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Structure of the haptoglobin–haemoglobin complex

Abstract

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 haemoglobinopathies1, 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 interaction2. 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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Figure 1: Crystal structure of porcine Hp–Hb.
Figure 2: The Hp-bound Hb dimer is in its oxy-state.
Figure 3: The Hb contact area overlaps with the Hb dimer contact area in Hb tetramers.
Figure 4: Hb residues prone to oxidative modifications and SAXS.

Accession codes

Primary accessions

Protein Data Bank

Data deposits

Atomic structure factors and coordinates have been deposited at the Protein Data Bank under accession number 4F4O.

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Acknowledgements

We are grateful to G. Ratz for technical assistance, R. Kidmose, N. S. Laursen and the staff at the Swiss Light Source beamlines for help with data collection. We thank G. V. Jensen for performing SAXS measurements and R. E. Weber for scientific discussion. The research was supported by The Lundbeck Foundation, The Novo Nordisk Foundation, The Research Council of Norway, The European Research Council and The Danish Council for Independent Research.

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C.B.F.A.: purification, crystallization, data collection, structure determination and analysis, manuscript preparation and study design. M.T.-J.: purification, crystallization, data collection and structure determination. M.J.N.: cloning, expression and purification. C.L.P.d.O.: SAXS measurements and analysis. H.-P.H.: ultraviolet–visible spectroscopy and analysis. N.H.A.: Raman spectroscopy and analysis. J.S.P.: SAXS measurements and analysis. G.R.A.: study design. S.K.M.: manuscript preparation and study design.

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Correspondence to Christian Brix Folsted Andersen or Søren Kragh Moestrup.

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Andersen, C., Torvund-Jensen, M., Nielsen, M. et al. Structure of the haptoglobin–haemoglobin complex. Nature 489, 456–459 (2012). https://doi.org/10.1038/nature11369

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