In silico model showing the effect of R17C mutation on disulfide bonding in THPO receptor-binding domain. (A) The THPO receptor-binding domain consists of a 4-helix bundle (red, shown as letters A-D) and a short antiparallel β sheet formed by 2 strands. The domain contains 4 highly conserved cysteine residues (C7, C29, C85, and C151), which form 2 disulfide bonds. Due to position constraints, the mutant C17 cannot form a disulfide bond with any of C7, C29, and C85 residues without disrupting the overall structure but can form a disulfide bond with C151 as shown in B.