer of haemoglobin, and adopts a dimeric 'dumbell-shaped' architecture (Andersen et al., 2012). "
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    ABSTRACT: The haptoglobin-haemoglobin receptor (HpHbR) of African trypanosomes allows acquisition of haem and provides an uptake route for trypanolytic factor-1, a mediator of innate immunity against trypanosome infection. Here we report the structure of Trypanosoma brucei HpHbR in complex with human haptoglobin-haemoglobin (HpHb), revealing an elongated ligand-binding site that extends along its membrane distal half. This contacts haptoglobin and the β-subunit of haemoglobin, showing how the receptor selectively binds HpHb over individual components. Lateral mobility of the glycosylphophatidylinositol-anchored HpHbR, and a ~50{degree sign} kink in the receptor, allows two receptors to simultaneously bind one HpHb dimer. Indeed, trypanosomes take up dimeric HpHb at significantly lower concentrations than monomeric HpHb, due to increased ligand avidity that comes from bivalent binding. The structure therefore reveals the molecular basis for ligand and innate immunity factor uptake by trypanosomes, and identifies adaptations that allow efficient ligand uptake in the context of the complex trypanosome cell surface.
    Full-text · Article · Dec 2014 · eLife Sciences
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    • "mg/mL [12,13]. These three Hp polymorphisms all contain the same Hb binding β globin (Hpβ), but differ in their α globin (Hpα1 or Hpα2) composition [14,15]. This results in dimeric (Hp 1-1) or multimeric (Hp 2-1, Hp 2-2) forms, as denoted by the number of α globin cysteines involved in disulfide bond formation. "
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    ABSTRACT: Endotoxemia plays a major causative role in the myocardial injury and dysfunction associated with sepsis. Extracellular hemoglobin (Hb) has been shown to enhance the pathophysiology of endotoxemia. In the present study, we examined the myocardial pathophysiology in guinea pigs infused with lipopolysaccharide (LPS), a Gram-negative bacterial endotoxin, and purified Hb. We also examined whether the administration of the Hb scavenger haptoglobin (Hp) could protect against the effects observed. Here, we show that Hb infusion following LPS administration, but not either insult alone, increased myocardial iron deposition, heme oxygenase-1 expression, phagocyte activation and infiltration, as well as oxidative DNA damage and apoptosis assessed by 8-hydroxy-2'-deoxyguanosine (8-OHdG) and terminal deoxynucleotidyl transferase dUTP nick end labeling (TUNEL) immunostaining, respectively. Co-administration of Hp significantly attenuated the myocardial events induced by the combination of LPS and Hb. These findings may have relevant therapeutic implications for the management of sepsis during concomitant disease or clinical interventions associated with the increased co-exposures to LPS and Hb, such as trauma, surgery or massive blood transfusions.
    Full-text · Article · Apr 2014 · Toxins
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    • "Haptoglobin (Hp) is a Hb-scavenging plasma glycoprotein that binds noncovalently to hemoglobin dimers that are generated by dissociation of acellular Hb tetramers after hemolysis [6]. Hp has been shown to reduce the oxidative toxicity of acellular Hb in part by facilitating its rapid removal from circulation through CD163- receptor-mediated endocytosis [7] [8]. Additionally, Hp binding impedes the renal clearance of Hb [9], dramatically diminishes the rate of heme dissociation from ferric Hb dimers [10] [11] [12], and "
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    ABSTRACT: Haptoglobin (Hp) is an abundant and conserved plasma glycoprotein, which binds acellular adult hemoglobin (Hb) dimers with high affinity and facilitates their rapid clearance from circulation following hemolysis. Humans possess three main phenotypes of Hp, designated Hp 1-1, Hp 2-1, and Hp 2-2. These variants exhibit diverse structural configurations and have been reported to be functionally non-equivalent. We have investigated the functional and redox properties of Hb-Hp complexes prepared using commercially fractionated Hp and found that all forms exhibit similar behavior. The rate of Hb dimer binding to Hp occurs with bimolecular rate constants of ∼0.9μM(-1)s(-1), irrespective of the type of Hp assayed. Although Hp binding does accelerate the observed rate of HbO2 autooxidation by dissociating Hb tetramers into dimers, the rate observed for these bound dimers is 3- to 4-fold slower than that of Hb dimers free in solution. Co-incubation of ferric Hb with any form of Hp inhibits heme loss to below detectable levels. Intrinsic redox potentials (E1/2) of the ferric/ferrous pair of each Hb-Hp complex are similar, varying from +54 to +59mV (vs NHE), and are essentially the same as reported by us previously for Hb-Hp complexes prepared from unfractionated Hp. All Hb-Hp complexes generate similar high amounts of ferryl Hb following exposure to hydrogen peroxide. EPR data indicate that the yields of protein-based radicals during this process are approximately 4% to 5%, and are unaffected by the variant of Hp assayed. These data indicate that the Hp fractions examined are equivalent to each other with respect to Hb binding and associated stability and redox properties, and that this result should be taken into account in the design of phenotype-specific Hp therapeutics aimed at countering Hb-mediated vascular disease.
    Full-text · Article · Jan 2014 · Free Radical Biology and Medicine
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