A unique loop extension in the serine protease domain of haptoglobin is essential for CD163 recognition of the haptoglobin-hemoglobin complex.

Haptoglobin and haptoglobin-related protein are homologous hemoglobin-binding proteins consisting of a complement control repeat (alpha-chain) and a serine protease domain (beta-chain). Haptoglobin-hemoglobin complex formation promotes high affinity binding of hemoglobin to the macrophage scavenger receptor CD163 leading to endocytosis and degradation of the haptoglobin-hemoglobin complex. In contrast, complex formation between haptoglobin-related protein and hemoglobin does not promote high affinity interaction with CD163. To define structural components of haptoglobin important for CD163 recognition, we exploited this functional difference to design and analyze recombinant haptoglobin/haptoglobin-related protein chimeras complexed to hemoglobin. These data revealed that only the beta-chain of haptoglobin is involved in receptor recognition. Substitution of 4 closely spaced amino acid residues of the haptoglobin beta-chain (valine 259, glutamate 261, lysine 262, and threonine 264) abrogated the high affinity receptor binding. The 4 residues are encompassed by a part of the primary structure not present in other serine protease domain proteins. Structural modeling based on the well characterized serine protease domain fold suggests that this sequence represents a loop extension unique for haptoglobin and haptoglobin-related protein. A synthetic peptide representing the haptoglobin loop sequence exhibited a pronounced inhibitory effect on receptor binding of haptoglobin-hemoglobin.