Relaxin structure. Quasi allosteric effect of the NH2-terminal A-chain helix

The NH2-terminal heptapeptide in the relaxin A-chain (Arg-Met-Thr-Leu-Ser-Glu-Lys) has been replaced by chemical means with three different helix-promoting peptides (Arg-Met-Ala-Ala-Ala-Ala-Ala-Ala-Ala-Ala-Ala-Ala, and the insulin segment Gly-Ile-Val-Glu-Gln). The partially protected NH2 terminally shortened relaxin derivative (N epsilon A16,N epsilon B8-bis(methyl-sulfonylethyloxycarbonyl)des-ArgA1,MetA2 , ThrA3,LeuA4,SerA5,GluA6,LysA7-B29-relaxin) has been prepared by a combination of cyanogen bromide digestion and Edman degradation of the epsilon-amino-protected derivative followed by mixed anhydride coupling with the synthetic peptides. All three derivatives have been isolated and purified by high performance liquid chromatography. Whole relaxins shortened at the NH2 terminus of the A-chain by 4 or more amino acid residues are biologically inactive in the mouse pubic symphysis assay (Büllesbach, E. E., and Schwabe, C. (1986) Biochemistry 25, 5998-6004). The introduction of the artificial peptides causes significant biological activity to reappear (about 30%). The loss of structural integrity of relaxins shortened by 4 or more residues of the A-chain NH2 terminus as observed by circular dichroism spectroscopy is largely reversed by the addition of the synthetic peptides. Our results suggest that no single amino acid in the NH2-terminal region of the A-chain is functionally important but that the presence of a helix is required for biological activity.