Functional importance of the A chain loop in relaxin and insulin.

Relaxin and insulin are disulfide homologues with divergent functions and antigenicity. We have synthesized human relaxin II and porcine insulin and several A chain loop variants of each and measured the effect of substitutions in vivo and in vitro. Substitution of IleA10 in insulin with glycine reduced the receptor binding ability by 2 orders of magnitude. Conversely, exchange of the glycine A14 residue in relaxin, which corresponds to the insulin position A10 for isoleucine, reduced the bio-activity and the receptor binding capacity of relaxin about 100-fold. Substitution of L-Ala in insulin as well as relaxin represented a compromise that allowed both hormones to recover about 30% of the native potency. X-ray analysis and computer-derived energy calculation confirm our receptor binding and biological potency studies, which suggest that the functional difference between derivatives and native hormones is based upon a structural change introduced into the A chain loop by substitution of the penultimate intrachain loop residue. In order to achieve a conformation that favors dynamic or passive interaction with the receptor, insulin and relaxin require a different A chain loop structure in spite of the striking overall similarity.