Structural contribution of the A-chain loop in relaxin.

Site-directed sequential disulfide bond formation has been used to synthesize relaxin analogs with modifications in the A chain loop (A10-A15). In the four different derivatives either the amino acid residues between the cysteines (A12-A14) were replaced or the intrachain disulfide bond (A10-A15) was eliminated. The substitution of the human relaxin II sequence (His-Val-Gly; A12-14) by the corresponding insulin sequence (Thr-Ser-Ile) or the hydrocarbon chain of omega-aminooctanoic acid (Aoc) caused significant loss of biological activity. Similar observations were made when the pair of cysteines (A10-A15) was replaced by either alanine or serine, whereby serine disturbs more than alanine. It is suggested that the structural features of the A chain loop not only make important contributions to the active conformation of relaxin but also that the structural requirements of insulin and relaxin are different.