Abstract

Glucagon can bind to terbium, resulting in a 1000-fold enhancement of terbium fluorescence. This process is critically dependent on pH and apparently requires that the NH2-terminal histidine residue of glucagon be in an unprotonated form. The terbium ion can be displaced by zinc but not readily by calcium. The binding of zinc or terbium to glucagon induces a large conformational change in the peptide resulting in a shift in the fluorescence emission maximum of glucagon from 352 to 339 mm and in a marked increase in the helix content of the peptide. Similar conformational changes occur with this peptide upon self-association or upon binding to lipids.