Fig. 1

Haptoglobin structure. a Schematic representation of Hp1 and Hp2. The common α-chain sequence of Hp1 and Hp2 is green. The blue colored amino acid sequence within the α-chain of Hp2 determines the distinct molecular phenotypes. This sequence corresponds to exons 3 and 4 of the ancestral HP2 allele and was deleted due to non-allelic homologous recombination during the structural evolution of HP2 to HP1. The cysteine residues that participate in protein multimerization are marked with a purple star. Both Hp1 and Hp2 further segregate into subtypes S and F or FS and SS, respectively, based onto two base mutations at positions 52 and 53 or 111 and 112. The C1r-LP cleavage sites with the red colored two base substitution sites in the mutated rHp are marked with black arrows. b Dimerization and proteolytic C1r-LP cleavage of pro-Hp 1 to Hp 1–1 dimer. c Protein quaternary structure of the Hp 1–1 homo-dimer with one inter-α-chain disulfide bond and three variants of Hp 2–2 cyclic homo-multimers with two inter-α-disulfide bonds