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.
Büllesbach
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Department of Biochemistry and Molecular Biology, Medical University of South Carolina, Charleston 29425-2211.
Schwabe
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D000818:Animals; D002851:Chromatography, High Pressure Liquid; D002942:Circular Dichroism; D006801:Humans; D007328:Insulin / Q000737:chemistry / Q000378:metabolism*; D051379:Mice; D011487:Protein Conformation; D011972:Receptor, Insulin / Q000378:metabolism; D043562:Receptors, G-Protein-Coupled; D018000:Receptors, Peptide / Q000378:metabolism; D012065:Relaxin / Q000737:chemistry / Q000378:metabolism*; D013552:Swine
D007328:Insulin; D043562:Receptors, G-Protein-Coupled; D018000:Receptors, Peptide; C033395:relaxin receptors; D012065:Relaxin; D011972:Receptor, Insulin