Relaxin: structures, functions, promises, and nonevolution.

During the last two decades synthetic chemistry and molecular biology have transformed the little-known parturition-mediating factor relaxin into a chemically defined entity. Relaxin is a disulfide bond analog of insulin that shows no cross-reactivity to the insulin receptors, causes widening of the birth canal in most mammals, and has additional or different functions in various species. The receptor interaction site in relaxin has been located quite precisely in the midregion of the B chain helix and is now known to involve two arginine residues that project from the alpha helix in an n, n + 4 configuration. The A chain of relaxin which appears to be uninvolved in receptor binding, is nonetheless essential. In fact, a major structural determinant in relaxin and insulin responsible for achieving either an insulin-like or a relaxin-like conformation is located in the penultimate position in the intrachain loop of the A chains.