Conformation and calcium binding properties of gastrin fragments of increasing chain length

Conformation and calcium binding properties of a series of gastrin-related peptides, in which the glutamic acid sequence at the N-terminal portion of the molecule has been elongated step by step, have been investigated using circular dichroism spectroscopy. A working hypothesis about the structure of these hormones in trifluoroethanol has been proposed. The structure comprises aβ-bend located at the level of the sequence Ala-Tyr-Gly-Trp. A correlation between chain elongation and increase of biological potency has been observed. All examined peptides strongly interact with calcium ions in trifluoroethanol. The variation of the circular dichroism spectra upon calcium addition provided some information about the groups involved in the coordination of the ions. Our results allow the hypothesis of the presence of one binding site, located at the C-terminal portion of the molecule in the gastrin octapeptide, and of an additional site at the N-terminus, in the longer fragments. The carboxyl function of Asp and Glu side-chains, at the two ends of the molecules, are probably involved in the interaction with the metal ions.