Haptoglobin (Hp)

Haptoglobin is an acute-phase protein in the blood plasma and consists of a complement control protein (CCP) domain and a serine protease domain (Polticelli et al., 2008). The serine protease domain of haptoglobin shows high similarity to chymotrypsin like proteases. The two anti-parallel beta barrels and two or three alpha helices each are found in Hp as well as in chymotrypsin like proteases (Figure 3). 

Figure 3: Haptoglobin Serine protease domain with β-barrels in yellow and α-helices in red.

The CCP domains of two haptoglobin monomers dimerise to form a fusion CCP domain. In normal monomeric conformation an antiparallel beta sheet is formed between the B1 and B2 beta strands of the CCP domain. For dimerisation B1 and B2 join to a single beta strand and form an antiparallel beta sheet with the B1/B2 strand of the other CCP domain. This process is called a beta strand swap. Additionally, a disulphide bond is formed between the two Cys33 residues of the CCP domains (Figure 4) (Andersen et al., 2012).

Figure 4: Beta swap of two haptoglobin CCP domains and covalent  disulphide bridge in yellow.