Tertiary structures of gastrin-like tetrapeptides

Statistical mechanics of protein structural transitions: Insights from the island model

The so-called island model of protein structural transition holds that hydrophobic interactions are the key to both the folding and function of proteins. Herein, the genesis and statistical mechanical basis of the island model of transitions are reviewed, by presenting the results of simulations of such transitions. Elucidating the physicochemical mechanism of protein structural formation is the foundation for understanding the hierarchical structure of life at the microscopic level. Based on the results obtained to date using the island model, remaining problems and future work in the field of protein structures are discussed, referencing Professor Saitô’s views on the hierarchic structure of science.

Computational analysis of conformational behavior of cholecystokinin fragments. I-CCK4, CCK5, CCK6 and CCK7 molecules

A conformational analysis has been performed on several peptide fragments (CCK4 to CCK7) of the cholecystokinin neuromodulator. The Monte-Carlo Metropolis method was used to explore the conformational space of all these flexible units and different electric charge distributions were introduced in order to mimic pH effects. Results agree reasonably well with experimental data from NMR and fluorescence experiments. The CCK4 fragment displays a peculiar conformational behavior when compared to all other longer peptides with short range interaction between the Trp and Phe aromatic side-chains. Several H-bonded conformers including C- or beta-turns are found for CCK5 to CCK7. These findings are correlated to the central and peripheral actions of these compounds and hypotheses concerning the best possible templates for each one are discussed.

Structural and conformational analogy between cholecystokinin and ergopeptines

The central nervous system (CNS) peptide cholecystokinin (CCK) and the ergopeptine alkaloids exhibit common pharmacology in the brain, particularly via catecholaminergic systems. We report here structural similarities between CCK and the ergot alkaloids, and the subsequent conformational analysis of the peptide undertaken to establish whether or not a three-dimensional relationship exists between the compounds. Two low-energy conformations of CCK that mimic the ergopeptine ergotamine are identified, one arising from an X-ray crystal structure and the other from a Dreiding model-based, computer-assisted search. The pharmacological, structural and conformational observations strongly support the hypothesis that CCK and the ergopeptines share common sites of action in the CNS.

High-resolution proton nuclear magnetic resonance studies of human gastrin

High-resolution 1H nuclear magnetic resonance (NMR) spectroscopy at 300 MHz has been used to study the behavior of human gastrin in aqueous solution. A large number of resonances have been assigned by analysis of one- and two-dimensional NMR spectra and the effects of pH and by comparison with the spectrum of des-less than Glu1-gastrin. In gastrin, the ratio of cis to trans conformations around the Gly-2 to Pro-3 peptide bond is 3:7. This is reflected in splitting of the resonances of several neighboring residues and of a residue distant in the sequence, Tyr-12. The pKa of Tyr-12 is 10.7. Sulfation of this residue perturbs the resonances of Tyr-12 and Gly-13 but has very little effect on the rest of the spectrum. A study of the temperature dependence shows that several perturbed resonances move toward their expected positions as the temperature is raised but with a linear dependence on temperature, consistent with a redistribution of populations among accessible local conformations rather than a cooperative conformational change. Addition of Na+ or Ca2+ causes only minor changes in the spectrum. The paramagnetic metal ion Co2+ produces a number of spectral changes, reflecting strong binding to at least one site involving the Glu residues and weaker binding to Asp-16.

Conformational calculations on gastrin C-terminal tetrapeptide

Energy optimizations were performed on some typical conformations of the gastrin C-terminal peptide amide NAc-Trp-Met-Asp-Phe-NH2. Two families of lowest energy conformations were found corresponding to: (a) alpha-helical structures; (b) conformations having beta-structure at the level of Trp residue, and C7-structure at the level of Asp residue. The two aromatic rings were folded on the peptide backbone and ca. 5 A distant from each other (centre to centre). The last family, favoured by energy and population probability, can better account for conformational experimental results and biological activity observations.