Mechanism

Alpha-beta chain haemoglobin (αβHb) heterodimers are rapidly bound by haptoglobin and form a tight complex. Formation of the complex is the result of an irreversible, non-covalent interaction between the two dimers. In the process, regions on the αβHb dimer linked to oxidative damage are buried in the interface of the two proteins, directly reducing harm. Loop 3 (see Fig. 6) in the serine protease domain of the haptoglobin is then recognized by CD163, a scavenger receptor protein located on the surface of macrophages responsible for endocytosis and degradation of haptoglobin-haemoglobin complexes. Efficiency of this degradation is enhanced by the fact that two receptors can simultaneously bind the loops on both ends of the symmetrical complex.

Fig. 7 Schematic illustration of haptoglobin-haemoglobin complex formation. Haemoglobin is shown in red and pink, haptoglobin in cyan and blue.

Electrostatic attractions and van der Waals interactions between exposed loops in the serine protease domain of haptoglobin, and helices and FG loops  on the alpha and beta chains of αβHb dimers stabilize the complex (see Fig 8). The latter regions are also involved in binding to the other haemoglobin chains to form the functional tetramer. This ensures haptoglobin only binds αβHb and prevents re-formation of tetrameric haemoglobin. The haptoglobin-haemoglobin complex  also buries a greater surface area at interface than the haemoglobin tetramer (2954Ų compared to 1980Ų), so formation of the of the former is favored at equilibrium.

Fig. 8 Selected van der Waals interactions between the haptoblogin serine protease domain (blue) and the αβHb dimer (red and pink). Residues involved are shown in orange.

In some cases, haptoglobin multimers are formed rather than the homodimer structure discussed so far. This is because, in humans, haptoglobin is encoded by two alleles: HP1 and HP2. HP1 codes for haptoglobin with a single CCP domain, while in HP2 the CCP domain is duplicated. This duplicated domain allows for interaction with further haptoglobin molecules via the central beta-sheet to form haptoglobin multimers. These multimers are capable of binding more αβHb dimers via their increased number of serine protease domains.